Overview of Upper Digestive Tract

Development of Gastrointestinal Tract

We will take a very short tour of early development prior to the trilaminar embryo stage, at which time we will follow the development of the gastrointestinal tract in detail. Thereafter, for each region of the gastrointestinal tract, we will begin with a short summary of the specific embryology relevant to the structures in that region. The single-celled zygote begins dividing roughly 30 hours after an oocyte is fertilized by a spermatozoa. It continues dividing without growing substantially until it reaches the 16-cell stage and is then referred to as a morula. The morula consists of an outer cell mass surrounding an inner cell mass, which will become the placenta and the embryo, respectively. For the purpose of this section we will focus on the inner cell mass as it morphs to create the body and the organs within.

As the zona pellucida (a protective covering of the oocyte and later the zygote) gradually disappears, fluid penetrates the morula and creates a space between the inner and outer cell masses. The inner cell mass remains in contact with the outer cell mass in one section, which will eventually form the connecting stalk and umbilical cord, connecting the embryo to the placenta. The fluid-filled space between the two cell masses is called the blastocyst cavity and at this time, roughly 4 days after fertilization, the entire structure is called a blastocyst. Normally, the blastocyst implants into the uterine lining starting on the sixth day and further development occurs within.

On the eighth day, another fluid-filled space forms between the inner cell mass and the rest of the blastocyst. This is the amniotic cavity ; despite its small initial size, it will eventually enlarge to surround the entire embryo. The portion of the inner cell mass that is in contact with the amniotic cavity, and amniotic fluid therein, is called the epiblast, and the portion that is in contact with the blastocyst cavity is called the hypoblast. The epiblast cells are tall columnar cells, and the smaller hypoblast cells appear cuboidal or squamous (flat). The epiblast and hypoblast layers constitute the bilaminar disc . By the ninth day, when the blastocyst has fully implanted into the uterus, the blastocyst cavity is referred to as the primary yolk sac. Cells that separate the primary yolk sac, bilaminar disc, and amniotic cavity from the developing placenta (cytotrophoblast and syncytiotrophoblast) form the extraembryonic mesoderm.

By the 12th day, fluid-filled gaps within the extraembryonic mesoderm converge and form yet another space, the extraembryonic cavity, which will compress the primary yolk sac before physically separating it and the bilaminar disc from the rest of the developing placenta except for a connecting stalk that will eventually become the umbilical cord. As the 13th and 14th days proceed, the primary yolk sac is compressed and pinched in two by the expanding extraembryonic cavity. One small remnant moves away from the bilaminar disc while the larger piece remains in contact with the hypoblast and is now called the secondary yolk sac. The secondary yolk sac is lined by cells that are derived from the hypoblast. In one region, these hypoblast cells enlarge and form the prechordal plate, a structure that marks the cranial/superior pole of the developing embryo. Opposite the prechordal plate, epiblast cells begin to proliferate near the embryo's caudal/inferior pole. These cells will form a structure called the primitive streak. This will eventually result in the process of gastrulation, during which the bilaminar disc is replaced by a trilaminar disc. Gastrulation begins on the 15th day and results in the replacement of the epiblast and hypoblast layers by three new germ cell layers, collectively called the trilaminar embryo. It consists of the embryonic ectoderm, embryonic mesoderm, and embryonic endoderm. As these layers are referred to hereafter, the word “embryonic” will be dropped.

To form the trilaminar disc, the primary streak extends from the caudal end of the epiblast toward the prechordal plate but does not quite reach it. As it extends cranially, replicating epiblast cells involute into it, invading the space between the epiblast and hypoblast, creating a fissure called the primitive groove. This process occurs along the entirety of the primitive streak, but there are some important features that occur at its cranial end, at an area called the primitive node. The epiblast cells that migrate through the primitive node migrate between the epiblast and hypoblast layers, moving directly toward the prechordal plate, forming a signaling structure called the notochordal process, an important structure in directing further development of the three germ cell layers. As gastrulation proceeds, the hypoblast is entirely replaced by cells that migrate through the primitive streak and settle in contact with the secondary yolk sac. This layer is the endoderm and will produce many of the body's glands as well as the cells that line the respiratory, urogenital, and gastrointestinal tracts. The cells of the former epiblast are now referred to as ectoderm; this layer will produce the epidermis, central nervous system, peripheral ganglia, and other cells of neural crest derivation. Between the endoderm and ectoderm is the mesoderm, a layer that will produce the kidneys and gonads, as well as the vascular, muscular, and connective tissue structures of the body. At this stage, we could choose to follow the development of any of the organ systems, but for the purpose of this volume we will focus on the development of the gastrointestinal system. Other systems will be mentioned in a more cursory manner when their development affects the gastrointestinal system.

The central region of the ectoderm pinches together to invade the mesoderm and form the midline neural groove on the 14th day of development. As development proceeds from the 16th to 18th day, the neural groove pinches together and invades the mesoderm as the neural tube, which differentiates to form the spinal cord, brainstem, and cerebral cortex. After the neural tube has detached from the ectoderm, other ectodermal cells, called the neural crest cells, migrate into the mesoderm. These cells migrate throughout the developing mesoderm to form the sympathetic chain ganglia, ganglia of the cranial nerves, and postsynaptic parasympathetic ganglia, among others. The mesoderm also undergoes several changes: the paraxial mesoderm is found to the immediate left and right of the neural tube and will form somites, which in turn form the axial skeleton, musculature, and dermis. Just lateral to the paraxial mesoderm is the intermediate mesoderm, which differentiates into gonads and precursors of the kidneys. Lateral to the intermediate mesoderm is the lateral plate mesoderm, which contributes to the body wall, limbs, and connective tissue structures that anchor the organs within the body cavities. In the case of the digestive system, the lateral plate mesoderm forms the abdominal wall that contains the contents of the peritoneal cavity but it also forms the smooth muscle and connective tissues that surround and support the gastrointestinal tract. It also creates the mesenteries that connect the digestive organs to the anterior and posterior abdominal wall. As mentioned already, the endoderm forms the lining of the gastrointestinal tract and several of the organs that develop from it. We will now describe how the trilaminar embryo morphs to create the abdominal cavity and organs within.

The lateral plate mesoderm is continuous on its lateral edge with the extraembryonic mesoderm that surrounds the developing embryo. It is sandwiched by the ectoderm and amniotic cavity dorsally; the endoderm and secondary yolk sac are located ventral to it. At 14 days of development, the lateral plate mesoderm constitutes a single mesodermal region, but shortly thereafter, gaps form within it that create a continuous, horseshoe-shaped space that extends from right to left, going around the cranial end of the embryo. This space is the intraembryonic cavity; as it enlarges, it becomes continuous with the extraembryonic cavity and it splits the lateral plate mesoderm into two layers. The parietal (somatic) layer of lateral plate mesoderm is the more superior of the two and is in direct contact with the ectoderm and amniotic cavity. The more inferior layer is the visceral (splanchnic) layer of lateral plate mesoderm and is in contact with the underlying endoderm and secondary yolk sac. This separation is complete but not really dramatic by the 16th day. However, as this space enlarges, it pushes the visceral layer and endoderm medially, creating a notable separation by the 18th day. The visceral layers of lateral plate mesoderm and endoderm on each side grow closer to each other, pinching the endoderm on the left and right, creating a tube that is separate from the rest of the secondary yolk sac. As this proceeds, the yolk sac stretches away from the developing gut tube and remains connected to it via the vitelline duct at the midgut, which will form the small intestine and part of the large intestine. Aside from its connection to the vitelline duct and secondary yolk sac, the rest of the endoderm and accompanying visceral lateral plate mesoderm fuse to form a complete tube that stretches from the oropharyngeal membrane (developing mouth) to the cloacal membrane (eventual anus and urogenital openings). This tube is the early gastrointestinal tract, and it will give rise to all the organs of digestion as well as the respiratory and urogenital tracts. From cranial to caudal, it is divided into the foregut (esophagus, stomach, proximal duodenum, liver, spleen, pancreas), midgut (distal duodenum, jejunum, ileum, vermiform appendix, cecum, ascending and transverse colon), and hindgut (descending colon, sigmoid colon, and rectum). In addition to the vitelline duct, another pouch of endoderm stretches away from the developing gut tube, the allantois. This pouch, originally a caudal extension of the primary yolk sac, extends off of the developing hindgut, and as development proceeds, it extends into the connecting stalk, cranial to the cloacal membrane. It contributes to the wall of the urinary bladder, but that is not our focus at this time. Eventually both the vitelline duct and allantois will extend alongside each other into the umbilical cord, and aberrations of each structure are associated with malformations of the midgut and urinary bladder, respectively.

While the gut tube is forming from the endoderm and visceral lateral plate mesoderm, a similar process is occurring with the ectoderm and parietal lateral plate mesoderm, so that a left and right lateral fold will form and fuse anteriorly to become the body wall. The left and right lateral folds first extend toward the yolk sac and then turn medially. As this happens, these layers pull the amniotic sac, which had previously covered a small area, to surround the entire developing embryo. Cross sections of the developing embryo at 18 days will appear remarkably different, depending upon whether or not the cross section includes the secondary yolk sac and vitelline duct. A cross section that includes the yolk sac will show an incompletely fused gut tube at the midgut, with the vitelline duct leading away from, and opening into, a ballooned yolk sac. The lateral folds have not yet migrated anteriorly enough to form a complete body wall. However, a cross section in a more posterior plane will exclude the yolk sac and show a fused anterior body wall surrounding a circular gut tube. The gut tube remains anchored to the anterior body wall by the ventral mesentery, which will largely disappear, and the dorsal mesentery, which will remain and transmit the vessels and nerves that connect the gut tube to the rest of the body. The gut tube, with its surrounding visceral layer of lateral plate mesoderm, separates the right and left peritoneal cavities, “descendants” of the intraembryonic coelom to either side. When the ventral mesentery disappears, there will be a single peritoneal cavity. By this time, the amniotic cavity almost entirely covers the developing embryo, with only a narrow span of mesoderm separating the right and left lateral folds.

Before 1 month of development has passed, the heart has descended into the thoracic region, bringing along a mesodermal structure, the septum transversum, which will contribute to the diaphragm. The septum transversum narrows the peritoneal cavity considerably, leaving two small openings between the pericardial cavity in the thorax and the peritoneal cavity in the abdomen. These are the pericardioperitoneal canals and they are normally closed as the diaphragm receives a left and right pleuroperitoneal membrane from the body wall. Contributions from the dorsal mesentery of the esophagus and muscle from the body wall assist in closing these canals and creating the diaphragm by the ninth week. Later, the musculature of the diaphragm develops as a secondary ingrowth from the body wall. The phrenic innervation from the cervical spinal cord to the diaphragm originates when the transverse septum first develops at the cervical level of the embryo. As the septum shifts to a low thoracic level, the phrenic nerves elongate. The commonest developmental abnormality of the diaphragm is a faulty growth of the left pleuroperitoneal membrane, resulting in an opening through which abdominal viscera may herniate into the left pleural cavity.

Caudal to the developing diaphragm is the foregut. The ventral and dorsal mesenteries remain in contact with the foregut, but the ventral mesentery disappears along the midgut and hindgut, leaving the developing gut tube suspended in the abdominal cavity. From the dorsal aorta, the celiac trunk supplies blood to the foregut, and its branches will supply all of the foregut organs as they develop. Extensions of the foregut stretch into the ventral and dorsal mesenteries to create the hepatic diverticulum and dorsal pancreatic bud, respectively. The hepatic diverticulum will form the liver and gallbladder but will also give rise to a ventral pancreatic bud, which will fuse with the dorsal pancreatic bud to form the entire pancreas. The ventral mesentery remains in contact with the developing liver, eventually forming the falciform ligament. The further development of this region will be covered in the sections related to the specific foregut organs, the esophagus, stomach, duodenum, liver, gallbladder, and pancreas.

During the sixth week the midgut has begun to elongate substantially and runs out of room within the peritoneal cavity. It moves into the umbilical cord, creating a physiologic umbilical herniation, which is a normal event in the development of the gastrointestinal system. The vitelline duct has narrowed but still connects the midgut to the secondary yolk sac, and this connection is one of the reasons that the physiologic herniation occurs, pulling the midgut into the umbilical cord. The vitelline duct will typically disappear roughly 10 weeks into development as the midgut starts returning to the peritoneal cavity. The superior mesenteric artery is derived from the vitelline artery and supplies all the developing midgut structures and, eventually, all organs of the midgut. The further development of this region will be covered in the sections related to the small and large intestines.

Development of the hindgut is intimately connected with the urinary and reproductive systems. All three systems initially empty into a common chamber, the cloaca, which is separated from the amniotic cavity by a cloacal membrane. The allantois extends from the cranial end of the cloaca and stretches into the umbilical cord alongside the vitelline duct. Between 4 and 7 weeks, the mesoderm located between the allantois and the vitelline duct/midgut, called the urorectal septum, extends caudally and separates the hindgut from the rest of the cloaca, which will hereafter be called the urogenital sinus. By the end of 7 weeks, the urorectal septum has totally partitioned the digestive and urogenital systems, leaving a urogenital membrane and anal membrane on the external surface of the body in the place of the cloacal membrane. The inferior mesenteric artery will supply all the hindgut organs. The further development of this region will be covered in the sections related to the large intestine and anal regions.

Although the foregut began as a simple, midline, tubular structure lined by epithelium derived from endoderm, it twists, expands, and elongates to create the adult relationships between each abdominal organ. Fusing and expansion of the dorsal mesenteries are key in this process. The portion of the foregut that will become the stomach first starts to expand in the sagittal plane, ballooning outward on its anterior and posterior surfaces. However, the expansion of the posterior surface quickly outpaces the other side and the stomach begins to bend. The enlarged expansion of the posterior side will become the stomach's greater curvature, and the anterior side will become the lesser curvature. As this is happening, the presumptive stomach rotates so that the posterior side shifts toward the left of the body while the anterior right side shifts to the right. The rotation and expansion of the posterior side are what give the stomach its characteristic shape, with the esophagus entering just to the right of the fundus and greater curvature, and the outlet of the stomach, the pyloric region, shifting to the right and slightly superior to the greater curvature. This moves the stomach from a superior/inferior axis to more of a right/left axis within the abdomen. The inner, circular layer of muscle at the terminus of the stomach enlarges significantly to form the pyloric sphincter.

The rotation and expansion of the stomach do not occur in isolation. The foregut is attached to the posterior body wall by a dorsal (posterior) mesentery, called the dorsal mesogastrium, in which the spleen and dorsal part of the pancreas will develop. The section of this mesentery between the developing spleen and the stomach will become the greater omentum. Anteriorly it is connected to the liver, and thereafter, to the anterior body wall by a ventral (anterior) mesentery. The section of the ventral mesentery that attaches the liver to the anterior body wall will become the falciform ligament, and the section between the liver, stomach, and duodenum will form the lesser omentum. As the stomach's posterior surface expands and rotates to the left, the attached mesentery follows, laying the spleen along the left side of the abdominal cavity. The dorsal mesentery between the stomach and spleen expands, folding onto itself and creating a large pocket between the two folds. The pocket thus formed is called the omental bursa. Continued rotation and expansion of the greater curvature bring this double-layered “apron” to extend inferiorly from the stomach, falling anterior to the transverse colon and small intestine. The motion of the developing stomach and growth of the liver shift the stomach to the left and the liver to the right side of the abdomen. This also brings the omental bursa to lie anterior to the pancreas, inferior to the inferior surface of the liver, and posterior to the stomach and lesser omentum, which can be subdivided into the hepatogastric and hepatoduodenal ligaments. Occasionally the omental bursa can extend superiorly and posteriorly to the liver as the superior recess of the omental bursa. In its mature form, the omental bursa is isolated from the rest of the abdominal cavity, except for a small opening called the omental foramen located immediately posterior to the right edge of the hepatoduodenal ligament.

Regions of the Abdomen

For the sake of convenience, the abdomen is traditionally divided into either four or nine regions. Of these two (somewhat artificial) divisions, the more simple one uses two imaginary planes, one passing vertically and the other horizontally through the umbilicus, dividing the abdomen into four quadrants, the right upper, left upper, right lower, and left lower quadrants.

A division of the abdomen into nine smaller areas is accomplished by the use of two vertical and two horizontal planes. The zone above the upper of the two horizontal planes is divided by the two vertical planes into a centrally placed epigastric region (epigastrium), with a right and left hypochondriac region on each side of it. The zone between the two horizontal planes is divided into a centrally placed umbilical region, with a left and right lumbar region on each side. The zone below the lower of the two horizontal planes has a centrally placed hypogastric region and a right and left inguinal region.

Different resources list different landmarks as the basis for drawing the lines of the nine-region scheme. The upper horizontal (superior transverse) line, or plane, may be drawn halfway between the superior border of the sternum and the superior border of the symphysis pubis. This plane has been considered as passing through the pylorus and has thus been called the transpyloric plane, which also has been described as being halfway between the xiphisternal junction and the umbilicus, and passing through the tip of the ninth costal cartilage, the fundus of the gallbladder, and the lower part of the body of the first lumbar vertebra. Another way of locating the upper horizontal plane is at the most inferior part of the costal margin (usually the most caudal part of the 10th costal cartilage). This plane is called the subcostal plane.

The lower horizontal (inferior transverse) line, or plane, may be assigned to the levels of the tubercles of the iliac crests and is called the transtubercular plane; it usually passes through the lower part of the fifth lumbar vertebra, or it may be located at the level of the anterior superior spine of the ilium and called the interspinous plane. It has also been located at the highest points of the iliac crests and called the supracristal plane.

The two vertical planes, or lines, one on each side, may be located halfway between the median plane and the anterior superior spine of the ilium (or halfway between the pubic tubercle and the anterior superior spine of the ilium or the midpoint of the inguinal ligament; right and left midinguinal planes ). The other common way of locating the vertical plane on each side uses the lateral border of the rectus abdominis muscle or the semilunar line, which, if followed inferiorly and medially toward the pubic tubercle, brings the entire inguinal canal into the inguinal region.

In attempting to use either quadrants or the smaller-sized nine regions in the localization of viscera, one finds that a substantial number of individual organs are not confined to any one region. Attention should be called to the fact that, because the diaphragm is the upper limit of the abdomen, most of the hypochondriac (as the name indicates) regions and parts of the epigastric region are under cover of the ribs. Because these three regions make up a good portion of the right and left upper quadrants, these quadrants also extend well up under the ribs.

Bony Framework of Abdominopelvic Cavity

The skeletal framework that serves as the attachment site for the muscles that make up the abdominal and pelvic walls consists of the lower ribs, costal cartilages, five lumbar vertebrae, and bony pelvis. The costal cartilages of the fifth, sixth, and seventh ribs angle obliquely upward and medially to join the sternum superior and lateral to the xiphisternal junction. The terminal portion of each of the 8th, 9th, and 10th costal cartilages tapers to a point and is attached to the lower border of the costal cartilage above. The 11th and 12th costal cartilages are quite short, with pointed tips, neither of which attaches to the cartilage above it. The lower border of the 10th costal cartilage is commonly the most inferior part of the caudal margin of the thoracic cage. From the beginning of the 10th costal cartilage to the junction of the 7th costal cartilage with the sternum, a cartilaginous border is formed, which is frequently referred to as the “costal arch” (costal margin), although this term is perhaps more correctly used to refer to the arch formed by the right and left cartilaginous borders as they are connected by the lower end of the sternal body from which the variable xiphoid process of the sternum projects. The latter serves as a landmark for the level of the body of the 10th (or 11th) thoracic vertebra.

The five lumbar vertebrae present the parts described for a typical vertebral body (centrum) and vertebral (neural) arch, supporting the two transverse processes, the spinous process, and the superior and inferior articular processes.

The bony pelvis is made up of the two hip bones, with the sacrum and coccyx wedged between them posteriorly. For descriptive purposes, the bony pelvis is divided by a plane passing through the sacral promontory and the crest of the pubis, into the major (false) pelvis above the plane and the minor (true) pelvis below this plane. This plane lies roughly in the inlet of the true pelvis, which is bounded by the sacral promontory, crest of the pubis, anterior margin of the ala of the sacrum, the arcuate line of the ilium, and the pecten pubis, all of which could be considered as forming the linea terminalis.

The hip bone (os coxae or innominate bone) is made up of the ilium, pubis, and ischium, which are separate bones in the young subject but fuse at the acetabulum in the adult. On the inner surface of the ilium, the arcuate line indicates the inferior border of the ala of the ilium, which ends superiorly in the palpable iliac crest, stretching from the anterior superior spine of the ilium to the posterior superior iliac spine. The crest also presents an external (lateral) lip, an internal (medial) lip, and an intermediate line and thickening on its lateral aspect a short distance posterior to the anterior superior spine, which is called the tubercle of the crest. The body of the pubis joins the pubic bone on the other side, by means of a fibrocartilaginous lamina, the symphysis pubis. The upper border of the body, which is thick, roughened, and turned anteroinferiorly, is called the crest, and at its lateral end is a prominence named the pubic tubercle. The superior ramus of the pubis, coursing superiorly and posterolaterally, enters into the formation of the acetabulum (acetabular portion, sometimes called the body) and presents a prominent pecten pubis, or pectineal line, which is continuous with the arcuate line of the ilium. The inferior ramus courses interiorly and posterolaterally, to join the ramus of the ischium and complete the margins of the obturator foramen. The main portion of the ischium extends interiorly and posteriorly from the acetabulum, to expand into the ischial tuberosity, which projects posteroinferiorly. From the posterior border of the inner side of the lower part of the acetabular portion of the ischium, the ischial spine projects posteromedially between the greater and lesser sciatic notches. A ramus of the ischium courses anteriorly from the lower end of the main portion of the bone, to become continuous with the inferior ramus of the pubis, forming what is often referred to as the ischiopubic ramus.

Anterolateral Abdominal Wall

Before describing the walls of the abdomen, it is necessary to mention different ways in which the word abdomen is used. In some cases, abdomen is synonymous with abdominopelvic cavity, but in other cases, it is used in a more specific sense to refer to that portion of the body cavity between the diaphragm and the pelvis minor (true pelvis). Abdomen is also used more loosely to refer to a general region of the body.

For purposes of specificity, it seems advisable to name that portion of the body cavity below the diaphragm the “abdominopelvic cavity” and then to divide this into the abdominal cavity proper and the pelvic cavity (pelvis minor), separated from each other by the plane of the pelvic inlet (the plane passing through the sacral promontory and the pubic crests). It must be remembered, however, that certain structures that are ordinarily referred to as abdominal structures (some of the coils of small intestine, for example) usually hang into the pelvic cavity, and that the inferior and posteroinferior support of the abdominal viscera is furnished by the walls of the pelvic cavity and not by the theoretical plane at the pelvic inlet. It is convenient to divide the borders of the abdominopelvic cavity into four general parts—the anterolateral abdominal wall, the posterior wall of the abdominal cavity, the diaphragm (superior wall or roof of the abdominal and abdominopelvic cavities), and the bowl of the pelvic cavity, which can be loosely called the floor of the abdominopelvic cavity. However, the limits of each boundary are not sharp, because we are dealing with curved contours, and certain arbitrary limits need to be defined for descriptive purposes. This has been done in part above and will be completed as necessary at appropriate places in the following descriptions.

The anterolateral abdominal wall fills in the gap in the bony-cartilaginous framework between the costal margin superiorly and the hip bones inferiorly. Following the curve of the body laterally, several muscles, nerves, vessels, and fascial layers will be encountered. For the present work, the quadratus lumborum muscle and the structures medial to it will be included with the posterior wall of the abdominal cavity. Owing to its muscular components, the anterolateral abdominal wall can contract and relax and, thus, help to accommodate the size of the abdominopelvic cavity to changes in volume of the contained viscera and to control intraabdominal pressure. The surgical approach to the abdominopelvic cavity is commonly made through this wall.

Starting from the outside, the layers of the anterolateral abdominal wall are skin, subcutaneous fat (superficial fascia), outer investing layer of deep fascia, the muscles with their related fasciae, transversalis fascia, extraperitoneal fascia, and parietal peritoneum. Abdominal skin is of average thickness (thicker posteriorly than anteriorly and laterally) and rather loosely attached to the underlying layers, except in the area of the umbilicus.

The subcutaneous fat is soft, movable, and contains a variable amount of fat, depending mostly on the state of nutrition of the individual and varying to some extent in distribution. The thickness of this layer can be roughly estimated by picking up a fold, the thickness of which, minus the double thickness of the skin, would be about twice the thickness of the layer. The superficial fascia, particularly of the part of the wall inferior to the level of the umbilicus, has been classically described as having a superficial fatty layer, called the Camper fascia, and a deep membranous layer (to some extent discontinuous), called the Scarpa fascia. This classical description is somewhat of a simplification of the actual situation, in which the layering is not always as clear-cut as indicated, but it serves as a means of description if this is kept in mind. The Camper layer is continuous with the fatty layer of surrounding areas, such as the superficial fascia of the thigh. The Scarpa layer fuses with the fascia lata along a line parallel to and just inferior to the inguinal ligament. Medial to the pubic tubercle, both layers continue into the urogenital region. This is significant in relation to the path that extravasated urine takes after injuries to the urethra or neck of the bladder. When entering the fasciae in the perineal region, this urine and blood may escape superiorly into the anterolateral abdominal wall. In the male, the two layers continue into the scrotum and blend into a single, smooth muscle-containing layer, the fat being rather abruptly lost as they enter into the formation of the scrotum. Just above the symphysis pubis a considerable addition of closely set strong bands to the Scarpa fascia form the fundiform ligament of the penis, which extends down onto the dorsum and sides of the penis.

The outer investing layer of the deep fascia (not readily distinguished from the muscular fascia on the external surface of the external abdominal oblique muscle and its aponeurosis ) is easily demonstrable over the fleshy portion of the muscle but is much more difficult to separate from the aponeurotic portion of the muscle. This layer is attached to the inguinal ligament and blends with the fascia coming out from under this ligament to form the fascia lata. It also joins with the fascia on the inner surface of the external oblique at the superficial inguinal ring to form the external spermatic fascia. External to the inferior end of the linea alba, the outer investing layer is thickened into the suspensory ligament of the penis, which anchors the penis to the symphysis pubis and the inferior pubic ligament. It is also continuous with the deep fascia investing the penis.

The external abdominal oblique muscle typically arises by eight digitations from the external surfaces of the lower eight ribs lateral to the costochondral junction, the middle group of digitations arising at a greater distance lateral to the junction than the ones above and below them. The upper five slips interdigitate with the serratus anterior muscle, and the lower three slips interdigitate with the latissimus dorsi muscle. The general direction taken by the fibers of this muscle is anteroinferior from their site of origin, and this leads the fibers from the lower two or three digitations to a fleshy insertion on the anterior half of the outer lip of the crest of the ilium, this portion of the muscle having a free posterior border that forms the anterior side of the lumbar triangle. The muscular portion from the remainder of the origin becomes the strong aponeurosis of this muscle along a line that courses vertically inferiorly through about the tip of the ninth costal cartilage to the level of the anterior superior iliac spine, where it curves rather sharply laterally to course toward this spine. The aponeurosis passes in front of the rectus abdominis muscle (where it partly fuses with the aponeurosis of the internal oblique) to blend with the one of the opposite side in the midline linea alba, gaining attachment to the xiphoid process at the upper end of the linea alba and to the pubis at the lower end. The lower margin of the aponeurosis is folded backward and slightly upward upon itself between the anterior superior iliac spine and the pubic tubercle. The folded edge, together with an extremely variable number of fibrous strands running along it, is called the inguinal ligament.

The nerve supply of the external abdominal oblique muscle is derived from the ventral rami of the 6th to 12th thoracic spinal nerves. The 6th to the 11th are intercostal nerves, which continue from the intercostal spaces into the anterolateral abdominal wall to lie in the plane between the internal abdominal oblique and transversus abdominis muscles. The 12th thoracic nerve is the subcostal nerve, and it follows a course similar to the intercostal nerves above. The iliohypogastric nerve from the anterior ramus of L1 also contributes to the supply. The nerves have a segmental distribution corresponding to the primitive segmental condition of the muscle, with the 10th thoracic extending toward the umbilicus and the 12th toward a point about halfway between the umbilicus and the symphysis pubis.

The external abdominal oblique muscle has several actions in common with the other large muscles of the anterolateral abdominal wall. These are to (1) support the abdominal viscera and, by compressing them, help to expel their contents; (2) depress the thorax in expiration; (3) flex the spinal column; and (4) assist in rotation of the thorax and pelvis in relation to each other. With the pelvis fixed in place, contraction of the external oblique of one side produces a rotation that brings the shoulder of the same side anteriorly.

The internal abdominal oblique muscle, smaller and thinner than the external oblique, arises from the posterior layer of the thoracolumbar fascia, from the anterior two thirds or more of the intermediate line (lip) of the iliac crest and the lateral one half to two thirds of the folded-under edge of the external oblique aponeurosis, together with the immediately adjacent and closely related iliac fascia. The majority of the fibers from the thoracolumbar fascia and the iliac crest course superiorly and medially, which means that their direction is perpendicular to the general direction of the fibers of the external oblique. The most posterior fibers insert on the inferior borders of the lower three (or four) ribs and their costal cartilages. The rest of these fibers end in an aponeurosis along a line which extends inferiorly and medially from the 10th costal cartilage toward the crest of the pubis. In the upper two thirds (to three fourths) of the abdomen, the aponeurosis splits at the lateral margin of the rectus into a posterior layer, which passes posterior to the rectus abdominis muscle, and an anterior layer, which passes anterior to it. These two layers join medial to each of the two rectus abdominis muscles and blend with those of the opposite side in the linea alba. In the lower one third of the abdomen, the aponeurosis of the internal abdominal oblique does not split but passes entirely anterior to the rectus abdominis muscle to reach the linea alba. The fibers arising from the margin of the external oblique aponeurosis and the related iliac fascia are paler and less compact and course downward and medially, arching superior to the spermatic cord in the male (round ligament in the female). This portion of the internal oblique is generally closely blended with the related portion of the transversus abdominis muscle and tends to fuse with it to create a common, more or less aponeurotic, insertion that passes anterior to the insertion of the rectus muscle on the pubic crest and for a variable distance on the pecten pubis as the conjoint tendon (inguinal falx). The nerve supply of the internal abdominal oblique is by way of the lowest two or three intercostal nerves, as well as the subcostal, iliohypogastric, and ilioinguinal nerves. The actions of the internal oblique are similar to those of the external oblique (see above), except that contraction of the muscle of one side would help to produce a rotation that would bring the shoulder of the same side posteriorly if the pelvis were fixed in place.

The cremaster muscle is well developed only in the male because it is an extension of the lower border of the internal abdominal oblique muscle that travels into the spermatic cord. Laterally it is thicker and fleshier and attaches to about the middle of the turned-under edge of the external abdominal oblique aponeurosis and to the inferior edge of the internal abdominal oblique muscle. From here, the somewhat scattered muscle fibers spread over the spermatic cord along with connective tissue (cremasteric fascia) running between them to end at the pubic tubercle and the anterior layer of the rectus sheath. The nerve supply of this muscle is from the genital branch of the genitofemoral nerve and also a branch from the ilioinguinal nerve. The action of the cremaster muscle is to lift the testis toward the superficial inguinal ring.

The transversus abdominis is a broad thin muscle that takes a nearly horizontal course around the inner side of the anterolateral abdominal wall. It arises from (1) the inner surfaces of the costal cartilages of the lower six ribs by fleshy slips, which interdigitate with the slips that make up the costal origin of the diaphragm; (2) an aponeurosis formed by the union at the lateral border of the erector spinae muscle of the layer of the thoracolumbar fascia attached to the tips of the transverse processes of the lumbar vertebrae and the layer of this fascia attached to the tips of the spinous processes of the same vertebrae (an indirect origin from the lumbar vertebrae); (3) the anterior one half to three fourths of the internal lip of the iliac crest; and (4) approximately the lateral one third of the folded-under margin of the external oblique aponeurosis and the closely related portion of the iliac fascia. The muscular fibers terminate in a strong (for most of its extent) aponeurosis along a line that extends from deep to the rectus muscle above and courses interiorly and slightly laterally to emerge lateral to the rectus at about the level of the umbilicus and then to extend variably toward the middle of the inguinal ligament. In the upper two thirds to three fourths of the abdomen, the aponeurosis passes posterior to the rectus muscle, fusing with the posterior layer of the aponeurosis of the internal abdominal oblique muscle, and ends by meeting the one of the opposite side in the linea alba. Insertion occurs also on the xiphoid process at the upper end of the linea alba. In the lower one fourth to one third of the abdomen, the aponeurosis passes anterior to the rectus muscle to reach the linea alba. The lower fibers of the transversus abdominis muscle have a common insertion with the lower fibers of the internal oblique, as described with the insertion of the latter muscle above. The transversus abdominis muscle is often described as having an inferior free border that arches over the spermatic cord in the male (round ligament in the female) from the origin on the external oblique aponeurosis to the pubic attachment. The nerve supply of the transversus muscle comes from the anterior rami of the lower five or six intercostal and subcostal nerves as well as the iliohypogastric, ilioinguinal, and genitofemoral nerves. The actions of the transversus muscle are the same as those listed as being common to the external oblique and other large muscles of the abdomen. Unilateral contraction of one side of the transversus abdominis muscle will not produce appreciable rotation.

The rectus abdominis is a flat, vertical muscle, located just lateral to the anterior midline, which is wider and thinner superiorly and becomes narrower and thicker inferiorly. It has a superior and an inferior attachment, each of which is called the origin of the muscle by some authors and the insertion by others. Several incomplete, zigzag, transversely running tendinous bands are present in the muscle, creating its distinctive appearance. These are better developed on the anterior surface of the muscle and are closely attached to the anterior wall of the rectus sheath. The one at the level of the umbilicus is segmentally related to the 10th rib. Two are usually present between the umbilicus and the xiphoid process, and, in about one third of the instances, one is found below the level of the umbilicus. The superior attachment of the rectus muscle is to the anterior surfaces of the fifth, sixth, and seventh costal cartilages, the xiphoid process, and the costoxiphoid ligament. These attachments fall more or less in a horizontal line. The inferior (caudal) or pubic attachment of the rectus muscle is by a short tendon, a broader lateral portion of which ends on a roughened area on the pubic crest, extending from the pubic tubercle to the pubic symphysis. The narrower medial portion of the tendon is attached to the front of the symphysis, where it interdigitates with the one of the opposite side. The nerve supply of the rectus abdominis muscle comes from the anterior branches of the anterior rami of the lower six or seven intercostal nerves that enter the deep surface of the muscle near its lateral edge to send cutaneous branches obliquely through the muscle as muscular branches enter into the formation of an intramuscular plexus. The branch from the 10th thoracic nerve usually enters the muscle below the tendinous inscription at the level of the umbilicus. The rectus abdominis muscle generally acts in conjunction with the previously described muscles to compress the abdominal organs and during respiratory expiration. However, it is particularly involved in producing flexion of the vertebral column, bringing the xiphoid and pubic bones closer together.

The pyramidalis is a small and seemingly unimportant muscle that is absent in 20% to 25% of the population. It arises from the crest of the pubis, just anterior to the line of attachment of the rectus muscle, and its fibers run superiorly and toward the linea alba, into which they insert as high as one third of the distance to the umbilicus. The pyramidalis is supplied by a branch from the subcostal nerve and, sometimes, also the iliohypogastric or ilioinguinal nerves. No biomechanical importance is ascribed to this muscle, although it does tense the linea alba, anchoring it to the pubic bones.

The rectus abdominis and pyramidalis muscles are wrapped in a sheath formed, for the most part, by the aponeuroses of the three large flat muscles of the anterolateral abdominal wall, the make-up of which differs in the lower one fourth to one third of the abdomen from the make-up of the rest of its length. In the upper two thirds to three fourths of the abdomen, the aponeurosis of the external abdominal oblique muscle fuses with the anterior lamella of the aponeurosis of the internal abdominal oblique muscle to form the anterior layer of the rectus sheath, and the aponeurosis of the transversus abdominis muscle fuses with the posterior lamella of the internal oblique aponeurosis to form the posterior layer of the rectus sheath. The anterior and posterior layers of the sheath fuse medial to the rectus muscle in the linea alba, and, at the lateral margin of the rectus muscle, the anterior and posterior layers come together at the line of the splitting of the aponeurosis of the internal oblique. The posterior layer of the sheath does not extend superior to the costal margin, so that the uppermost part of the rectus muscle lies directly on the chest wall. In the lower part of the abdomen, the aponeurosis of the internal oblique muscle does not split into two layers, and both it and the greater part of the aponeurosis of the transversus muscle pass anterior to the rectus muscle, so that only the transversalis fascia forms the posterior layer of the rectus sheath in this area. Usually, the inferior margin of the definitely aponeurotic part of the posterior layer of the sheath is an obvious margin, called the arcuate line.

The transversalis fascia is thin and adherent in some areas and thickened and more independent in others. At the arched lower border of the transversus muscle, the transversalis fascia is thought to fuse with the fascia on the external surface of the transversus and to form a sheet extending to the inguinal ligament. This fascia extends deep to the inguinal ligament to form the anterior wall of the femoral sheath. Lateral to this, in the area where the transversus abdominis arises from the turned-under edge of the external oblique aponeurosis and the related iliac fascia, the transversalis fascia fuses with the iliac fascia.

The extraperitoneal fascia (subserous fascia) is thin and comparatively free from fat on the roof and anterolateral abdominal wall, except in the lowest portion, where it is loose and fatty to allow for the expansion of the bladder. In contrast to the situation on the roof and most of the anterolateral abdominal wall, the extraperitoneal tissue on the posterior wall of the abdominal cavity is large and quite fatty, particularly around the great vessels and kidneys.

Inguinal Canal

The space occupied by the spermatic cord and its coverings as it passes obliquely through the anterolateral abdominal wall in the male is called the inguinal canal. A similar inguinal canal is present in the female; it transmits the round ligament of the uterus toward its termination in the labia majora. For the sake of convenience, the description given here will be based on the male. In general, it can be said that the canal and the structures described in relation to it are much the same in the female, although somewhat narrower.

The inguinal canal is an oblique tunnel, 3 to 5 cm long, through the muscular and deep fascial layers of the anterior abdominal wall that lie parallel to and just above the inguinal ligament. The canal extends between the deep inguinal ring, located in the transversalis fascia approximately halfway between the anterior superior spine of the ilium and the pubic symphysis, and the superficial inguinal ring, located in the aponeurosis of the external abdominal oblique muscle just superior and lateral to the pubic tubercle. The deep inguinal ring can be described as a funnel-shaped opening in the transversalis fascia, because it is the site at which this fascia is continued onto the spermatic cord to become the innermost covering of the cord, the internal spermatic fascia. The inferior epigastric vessels are just inferomedial to the deep inguinal ring, and the most lateral part of the inferior border of the transversus muscle is just superolateral to this ring. The superficial inguinal ring is formed by a splitting apart of the fibers of the external abdominal oblique aponeurosis, with those fibers that pass superomedial to the ring going to intermingle with similar ones of the opposite side and attach to the anteroinferior surface of the symphysis pubis. This portion of the external oblique aponeurosis is called the medial crus of the superficial ring. The fibers of the external oblique aponeurosis that pass inferolateral to the superficial inguinal ring are the lateral crus of the ring, which, in a sense, is the medial end of the inguinal ligament.

The lower border of the external abdominal oblique aponeurosis is folded under upon itself, with the edge of the fold (and variable added fibrous strands) forming the inguinal ligament. The fascia lata on the anterior aspect of the thigh is closely blended to the full length of the inguinal ligament. Its lateral half, folded deep to the aponeurosis, is firmly fused with the iliac fascia as the iliacus muscle passes into the thigh. As to the medial half of the inguinal ligament, the folded edge is actually formed by the fibers of the aponeurosis rolling under in such a way that the fibers forming the inferolateral margin of the superficial inguinal ring become the most inferior fibers at the attachment to the pubic bone and thus attach most interiorly on the pubic tubercle, whereas the fibers that were originally more inferior attach higher up on the tubercle and in sequence along the medial part of the pecten pubis for a variable distance, with the lowest fibers in the aponeurosis attaching farthest laterally on the pecten. The portion of the aponeurosis that runs posteriorly and superiorly from the folded edge to the pecten pubis can be called the pectineal part of the inguinal ligament, or the lacunar ligament. The fibers of the external oblique aponeurosis, described above, are attached to the pubic tubercle and the pecten pubis and continue, to a varying extent, beyond these points of attachment. Those which continue from the pecten pubis superiorly and medially superficial to the conjoined tendon reach the midline and blend somewhat with the external oblique aponeurosis of the opposite side. They are called the reflected inguinal ligament.

Lateral to the superficial inguinal ring, variable fibrous strands course roughly perpendicular to the fibers of the external oblique aponeurosis and are blended with the fibers of the superficial surface of this aponeurosis. These fibers, called the intercrural fibers, can be thought of as helping to prevent the split between the two crura of the external oblique aponeurosis (the superficial inguinal ring) from extending farther laterally.

Another structure that is frequently described as being formed by fibers from the external abdominal oblique aponeurosis, and which has considerable clinical significance as a firm structure to which sutures can be anchored in the surgical repair of hernia, is the pectineal ligament (Cooper ligament). This ligament runs along the sharp edge of the pecten pubis and has the effect of heightening this ridge. It is often described as being formed by fibers of the lateral part of the pectineal portion of the inguinal ligament (lacunar ligament) which, as they approach the pecten, turn sharply superolaterally to run along it. The pectineal ligament can also be interpreted as a building up of the periosteum along the pecten pubis, which is more in keeping with what appears to be the situation in many cadavers.

The origins and insertions of the internal abdominal oblique muscle and the transversus abdominis muscle have been described previously, but certain details in regard to the portions of these muscles related to the inguinal canal merit additional description. The exact amount of the turned-under edge of the external abdominal oblique aponeurosis (and the adjacent iliac fascia to which this edge of the aponeurosis is closely related) from which these two muscles take origin is quite variable, and it may be difficult to separate muscles in this area. The origin of the internal oblique muscle, more times than not, extends far enough medially so that some fasciculi of the muscle are anterior to the spermatic cord as its constituent structures come together at the deep inguinal ring, thus reinforcing this area to a certain extent. The origin of the transversus abdominis muscle (if it can be adequately separated) usually does not extend medially beyond the lateral border of the superficial inguinal ring, if it extends even that far. Because the conjoined tendon inserts on the pecten pubis and the crest of the pubis and thus along a line that angles from the pecten onto the crest, the part of this tendon inserting on the pecten is in one plane and that inserting on the crest is in a somewhat different plane. The part of the conjoined tendon inserting on the pecten pubis is partially fitted to the contour of the spermatic cord, and it approaches the pecten from posterior to the spermatic cord to meet the lacunar ligament (pectineal part of the inguinal ligament), which approaches the pecten from below the spermatic cord.

The inguinal canal and the structures within it can be further elucidated by thinking of this tubular tunnel as having a roof, a floor, and anterior and posterior walls, although, of course, because the tunnel is shaped to accommodate a cylindrical structure (the spermatic cord), no sharp boundary between any of the four walls can be established. It should be further remembered that the openings at the ends of the tunnel are not in planes perpendicular to the long axis of the tunnel but are in planes that form an acute angle with the long axis of the tunnel, so that the posterior wall of the canal extends farther medially than does the anterior wall and the anterior wall extends farther laterally than does the posterior wall. The two openings, of course, are the deep inguinal ring in the transversalis fascia at the internal end of the canal and the superficial inguinal ring in the aponeurosis of the external abdominal oblique muscle at the external end of the canal. The external abdominal oblique aponeurosis, strengthened by the intercrural fibers, is present in the entire length of the anterior wall of the canal. For approximately the lateral one quarter to one third of the canal, fibers of the internal oblique muscle, which arise from the inguinal ligament and related iliac fascia, form the anterior wall of the canal deep to the external oblique aponeurosis. Superficial to the external oblique aponeurosis lie the superficial fascia and the skin, which continue medially beyond the anterior wall of the canal above the superficial inguinal ring. The floor (inferior boundary) of the canal is formed in its medial two thirds to three quarters by the rolled-under portion of the external oblique aponeurosis together with the lacunar ligament (pectineal portion of the inguinal ligament), forming a shelf upon which the spermatic cord rests. The transversalis fascia is present for the entire length of the posterior wall of the canal. Toward the medial end of the canal, and thus reinforcing the part of this wall posterior to the superficial inguinal ring, is the reflected inguinal ligament to the extent present just anterior to the conjoined tendon of the transversus and internal oblique muscles. A quite variable expansion from the tendon of the rectus abdominis muscle (called by some authors the inguinal falx) fuses, to a variable extent, with the posterior aspect of the conjoined tendon. All of the reinforcing structures just described are, of course, anterior to the transversalis fascia. Posterior or deep to the transversalis fascia are the loose extraperitoneal fascia and peritoneum, which continue across posterior to the deep inguinal ring. At the lateral end of the canal, the inferior epigastric artery and vein are posterior to the canal in the extraperitoneal fascia as they are in relation to the medial (inferomedial) margin of the deep inguinal ring. Overlying these vessels, a thickening in the transversalis fascia is variably present. A slight depression in the parietal peritoneum, as seen from within, is apt to be present at the site of the deep inguinal ring. The roof of the inguinal canal can be said to be formed by the most inferior fasciculi of the internal oblique muscle as they gradually pass in a slightly arched fashion, from a position at their origin anterior to the canal to a position at their insertion (by way of the conjoined tendon) posterior to the canal. At the lateral end of the canal, the lower fasciculi of the transversus abdominis arch similarly over the canal. It should be pointed out that, although the description above of a roof and a floor of the canal can serve a useful purpose in talking about the canal, the anterior and posterior walls of the canal, in a sense, come together superior and inferior to the canal, and the roof and much of the floor are, perhaps, manufactured for descriptive purposes.

The weakest area in the anterolateral wall in relation to the inguinal canal is the superficial inguinal ring, which, to a varying extent, is reinforced by the reflected inguinal ligament, the conjoint tendon, and the expansion laterally and inferiorly from the tendon of the rectus abdominis muscle to the pecten pubis. This generally weakened area, the inguinal (Hesselbach) triangle, through which a direct inguinal hernia will pass, is a triangle bounded superolaterally by the inferior epigastric vessels, superomedially by the lateral margin of the rectus, and inferiorly by the inguinal ligament.

Developmentally, the inguinal canal is established as an outpouching in the inferior part of the anterior abdominal wall, the processus vaginalis, containing all of the layers from the parietal peritoneum outward, in preparation for the descent of the testes from their origin along the posterior abdominal wall through the inguinal canal and into the scrotum. Originally, the process was straight in an anterior-posterior direction, but further regional development causes it to become oblique. The processus vaginalis normally loses its connection with the parietal peritoneum of the abdominopelvic cavity, and all that remains of this is the double-walled serous sac, the tunica vaginalis , that partially surrounds the testis. The outpouchings of the other layers remain as coverings of the spermatic cord and testis which are picked up by the spermatic cord as it passes through the successive layers of the anterolateral abdominal wall. The covering acquired from the transversalis fascia is called the internal spermatic fascia. The spermatic cord is typically described as having passed inferior to the lower border of the transversus abdominis. The covering derived from the internal abdominal oblique muscle is the cremasteric muscle and fascia. The covering of the spermatic cord and testis procured from the external abdominal oblique muscle is the external spermatic and intercrural fasciae.

Posterior Wall of Abdominal Cavity

The bodies of the five lumbar vertebrae, together with the related intervertebral discs, form a distinct, longitudinal, midline elevation in the posterior wall of the abdominal cavity, which may actually come within a relatively short distance (a few centimeters) of the inner surface of the anterior abdominal wall. The intervertebral disks produce bulges in this elevation, and the anterior longitudinal ligament, with the closely attached crura of the diaphragm, covers its anterior surface. Just lateral to the lumbar vertebrae are the psoas major and minor muscles (if present). Lateral to the psoas major muscle in the area inferior to the crest of the ilium is the iliacus muscle, and in the area between the 12th rib and the crest of the ilium is the quadratus lumborum muscle.

The psoas major muscle arises from (1) the anterior surfaces of the bases and the inferior borders of the transverse processes of all of the lumbar vertebrae; (2) the lateral aspect of the intervertebral disc above each of the lumbar vertebrae and from the adjacent parts of the vertebra above and the vertebra below each of these discs; and (3) from tendinous arches stretching across the concavity at the side of the body of each of the first four lumbar vertebrae. This muscle courses interiorly along the brim of the pelvis and passes inferior to the inguinal ligament to enter the thigh and insert on the lesser trochanter of the femur.

Present in 40% to 60% of the cases, the psoas minor muscle arises from the lateral aspect of the bodies of the 12th thoracic and 1st lumbar vertebrae and the intervertebral disc between them. It courses inferiorly on the anterior aspect of the psoas major, ending in a long flat tendon that inserts into the pecten pubis and the iliopectineal eminence.

The iliacus muscle fills much of the iliac fossa (which actually forms the lateral wall of this part of the abdominopelvic cavity) and arises from the upper two thirds of this fossa, the inner lip of the crest of the ilium, the anterior sacroiliac and iliolumbar ligaments, and the base of the sacrum. Its fibers converge as they course inferiorly to insert, for the most part, into the lateral side of the tendon of the psoas major muscle. Some of the fibers also insert onto the femur just inferior and anterior to the lesser trochanter. Because most of the iliacus muscle inserts into the tendon of the psoas major, the two muscles are often described as the iliopsoas muscle and, of course, have a common action.

The quadratus lumborum muscle arises from the posterior part of the iliac crest (inner lip), the iliolumbar ligament, and the transverse process of the 5th lumbar vertebra (and perhaps 4th, 3rd, and 2nd lumbar vertebrae for a sometimes-demonstrable anterior layer of the muscle), and inserts into the lower border of the medial part (half or so) of the 12th rib, with some insertion into the transverse processes of the upper four lumbar vertebrae. It draws the last rib inferiorly and thus acts as anchorage for the diaphragm to the crest of the ilium, and it also bends the lumbar portion of the lumbar spine laterally. Its anterior surface is covered by a variably thin, anterior layer of the thoracolumbar fascia. Its posterior side is in contact with the thoracolumbar fascia's middle layer.

The posteroinferior part of the diaphragm may be considered as part of the posterior abdominal wall. Depending on where the posterior limit of the anterolateral abdominal wall is arbitrarily located, the portions of the transversus abdominis muscle (overlain by the internal and external oblique muscles) just lateral to the quadratus lumborum muscle can also be considered as helping to form the posterior abdominal wall.

The superficial structures of the lower part of the back are, of course, external to what has been described here and would have to be traversed were the abdominal cavity approached from behind.

Diaphragm

The dome-shaped roof of the abdominopelvic cavity is formed by a musculoaponeurotic septum, the diaphragm, which also forms the floor of the thoracic cavity and thus is the partition between these two cavities. This unique muscular structure takes origin by its entire circumference from the inner aspect of the lower margin of the thoracic cage. The muscular fibers course superiorly and inward to insert in the margins of the diaphragm's central tendon. The sternal origin is, by way of a fairly short, fleshy slip (a right and a left one), on the posterior aspect of the xiphoid process, which, on its way to the anterior margin of the central tendon, does not ascend nearly as much as do the fibers from the other two areas of origin. In fact, depending on the position of the individual and the degree of contraction of the diaphragm, the fibers from the sternal origin may even course inferiorly.

The costal origin in general arises from the inner surfaces of the costal cartilages and the adjacent parts of the lower six ribs by fibers from each that interdigitate with the origin of the transversus abdominis muscle. The lumbar origin consists of a crus and a medial arcuate ligament and a lateral arcuate ligament on each side. The crura begin as tendinous structures, which attach to the anterior and lateral sides of the upper lumbar vertebrae (one to three or four for the right and one to two or three for the left) and related intervertebral disks, blending with the anterior longitudinal ligament. The right crus is stouter as well as longer than the left and, as it becomes muscular, usually splits to send a portion to the left of the esophageal hiatus. The medial margins of the two crura converge to meet in the midline to form an arch across the anterior aspect of the aorta, the median arcuate ligament. The medial arcuate ligament is a tendinous arch that appears as a thickening in the fascia over the superior part of the psoas major muscle, extending from the side of the body of the second (or first) lumbar vertebra, where it blends with the lateral margin of the corresponding crus to the end of the transverse process of the first (or second) lumbar vertebra. The lateral arcuate ligament is a thickening in the fascia that covers the quadratus lumborum muscle and reaches from the end of the transverse process of the 1st (or 2nd) lumbar vertebra to the tip and lower margin of the 12th (or 11th) rib.

The central tendon is a thin but strong and dense aponeurosis, closer to the sternal origin than the costal and lumbar origins. It is shaped somewhat like a thick and widely opened V, with slight indentations, which produce three leaflets. The fibrous pericardium is blended with its superior surface.

Several openings (hiatuses) permit the passage of structures between the thoracic and abdominal cavities. The inferior vena cava passes through the caval opening at the junction of the right and middle leaflets of the central tendon in the most anterior and highest of the three large openings, being at the level of the disc between T8 and T9. It often transmits a branch of the phrenic nerve. The esophageal hiatus is in the muscular portion of the diaphragm just posterior to the central tendon, a little to the left of the midline and about at the level of T10, and transmits the esophagus as well as the anterior and posterior vagal trunks to the abdomen. The aortic hiatus (really a notch in the posterior margin of the diaphragm) is at the level of T12 and transmits the thoracic duct and azygos vein in addition to the aorta. The greater and lesser splanchnic nerves pierce the crura, and, in addition, the left crus is pierced by the hemiazygos vein.

Intervals at the origin where muscle is replaced by areolar connective tissue occur at the sternocostal triangle and, with great variation, at the lateral arcuate ligaments.

Floor of Abdominopelvic Cavity

The outlet of the pelvis (inferior aperture of the pelvis) is, for the most part, closed by the slinglike structure known as the pelvic diaphragm that, together with the urogenital diaphragm, gives the inferior and posteroinferior support to the abdominopelvic viscera. As usually described, the pelvic diaphragm consists of the right and left levator ani muscles, the right and left coccygeus muscles, and the fascia on both surfaces of these muscles.

The levator ani muscle arises (1) from the pelvic surface of the pubis along a line from a little lateral to the inferior part of the pubic symphysis toward the obturator foramen, (2) from the tendinous arch of the levator ani, which is a thickening of the fascia on the pelvic surface of the obturator internus muscle along a line extending from the lateral end of the pubic origin of the levator ani to the ischial spine, and (3) from the pelvic surface of the ischial spine. In general, the fibers of the right and left levator ani muscles run posteriorly, inferiorly, and medially, with varying degrees of obliquity, to come into relationship with each other in the midline by inserting into the perineal body, as well as the anterior and lateral sides of the coccyx, or to blend closely with the midline viscera, interposed between the two muscles. The levator ani can be described in at least three parts: (1) The puborectalis muscle, has fibers inserting into the perineal body, clasping the prostate in the male and the vagina in the female. The remaining fibers form a sling around the anorectal junction. (2) The intermediate set of fibers, found lateral and slightly superior to the puborectalis, form the pubococcygeus muscle. It originates from the posterior aspect of the pubic bone and the tendinous arch of the levator ani to insert into the coccyx and anococcygeal ligament. (3) The remainder of the levator ani muscle is the iliococcygeus muscle, which originates from the tendinous arch of the levator ani and inserts onto the lateral aspect of the coccyx. The nerve supply of the levator ani muscle is from the fourth sacral nerve by way of the perineal branch of the pudendal nerve.

The coccygeus muscle, which is closely applied to the deep surface of the sacrospinous ligament and is in much the same plane as the iliococcygeus muscle, is a flat triangular muscle arising from the ischial spine and inserting into the margin of the lower two sacral segments and the first two segments of the coccyx. It receives its nerve supply from the anterior primary ramus of the fourth sacral nerve.

The bony framework of the true pelvis (pelvis minor), supplemented by the sacrospinous and sacrotuberous ligaments, presents several openings, each of which is, to a great extent, closed by muscle. The obturator internus, a muscle of the lower extremity, covers the obturator foramen except for the much smaller obturator canal through which the obturator vessels and nerve pass. It arises from the pelvic aspect of the obturator fascia and the adjacent bone and passes through the lesser sciatic foramen (mostly filling this opening) on its way to the medial side of the greater trochanter of the femur, where it inserts. Anterosuperior to the origin of the levator ani muscle, the obturator internus muscle forms a portion of the wall of the pelvic cavity. The piriformis, also a muscle of the lower extremity, arises from the pelvic surface of the sacrum between, and lateral to, the second, third, and fourth anterior sacral foramina and mostly fills the greater sciatic foramen in traversing this foramen on its way to insert on the summit of the greater trochanter of the femur.

Peritoneum

The peritoneum is the extensive serous membrane that, in general, lines all borders of the abdominopelvic cavity and reflects from the body wall onto the organs that are contained within it. A general concept that one might have of the pleura or the serous pericardium can be carried over to the peritoneum. In all of these situations, the serous membrane lining the body wall is continuous with that on the surfaces of the viscera contained within the portions of the body cavity involved, and although one refers separately to parietal and visceral portions of the serous membrane, they are continuous. Also, under normal circumstances, the organs fill the respective portion of the body cavity so completely that the visceral and parietal portions of adjacent structures are separated from each other by only a thin film of fluid. The peritoneal cavity of the female is the only place where an organ's lumen is in direct contact with the peritoneal space, as the opening of each uterine tube is open to the peritoneal cavity.

The peritoneum is much more complicated in its arrangement than either the visceral pleura or the serous pericardium. This is essentially due to the fact that parts of several viscera deform the peritoneal serous membrane to various degrees in the course of fetal development. The rotations of the gut, combined with the propensity of one free peritoneal surface to fuse with another free surface, result in complex changes of the arrangement that lead to the mature appearance of the abdominal organs. The stomach (to cite only one example of the manifold rearrangements in the visceroperitoneal relations) in its primary vertical position was attached by one double layer of peritoneum, the ventral mesogastrium, to the ventral body wall, and by another double layer, the dorsal mesogastrium, to the posterior wall. When the stomach rotated, its original left side became the anterosuperior surface and the original right side the posteroinferior surface; the dorsal mesogastrium was swept toward the left to form an outpouching of the peritoneal cavity, which presents itself at an early developmental stage (6 weeks) as the omental bursa (lesser sac), communicating with the rest of the peritoneal cavity (greater sac) by only a small opening, the omental foramen (of Winslow), located a little to the right of the midline posteroinferior to the liver.

The best way to obtain a general concept of the arrangement of the peritoneum is to trace it in three planes, a midsagittal plane and two horizontal planes, one at the level of the pylorus and the other at the level of the umbilicus, in a preferably fresh specimen at the autopsy table. Lacking this opportunity, the use of these three planes still remains methodically the most informative approach for studying the peritoneal continuity and its relationship to the abdominopelvic viscera.

In the midsagittal plane, the greater and lesser peritoneal sacs must be pursued separately, because they are not continuous anywhere in this plane. In following the cut edge of the greater sac, one can start with the parietal peritoneum on the inner surface of the anterior wall at the level of the umbilicus. Progressing superiorly, the peritoneum continues onto the inferior surface of the diaphragm and along it until it is reflected to the liver as the superior (anterior) layer of the left triangular ligament. From here it extends along the anterosuperior surface of the liver, around the free margin of the liver, and onto its visceral surface, until it is reflected toward the lesser curvature of the stomach as the anterior layer of the lesser omentum, which then advances onto the anterior surface of the stomach, leaving the latter as the anterior surface of the greater omentum. At the free margin of the greater omentum, this layer turns superiorly to become the posterior surface of the greater omentum, which proceeds superiorly to the transverse colon, where it appears to continue onto the posterior surface of the transverse colon and then as the posterior layer of the transverse mesocolon. From the posterior layer of the transverse mesocolon, the peritoneum turns interiorly from the lower border of the pancreas across the anterior surface of the third portion of the duodenum and becomes the right (superior) layer of the intestinal mesentery. At its free margin the mesentery entirely (except for the area of mesenteric attachment) surrounds the small intestine and continues to the posterior body wall as the left (inferior) layer of the mesentery. On reaching the body wall, it runs as the parietal peritoneum of the posterior wall interiorly on the anterior surface of the aorta and then on the vertebral column to about the second sacral level, where it comes to lie on the anterior surface of the rectum, from which, in the male, it is reflected onto the posterosuperior surface of the bladder, bounding the rectovesical pouch. In the female, the peritoneum passes from the anterior surface of the rectum to the posterior vaginal fornix, bounding the rectouterine pouch (of Douglas). It passes up the posterosuperior aspect of the uterus, over the fundus of the uterus, and down on its anteroinferior aspect to about the junction of the body and cervix, from whence it reflects onto the posterosuperior aspect of the bladder, bounding the vesicouterine pouch. In both the male and the female, the peritoneum passes from the superior surface of the bladder to the inner surface of the anterior body wall, a variable distance above the symphysis pubis, depending on the degree of distention of the bladder. From here it continues superiorly to the point at which this tracing of the peritoneum was started.

In following the cut edge of the omental bursa's peritoneum in a midsagittal plane, a start can be made on the anterior surface of the pancreas, and the peritoneum can be traced superiorly from here onto the surface of the diaphragm until it reflects from the diaphragm to the liver as the inferior (posterior) layer of the left triangular ligament. From here it can be traced along the posterior and then inferior surfaces of the liver to the point at which it leaves the liver to go to the lesser curvature of the stomach as the posterior layer of the lesser omentum, which continues onto the posteroinferior surface of the stomach and to the greater curvature, where it leaves the stomach to extend for a variable distance into the greater omentum. This distance depends on the degree of fusion of the peritoneum which has taken place, typically not reaching beyond the transverse colon. The peritoneum turns superiorly on the anterior surface of the transverse colon, and then, in the adult, it usually forms the anterior layer of the transverse mesocolon if the fusion of the primitive dorsal mesogastrium with the primitive mesentery of the transverse colon has been complete. The transverse mesocolon comes to the posterior body wall just inferior to the point at which the tracing of the lesser sac peritoneum was started.

In tracing the peritoneum in a horizontal section at the level of the omental foramen, a start can be made with the greater sac peritoneum on the inner surface of the anterior abdominal wall in the midline. Following the cut edge of the parietal peritoneum to the left along the inner surface of the anterolateral wall to the region of the posterior wall, it will pass onto the anterolateral surface of the left kidney, from where it reflects to the hilar area of the spleen, forming the external layer of the splenorenal ligament, and then completely surrounds the spleen except at the hilar area. From the anterior margin of the hilum of the spleen, the peritoneum passes to the stomach as the external layer of the gastrosplenic ligament. The peritoneum can then be followed along the anterosuperior surface of the stomach to the lesser curvature, where it leaves the stomach as the anterior layer of the lesser omentum, which can be followed to the right until the free margin is reached a short distance to the right of the midline. Here the peritoneum passes around the free margin of the lesser omentum (anterior boundary of the omental foramen) to become the peritoneum of the omental bursa, which continues to the left, as the posterior layer of the lesser omentum, to the lesser curvature of the stomach, where it continues onto the posteroinferior surface of the stomach, which it follows until it leaves the stomach to form the internal (lesser sac) layer of the gastrosplenic ligament. From the spleen the peritoneum forms the internal layer of the splenorenal ligament and then travels to the right anterior to the aorta and the inferior vena cava. At the right margin of the inferior vena cava, the peritoneum again becomes continuous with the greater sac and continues to the right onto the anterior aspect of the right kidney. From here the tracing of the peritoneum could differ, depending on whether the bare area of the liver were to extend down just far enough to be encountered in the plane of the tracing or whether the plane of tracing passes just inferior to the bare area of the liver. In the former case, the peritoneum would pass from the kidney as the inferior layer of the coronary ligament to the liver, and would follow around the liver to its anterosuperior surface, where it would leave the liver as the left layer of the falciform ligament, to go to the inner surface of the anterior body wall and to the left to the point from which the tracing started. To complete the tracing in this plane, one must follow the peritoneum from the right layer of the falciform ligament onto the anterosuperior surface of the liver, and to the right along this surface to the superior layer of the coronary ligament, along this to the diaphragm, and then anteriorly to the right layer of the falciform ligament. If the plane of section passes just inferior to the bare area of the liver as the peritoneum leaves the anterior surface of the inferior vena cava (the posterior boundary of the omental foramen), it passes across the anterior surface of the right kidney, then to the diaphragm, and forward on the inner surface of the body wall to the falciform ligament.

In tracing peritoneum in a horizontal section at about the level of the umbilicus, one can start at the midline of the inner surface of the anterior abdominal wall and follow from this point the parietal peritoneum to the left along the inner surface of the wall to the posterior wall, where it reflects onto the left side of the descending colon to cover also the anterior surface and right side of this structure, from which it passes to the posterior body wall. In early development the descending colon was suspended by the primitive dorsal mesentery, but peritoneal fusion during embryologic development brings it into the adult relationship to the peritoneum just described. The peritoneum continues to the right on the posterior body wall to about the midline, where it reflects forward to form the left (inferior) layer of the intestinal mesentery. The small intestine is completely surrounded (except at its mesenteric attachment) in the free margin of the mesentery; from here the peritoneum is traced posteriorly to the posterior body wall as the right (superior) layer of the mesentery. Thereafter, the peritoneum can be followed to the right onto the posterior body wall, until it reflects from here to cover the left, anterior, and right surfaces of the ascending colon. This structure was also suspended originally by the primitive dorsal mesentery. From the right side of the ascending colon, the peritoneum passes to the posterior body wall and then forward on the inner surface of the anterolateral abdominal wall until it reaches the midline, from where the tracing was started. Also in a section at about the level of the umbilicus, one would expect to find the greater omentum cut, which is present as an island of peritoneum not connected in this section to the rest of the peritoneum. If the transverse colon is hanging low enough, it too would be cut as an island with its peritoneum continuous with that of the greater omentum.

Worthwhile additions to the general concept of the distribution of the peritoneum, gained by tracing it in several planes as done above, can be obtained by careful study of a view of the posterior half of the abdominopelvic cavity, in which all of the viscera (except the bladder and rectum) that invaginate the peritoneum to any degree have been removed, cutting the peritoneum along its lines of reflection from the posterior body wall or the anterior surfaces of the viscera and vessels that do not project into the peritoneum. The right and left kidneys, the pancreas (except for the tip of its tail), the second, third, and most of the fourth parts of the duodenum, and the aorta and inferior vena cava do not project into the peritoneal cavity to a significant degree. The peritoneum covers the inner surface of the abdominopelvic body walls as parietal peritoneum, except where it is lifted away from them by the structures just listed (the bare area of the liver against the diaphragm, the ascending and descending colon, the roots of the mesentery, the transverse mesocolon and sigmoid mesocolon, the ureters and inferior mesenteric vessels, and the rectum and bladder and, in the female, the uterus and broad ligaments, other folds in the pelvis, and folds on the inner surface of the anterior abdominal wall). The folds on the inner surface of the anterior abdominal wall are the falciform ligament of the liver (a remnant of the ventral mesentery, ventral to where the liver grew into it), running superiorly and a little to the right from the umbilicus, with the ligamentum teres (obliterated umbilical vein) of the liver in its free margin; the median umbilical fold, projecting from the superior aspect of the urinary bladder, running superiorly up the midline to the umbilicus; the medial umbilical folds, also running to the umbilicus and containing the obliterated right and left umbilical veins; and the right and left lateral umbilical folds, containing the inferior epigastric artery and vein on each side (which may produce a slight elevation remindful of a fold by pulling the peritoneum a little away from the body wall). The depression between the median and medial umbilical folds is called the supravesical fossa, whereas the one between each medial and lateral umbilical fold is the epigastric fold. Lateral of the lateral umbilical fold is the lateral inguinal fossa. Parietal peritoneum is thus seen to be applied to prac­tically the entire extent of the inner surface of the anterolateral abdominal wall, and virtually any incision through this wall will open into the peritoneal cavity.

Much of the diaphragm has parietal peritoneum on its abdominal surface, but much less of the muscular portion of the posterior abdominal wall is directly lined by peritoneum on its inner surface. This is because several viscera, major vessels, and a significant amount of adipose tissue lie behind the peritoneum and most of the abdominal viscera project from the posterior wall into the peritoneal cavity.

From the preceding description it is obvious that the degree to which the various abdominal viscera are covered by peritoneum (visceral peritoneum) varies along a spectrum in which peritoneum may cover just part of one surface of the viscus in question to the other extreme in which peritoneum covers the viscus entirely, except for the area of attachment of a suspending double-layered fold of peritoneum. “Retroperitoneal” is a very commonly used descriptive term having the general meaning of “behind the peritoneum,” which is well agreed upon, but some authors refer to certain organs as retroperitoneal that other authors would not designate in this fashion. Generally, “primarily retroperitoneal structures” (e.g., ureter, kidney) are those that develop posterior to the peritoneal lining and never develop a mesentery. “Secondarily retroperitoneal structures” once had a mesentery but lost it when the organ was laid back along the body wall and the mesentery fused to a degree with the parietal peritoneum (e.g., ascending colon, second portion of the duodenum). “Intraperitoneal structures” are those that are suspended from the posterior body wall by a mesentery containing blood vessels and nerves associated with the organ (e.g., stomach, ileum). Additional details will be given in the sections dealing with each organ or region.

The mesentery is commonly taken to mean the mesentery of the small intestine (i.e., the jejunoileal portion of the small intestine which is the portion having a mesentery or a double-layered fold of peritoneum suspending it from the posterior abdominal wall). The root of the mesentery is about 15 cm in length, and its line of attachment varies a bit with the shape of the duodenum, but, in general, it courses from a little to the left of the second lumbar vertebra inferiorly and to the right, crossing the third part of the duodenum, the aorta, the inferior vena cava, the right ureter, and the right psoas major muscle to reach a point near the right sacroiliac joint. The free or unattached border, which contains the loops of the small intestine, is frilled out to such an enormous degree that it may attain a length varying from 3 m to more than 6 m. The distance from the attached border to the free border measures 15 to 22 cm; it may definitely increase with age, probably owing to stretching of the mesentery due to laxity of the anterior abdominal wall. Between the two layers of peritoneum on the two surfaces of the mesentery are the superior mesenteric artery and its branches, the accompanying veins, lymphatics, approximately 100 to 200 lymph nodes, autonomic nerve plexuses, connective tissue, and varying amounts of adipose tissue, which is present in greater amounts near the root. The mesentery divides the area below the transverse mesocolon into two compartments, which are important in determining collections of fluid and the localization of infection.

The transverse mesocolon is the broad peritoneal fold suspending the transverse colon from the posterior body wall. The root of the transverse mesocolon crosses the anterior surface of the right kidney, the second portion of the duodenum, and the head of the pancreas, and then passes along the lower border of the body and tail of the pancreas superior to the duodenojejunal flexure, to end on the anterior surface of the left kidney. It contains the middle colic artery, branches of the right and left colic arteries, accompanying veins, lymphatic structures, autonomic nerve plexuses, as well as a considerable thickness of connective tissue.

The sigmoid mesocolon is the mesentery of the sigmoid colon. When the peritoneum begins to surround the large intestine near the crest of the ilium, the attachment of the sigmoid mesocolon follows a fairly straight line from the posterior part of the left iliac fossa inferiorly and medially to reach the third sacral segment. If, as is the case in the other extreme of the range of variation, the colon is closely bound down in the iliac fossa, the line of attachment of the sigmoid mesocolon goes posteriorly along the pelvic brim until it crosses the anterior side of the sacroiliac joint, and then descends along the anterior aspect of the sacrum to the level of its second to third segment. The sigmoid colon is enwrapped by the free margin of the sigmoid mesocolon, which has its greatest width (distance from attached to free border) at its attachment to the first sacral segment. This width varies from about 5 to 18 cm, although it occasionally may be as much as 25 cm between the layers of the sigmoid mesocolon through which run the sigmoidal and superior rectal arteries, accompanying veins, lymphatics and autonomic nerve plexus, and connective tissue, which, of course, includes varying amounts of adipose tissue.

The greater omentum is the largest peritoneal fold; it may hang down like a large apron from the greater curvature of the stomach in front of the other viscera as far as the brim of the pelvis or even into the pelvis. It may even extend into an inguinal hernia, most commonly on the left side. It also may be much shorter than this, appearing as just a fringe on the greater curvature of the stomach, or it may be of some length and found folded in between coils of the small intestine, tucked into the left hypochondriac area or turned superiorly just anterior to the stomach. The superior end of the left border is continuous with the gastrosplenic ligament, and the superior end of the right border extends as far as the beginning of the duodenum. The greater omentum is usually thin, with a delicate layer of fibroelastic tissue as its framework, and somewhat cribriform in appearance, although it usually contains some adipose tissue and may accumulate a large amount of fat in an obese individual. In the make-up of the greater omentum, the peritoneum of the omental bursa on the posteroinferior surface of the stomach and the greater sac peritoneum on the anterosuperior surface of the stomach meet at the greater curvature of the stomach and course inferiorly to the free border of the greater omentum, where they turn superiorly to the transverse colon. Early in development, these two layers of elongated dorsal mesogastrium course superiorly in front of the transverse colon and transverse mesocolon to the anterior surface of the pancreas. Owing to fusions of these two layers of peritoneum to each other and to the peritoneum on the transverse colon, and the anterior surface of the primitive transverse mesocolon, it appears, in the fully developed state, as though the “two layers” of peritoneum, running superiorly as the posterior layer of the greater omentum, separate from each other to surround the transverse colon and continue as the two layers of the transverse mesocolon. Frequently, there is enough fusion in the four-layered primitive greater omentum inferior to the transverse mesocolon that no extension of the omental bursa is present between layers. Close to the greater curvature of the stomach, the right and left gastroomental vessels course, anastomosing with each other in the greater omentum. The greater omentum, if of any length, has a great deal of mobility and can shift around to fill what would otherwise be temporary gaps between viscera or to build up a barrier against bacterial invasion of the peritoneal cavity by becoming adherent at a potential danger spot.

The lesser omentum, which can be subdivided into hepatogastric and hepatoduodenal ligaments, extends from the posteroinferior surface of the liver to the lesser curvature of the stomach and the beginning of the duodenum. It is extremely thin, particularly the part to the left, which is sometimes fenestrated. The right side is thicker and ends in a free, rounded margin, which contains the common bile duct to the right, the hepatic artery to the left, and the portal vein posterior to these two, and forms the anterior border of the omental foramen. In addition to the structures just listed, the lesser omentum contains the right and left gastric arteries (close to the lesser curvature of the stomach) and the accompanying veins, lymphatics, and autonomic nerve plexuses. The lesser omentum reaches the liver at the porta hepatis, and to the left of the porta hepatis it extends to the bottom of the fossa for the ligamentum venosum, the obliterated ductus venosus, which carried oxygenated blood from the umbilical vein to the inferior vena cava.

The omental bursa (lesser sac of the peritoneum) is a large fossa, or outpouching, from the general peritoneal cavity. It is bounded in front, from superior to inferior, by the caudate lobe of the liver, lesser omentum, posteroinferior surface of the stomach, and anterior layer of the greater omentum (at least in part). Posteriorly, from inferior to superior, are the posterior layer of the greater omentum (the amount of this depends on the variable inferior extent of the bursa), transverse colon, anterior layer of the transverse mesocolon, anterior sur­face of the pancreas, left suprarenal gland, superior end of the left kidney, and, to the right of the esophageal opening into the stomach, that part of the diaphragm supporting the caudate lobe of the liver. The horizontal extent of the bursa stretches from the omental foramen at the right to the hilum of the spleen at the left, where it is limited by the splenorenal and gastrosplenic ligaments. Inferiorly, the bursa may extend about as far as the transverse colon, its cavity having originally reached as far down as the free margin of the greater omentum before becoming obliterated by fusion of its layers. The portion of the bursa between the caudate lobe of the liver and the diaphragm is called the superior recess, and the narrow portion from the omental foramen across the head of the pancreas to the gastropancreatic fold is called the vestibule of the bursa.

The omental foramen (of Winslow) is the opening by which the omental bursa communicates with the general peritoneal cavity (greater sac). It is somewhat circular and is usually large enough to admit one or two fingers. Anterior to the foramen is the free margin of the lesser omentum, containing the common bile duct, hepatic artery, and portal vein. Its posterior border is the peritoneum covering the inferior vena cava; superior to it is the peritoneum on the caudate process of the liver, and its inferior boundary is the peritoneum that covers the beginning of the duodenum and the hepatic artery.

Many extremely variable and inconstant fossae or recesses have been described which are of interest to the surgeon because of the possibility of herniation of a loop of intestine into any one of them. The more common ones are located either in the region of the fourth portion of the duodenum or in the region of the cecum and ileocecal junction. A relatively common “intersigmoid recess” is found on the left side of the line of attachment of the sigmoid mesocolon at the angle that is present in this line when the colon is tightly bound down in the iliac fossa.

A characteristic of the peritoneum covering the surfaces of the various parts of the colon is the presence of little outpouchings of peritoneum containing adipose tissue, which are called omental appendages (appendices epiploicae).

The parietal peritoneum is supplied by the nerves to the adjacent body wall and is thus pain sensitive. The visceral peritoneum is insensitive to ordinary pain stimuli but does respond to ischemia, distention, and inflammation. When moist surfaces of peritoneum which are in contact become irritated, adhesions tend to form that often become permanent.

Pelvic Fascia and Perineopelvic Spaces

The steadily changing pressure and filling conditions in the pelvis require adaptability of those structures that support the viscera within the funnel-like frame of the pelvis. Part of this support derives from the anorectal musculature and the levator ani. But since these muscles are, to a great extent, involved in the sphincteric and emptying functions of the anorectal canal, their supporting tasks are assisted by the connective tissue structures of the pelvic fascia, which have substantial tensile strength. The anatomic relationships of the pelvic fascia and associated muscles are physiologically and surgically significant. The pelvic fasciae are divisible into a visceral and a parietal portion. The former lies entirely superior to the pelvic diaphragm, forming the fascial investments of the pelvic viscera, the perivascular sheaths, and the intervisceral and pelvovisceral ligaments, which are described below.

The parietal portion of the pelvic fascia may be divided into parts that lie either superior or inferior to the levator ani muscle. Superior to the levator ani, parietal pelvic fascia is a continuation of the parietal abdominal fascia. The iliopsoas fascia and the transversalis fascia of the abdomen are attached along the linea terminalis to the bony pelvis and then extend inferiorly into the pelvis over the inner surface of the obturator internus muscle as the obturator fascia. Anteriorly, the transversalis fascia is attached to the inner surface of the pubic bones and symphysis. The prevertebral fascia of the abdomen continues inferiorly into the pelvis as the presacral fascia.

The superior layer of the pelvic diaphragm arises from the arcus tendineus of the levator ani muscle, which is a thickening in the obturator fascia, running arc-wise and convex inferiorly from the posterior surface of the pubic ramus (1 or 2 cm in front of the obturator foramen) to a point just superior to the ischial spine. From this arcus, the superior fascia of the pelvic diaphragm spreads out to cover the superior (inner) surface of the levator ani and coccygeus muscles.

Anteriorly, this fascia spans the infrapubic interval in front of the transverse perineal ligament. The fascia descends just a few millimeters to form a small fossa, the bottom of which is pierced by the dorsal vein of the penis or clitoris, respectively. On each side of this small fossa, a thickening in the fascia extends posteriorly from each side of the lower end of the symphysis pubis to the prostate in the male and to the bladder in the female. These thickened parts are the medial puboprostatic ligaments (or anterior true ligaments of the prostate) in the male, to which correspond the medial pubovesical ligaments (pubourethral or anterior true ligaments) of the bladder, in the female. The lateral puboprostatic or pubovesical ligaments (or lateral true ligaments of prostate or bladder) lie just posterior to this and consist of lateral reflections from the fascia to the prostate or bladder, respectively.

The thickenings in the superior fascia of the pelvic diaphragm, which make up the medial puboprostatic or pubovesical ligaments, continue backward in a slight curve, concave downward, gradually diverging to the region of the ischial spine. This constitutes on each side the arcus tendineus of the pelvic fascia, which lies considerably more medially and below the arcus tendineus of the levator ani. The superior fascia of the pelvic diaphragm also continues medially and below its arcus tendineus. Anterior to the rectum, it spans the interval between the crura of the pubococcygeus muscles and, coursing around their free margins, fuses with the deep (superior) layer of the urogenital diaphragm. Here also it is reflected upon the prostate and bladder in the male and the vagina in the female as the visceral fascial sheaths of these respective organs.

Posteriorly, the superior fascia of the pelvic diaphragm surrounds the rectum as it passes through the pelvic diaphragm. It is reflected there as a sheath upon the rectum as the visceral (rectal) fascia, but it also blends with the longitudinal rectal musculature and contributes fibrous extensions to the formation of the fibromuscular, conjoined longitudinal muscle of the anal canal. The reflection takes place largely at the arcus tendineus of the pelvic fascia, but also more medially and more inferiorly in the region where the viscera begin to penetrate the pelvic diaphragm.

Inferior to the levator ani, the obturator fascia continues inferiorly on the medial walls of the pelvis below the arcus tendineus of the levator ani muscle. It covers the obturator internus muscle and is attached to the bony pelvis about the margins of that muscle. In its lower portion the fascia is split to form the more-or-less horizontal pudendal canal (Alcock canal), in which course the internal pudendal vessels and the pudendal nerve. Depending on when it leaves the pudendal nerve, the canal may also include the dorsal nerves of the penis. The inferior fascia of the pelvic diaphragm is a comparatively thin sheet that extends from the arcus tendineus of the levator ani muscle and covers the inferior surface of this and the coccygeus muscle. It continues around the lower rectum and the anal canal. It is reflected into the anterior recess of the ischioanal fossa.

The perineal fascia consists of a superficial subcutaneous and a deep membranous layer. The former is continuous with the subcutaneous fat (Camper fascia) of the abdominal wall; the latter is the superficial perineal fascia (Colles fascia), corresponding to the Scarpa fascia of the abdomen. The superficial layer varies considerably throughout the perineum. Over the anal triangle it forms the fatty layer of the deep part of the ischioanal fossa, whereas laterally over the ischial tuberosities, it is made up of fibrous fascicles that connect to the underlying bone and form, directly over the ischial tuberosities, fibrous bursal sacs. The main part of the superficial perineal fascia has a firm attachment to the pubic rami and to the posterior margin of the urogenital diaphragm. It spreads medially across the urogenital triangle, constituting the floor of the superficial perineal space, which lies between it and the inferior layer of the urogenital diaphragm and contains the superficial perineal musculature.

The visceral fascia invests, one by one, each of the pelvic organs, forming their fascial capsule (e.g., vesical fascia, prostatic fascia, vaginal-uterine fascia, rectal fascia ). It also contains the ligaments that connect these viscera with each other and with the pelvic walls and floor, as well as the perivascular sheaths. The latter consist of the hypogastric sheath, which arises on each side from the parietal pelvic fascia over a roughly triangular area in the posterolateral angle of the pelvis and extends inferiorly to the spine of the ischium. This sheath contains the internal iliac vessels (and a variable number of their branches) and the ureter, as well as its accompanying nerves and lymphatics. Anteriorly, the sheath is continuous with the tendinous arch of the pelvic fascia, which extends anteriorly to the superolateral border of the bladder, where it splits into superior and inferior layers. These blend, respectively, with the superior and lateral aspects of the vesical fascia. Anteriorly, the arch carries the obliterated umbilical artery and superior vesical vessels to the urinary bladder as the lateral ligament of the bladder. Posteriorly, in the female, the hypogastric sheath fuses with the suspensory ligament of the ovary containing the ovarian vessels.

The uterosacral ligament extends inferiorly from the hypogastric sheath. Laterally, it blends with the superior fascia of the levator ani and medially with the inferolateral aspects of the bladder or prostatic fascial capsule. In a sense, it thus constitutes a reflection from the superior fascia of the levator ani to the vesical (visceral) fascia along the tendinous arch of the levator ani, its anterior portion containing the lateral true ligaments of the bladder or prostate. Posteriorly, the transversely placed transverse cervical (cardinal) ligament of the uterus extends from the uterosacral ligament, carrying the ureter, inferior vesical vessels, uterine vessels, and autonomic nerves.

The presacral fascia extends medially from the hypogastric sheath sitting anterior to the sacrum and anterior sacrococcygeal ligament, lying in a more or less vertical plane, in contrast to the superior and inferior wings, which unfold in a nearly horizontal plane. Upon reaching the sides of the rectum, the presacral fascia splits into two leaves that encircle the rectum as the rectal (visceral) fascia. This fascia carries the superior and middle rectal vessels, inferior hypogastric or pelvic nerve plexus, and many lymphatics.

The course of the pelvic muscles and the anorectal musculature, together with the superior and inferior fascia of the levator ani, give rise to a number of perineopelvic spaces, which require more than mere anatomic recognition because they have a fundamental importance for an adequate concept of infectious and malignant processes of the pelvis and perineum. As with the fasciae, these spaces are conveniently separated by the levator ani muscle. Superior to the levator ani, in the male, there are four main spaces: (1) the prevesical space (of Retzius), (2) the rectovesical space, (3) the bilateral pararectal spaces, and (4) the retrorectal space.

The prevesical space of Retzius is, in both sexes, a potentially large cavity surrounding the anterior and lateral walls of the bladder. The main cavity in front of the bladder contains two superimposed anteromedian recesses and two lateral compartments. The upper anteromedial recess lies posterior to the anterior abdominal wall (i.e., behind the most medial parts of the transversalis fascia ) and is roofed by the peritoneal reflection from the dome of the bladder supported by the urachus and the umbilical prevesical fascia. Its lateral borders are demarcated by the obliterated umbilical arteries. The lower recess, continuous with the one above, lies posterior to the symphysis and pubic bones, anterior to the bladder, with a floor formed by the pubovesical ligaments in the female or the puboprostatic ligaments in the male. The lateral recesses of the prevesical space are bounded by a lateral wall formed by the obturator fascia and the superior fascia of the levator ani, and a median wall presented by the bladder and the lateral ligaments of the bladder. They contain the ureter and the main neurovascular supply to the bladder and, in the male, the prostate. The floor of the lateral recess is the superior fascia of the levator ani. Posteriorly, the lateral recess of the prevesical space extends to the hypogastric sheath in the region of the ischial spine. The roof is formed by the tendinous arch of pelvic fascia covered by the peritoneum, where these tissues are reflected from the lateral pelvic wall.

The retrovesical compartment in the male, divisible into three subspaces, lies between the bladder and the prostate, covered by the vesical and prostatic fasciae anteriorly, and the rectal fascia covering the rectum posteriorly. Its roof is formed by the rectovesical recess or pouch of the peritoneum, which comes into existence by the continuity of the peritoneal reflection from the rectum to the bladder. Its floor is the posterior part of the urogenital diaphragm. The rectoprostatic (Denonvilliers) fascia, originating from the undersurface of the rectovesical peritoneal pouch and extending inferiorly in a coronal plane, divides into two leaves, an anterior leaf, blending with the prostatic fascia or capsule, and a posterior leaf, attaching below to the urogenital diaphragm medially and to the hypogastric sheath laterally. Thus the retrovesical compartment can become subdivided into the retrovesical space and retroprostatic space anteriorly and the prerectal space posteriorly. The inferior aspect of the hypogastric sheath marks the lateral boundary of the two anterior spaces and also the separation from the lateral recess of the space of Retzius. Inferiorly, the prerectal space terminates where the rectal fascia attaches itself to the urogenital diaphragm or its thin superior fascia. The retroprostatic space (Proust space) terminates inferiorly in the same region but varies, depending on the very inferior limit of the rectoprostatic fascia and its attachments to the prostatic capsule.

In the female, as in the male, the area between the bladder and the rectum is divided into three spaces. The dominant dividing structure, however, is not the rectoprostatic fascia but the much more substantial vagina, cervix, and uterus. Anterior to these structures, two spaces come into existence, the vesicocervical space superiorly and the vesicovaginal space inferiorly. They are separated by a fascial septum, the supravaginal septum or vesicocervical ligament, which forms the floor of the vesicocervical space and the roof of the vesicovaginal space. The vesicocervical space is roofed by the uterovesical fold of the peritoneum and extends inferiorly to the point where the urethra and vagina are in apposition superior to the urogenital diaphragm. In the floor of this space, the medial and lateral pubovesical ligaments surround the urethra. Laterally, the vesicovaginal space is limited by the strong fascial connections between the bladder and the cervix.

In the female, the rectovaginal space is farther from the anterior compartments because the substantial mass of the cervix, uterus, and vagina provide more separation than in the male. Whether or not the small area between the rectum and the genital organs can be divided into a retrovaginal and a prerectal space is a controversial question of no practical significance. Of more practical importance is the fact that the rectovaginal space is roofed by a deep peritoneal fold that forms the rectouterine pouch (of Douglas). The boundaries of this space are, anteriorly, the vaginal fascia and, posteriorly, the rectal fascia. Laterally, the space extends to the fusion of the vaginal and rectal fascial collars, which, in this region, form the wings of the vagina. The space terminates inferiorly at the line of fusion between the posterior vaginal wall and the anal canal. In this region numerous fascial and muscular elements fuse, terminating inferiorly at the perineal body, also called the “central point of the perineum.”

The pararectal space extends on each side from the rectoprostatic fascia (male) or the cardinal ligament (female) to the presacral fascia. It lies on the supraanal fascia covering the superior surface of the pubococcygeus muscle, alongside the inferolateral parts of the rectum or its fascial enclosure. Its roof is made up, in both sexes, of the peritoneum reflected from the lateral aspects of the rectum to the pelvic walls, forming the floor of the pararectal peritoneal fossa.

The presacral space, similar in both sexes, constitutes the interval between the parietal pelvic fascia, covering the sacrum as well as the piriformis, coccygeus, and pubococcygeus muscles, and the presacral fascia, which envelops the rectum as the rectal fascia. Where the posterior rectal wall lies almost horizontally, the ventral lining of the presacral space is produced by the rectal fascial collar. Superiorly, the space becomes continuous with the prevertebral-retroperitoneal areolar tissue. A strong lateral barrier for this space is provided by the attachment of the hypogastric sheath to the parietal fascia, a fact that explains why retrorectal abscesses are more apt to rupture into the rectum than to penetrate into the space superior to the levator ani.

In the spaces inferior to the levator ani, the submucous space, encircling the sphincteric portion of the rectum and extending from the anorectal muscle ring to the dentate line, is the highest or most cranial. Its practical significance is explained by its contents: the terminal anastomotic network of the internal rectal venous plexus and a rich lymphatic plexus, both embedded in a supportive fibroelastic connective tissue.

A potential but not truly anatomic space, with somewhat ill-defined borders, lies within the conjoined longitudinal muscle between the internal and external anal sphincters. This intermuscular space surrounds the entire circumference of the anal canal, from the junction of the external sphincter with the levator ani to the intramuscular groove. Abscesses in this intermuscular space may develop as a result of infection of the perianal glands expanding within it. Both the submucous and intermuscular spaces are not interfascial but, rather, intravisceral.

The perianal space is located between the skin and the transverse septum of the ischioanal fossa. Its boundaries, projected to the surface, correspond to the anal triangle. Anteriorly, the space extends to the posterior border of the superficial transverse perineal muscle and laterally as far as the ischial tuberosities. Medially, the perianal space is confined by the anoderm superiorly as far as the latter's firm attachment to the internal anal sphincter. Numerous fibrous extensions from the conjoined longitudinal muscle, which pass through the subcutaneous external anal sphincter, transverse the perianal space. It is important to note that, circumanally, the perianal space reaches to the inferior end of the internal sphincter, within the subcutaneous external anal sphincter. The space contains the external rectal venous plexus and superficial perianal lymphatics. Posteriorly, extending as far as the coccyx, the perianal space changes its name and becomes the superficial postanal space, which extends from the anal canal to the subcutaneous tissue inferior to the extensions of the superficial external anal sphincter, known as the anococcygeal ligament, as it attaches to the posterior surface of the coccyx. It is noteworthy that the perianal space of each side communicates with its counterpart of the opposite side via this superficial postanal space inferior to the anococcygeal ligament in just the same fashion as the ischioanal fossae of each side communicate superior to this ligament via the deep postanal space. Posteriorly, the relationships to the extensions of the conjoined longitudinal muscle and the fibers of the corrugator cutis ani confine abscesses and fistulas complicating anal fissures to the superficial tissues.

The largest and most important of the spaces inferior to the levator ani muscle are the paired ischioanal fossae (average 6 to 8 cm anteroposteriorly, 2 to 4 cm wide, 6 to 8 cm deep). Each of these is irregularly wedge-shaped, with the apex at the pubic angle and the base at the gluteus maximus muscle. The superomedial wall is formed by the circumanal and infraanal fasciae covering the superficial and deep portions of the external anal sphincter and the superimposed puborectalis and pubococcygeus portions of the levator ani muscle. The attachments of this muscle and the infraanal fascia to the urogenital diaphragm mark the medial wall of the anterior extension (Waldeyer space), which extends anteriorly into the space above the urogenital diaphragm. At the most cranial point of the ischioanal fossa, the inner wall joins the outer wall, which is formed by the obturator fascia, overlying the obturator internus muscle, and farther inferiorly by the ischial tuberosity. The infraanal fascia covering the iliococcygeus muscle is the roof of the ischioanal fossa. The coccyx, sacrospinous ligament, sacrotuberous ligament, and overlapping gluteus maximus muscle constitute the base or posterior wall of the fossa. These structures thus confine the posterior extension of the ischioanal fossa, which has, posteriorly to the anal canal, no medial walls. The fossae of each side communicate with each other by what is known as the deep postanal space, which lies superior to the anococcygeal ligament or posterior extension of the external anal sphincter and inferior to the levator ani muscle. This deep postanal space is also known as the posterior communicating space, because through it communicate the right and left ischioanal fossae. The deep postanal space is thus the usual pathway for purulent infections to spread from one ischioanal fossa to the other, resulting in the semicircular or “horseshoe” posterior anal fistula. The floor of the ischioanal space posterior to the urogenital diaphragm is the transverse septum of the ischioanal fossa. In the anterior recess the floor is formed by the urogenital diaphragm. The ischioanal space is filled with adipose tissue in a matrix of thin collagenous fibrils. The inferior rectal vessels and nerves cross each space obliquely from its posterolateral angle en route from the pudendal vessels and nerves in the obturator canal to the anal canal.

The superficial and deep compartments of the urogenital diaphragm occupy the space within the pubic arch and contain the urogenital musculature that is in close functional relationship to the pelvic diaphragm and the anorectal sphincters.

Blood Supply of the Abdomen

The aorta enters the abdomen by passing posterior to the median arcuate ligament of the diaphragm at the level of T12. Its first branches are the paired inferior phrenic arteries, which commonly originate between the diaphragmatic crura and course to the inferior aspect of the dome of the diaphragm, where they divide into anterior and posterior branches. The latter of these anastomose with the intercostal arteries, whereas the former anastomose with twigs of the inferior phrenic artery, as well as the musculophrenic, pericardiacophrenic, and internal thoracic arteries. Communications also exist, through the coronary ligament and bare area of the liver, with the hepatic arterial system. The size and origin of the inferior phrenic arteries vary greatly. Their caliber ranges from 1 to 4 mm. They may exit bilaterally (60%) from either the aorta or celiac artery, or one from the former and the other from the latter. They may emerge as a common trunk (40%), either from the aorta (20%), from the celiac artery (18%), or from the left gastric artery (2%), before branching into left and right inferior phrenic arteries.

From the trunk of the posterior branch of the inferior phrenic artery, multiple superior suprarenal arteries arise, which, with the middle suprarenal artery (from the aorta) and inferior suprarenal artery (from the renal or accessory renal arteries), will supply blood to the suprarenal (adrenal) gland. Another important vessel is the recurrent esophageal branch, which is given off by the left inferior phrenic artery shortly after it has passed posterior to the esophagus. The right inferior phrenic artery gives off several branches that supply oxygenated blood to the inferior vena cava.

From the posterior surface of the aorta, opposite the four upper lumbar vertebral bodies, arise four lumbar arteries, either via a common trunk or separately on each side. Because the aorta ends at the L4 level, a fifth pair of lumbar arteries frequently originate from the middle sacral or internal iliac arteries. The lumbar arteries curve around the vertebral bodies and pass posterior to the sympathetic trunk, psoas major, and quadratus lumborum muscles, except for the fourth lumbar segmental artery, which often traverses anterior to the latter. The right lumbar arteries travel posterior to the inferior vena cava, and L1 and L2 arteries run posterior to the cisterna chyli. Each lumbar artery gives off a long posterior branch, which, via medial, lateral, and spinal rami, supplies the skin and muscles of the back, the spinal ligaments, and the spinal cord. Leaving the lateral border of the quadratus lumborum muscle, the lumbar arteries continue between the transversus abdominis and internal abdominal oblique muscle layers. As they travel toward the rectus abdominis muscle, they release lateral cutaneous and anterior cutaneous branches and anastomose with the lower intercostal, iliolumbar, and superior and inferior epigastric arteries and the ascending branch of the deep circumflex iliac artery. The lumbar arteries participate in an arterial circle formed by adipose capsular branches from the renal, suprarenal, and gonadal arteries.

The unpaired, visceral branches from the abdominal aorta that feed the foregut, midgut, and hindgut are the celiac, superior mesenteric, and inferior mesenteric arteries, respectively. The renal arteries exit the aorta at the level of L1 or between L1 and L2 and supply the kidneys, suprarenal glands, and proximal ureters. The gonadal (testicular or ovarian) vessels exit the anterior surface of the aorta inferior to the renal arteries at a level varying from L1 to L3, but they may occasionally arise from a suprarenal, phrenic, superior mesenteric, lumbar, common iliac, or internal iliac artery. They may appear as a duplicated artery (17%) on one side or, less frequently, on both sides. An important abnormality concerns an arched gonadal artery (arched testicular artery of Luschka), which originates from the aorta posterior and inferior to the renal vein but ascends to curve superiorly and descends anterior to the renal vein.

The aorta divides at the level of the lower third of the L4 vertebra into the approximately 6 mm–wide common iliac arteries, the lengths of which vary from 1 to 9 cm. Up to the point at which they divide into external and internal iliac arteries, the common iliac arteries have no branches, except for small, unnamed branches to the peritoneum and subperitoneal tissue.

The superior epigastric and musculophrenic arteries (both terminal branches of the internal thoracic artery) supply the anterolateral wall superiorly. The latter vessels travel inferiorly in a space posterior to the lower costal cartilages and send branches to the seventh to ninth intercostal spaces, the lower pericardium, and the superior region of the abdominal muscles. Terminating at the 10th and 11th intercostal spaces, they anastomose with the intercostal and subcostal arteries, with additional small connections to the lumbar and deep circumflex iliac arteries. A branch piercing the diaphragm communicates with the anterior ramus of the inferior phrenic artery. The superior epigastric artery, entering the rectus sheath posterior to the seventh costal cartilage and descending posterior to the rectus abdominis muscle, ramifies to supply this muscle and gives off a number of small cutaneous branches. It anastomoses with the inferior epigastric artery.

The main vessels that feed the inferior abdominal wall are the inferior epigastric and deep circumflex iliac arteries. Both arise from the external iliac artery, the former on its medial side and the latter on its lateral side just superior to the inguinal ligament. The inferior epigastric artery runs superiorly toward the umbilicus, supplying blood to the nearby peritoneum, transversalis fascia, and rectus sheath. It has several branches that supply the abdominal muscles and overlying subcutaneous tissue and skin. Typically it anastomoses heavily with the superior epigastric and lower intercostal arteries. Shortly after its origin, the inferior epigastric artery releases the cremasteric artery and a small pubic artery. The latter artery anastomoses with a branch of the obturator artery to supply structures on the posterior aspect of the pubic bone. The cremasteric artery accompanies the spermatic cord to supply the cremasteric muscle and fascia, ultimately anastomosing with the testicular artery. In the female, this artery accompanies the round ligament.

The deep circumflex iliac artery courses in a sheath formed by the union of the transversalis and iliac fasciae (or between the latter and the peritoneum) laterally and superiorly toward the anterior superior iliac spine. After piercing the transversalis fascia along the inner lip of the iliac crest, it continues to the crest's midpoint and passes through the transverse abdominal muscle to pursue a posterior course between this and the internal abdominal oblique muscle. An ascending branch, leaving the main artery near the anterior superior iliac spine, anastomoses with the subcostal, lumbar, and lower intercostal arteries; other branches communicate with the superficial circumflex iliac, inferior epigastric, iliolumbar, and superior gluteal arteries.

The final three arteries that supply blood to the abdominal wall are branches of the femoral artery. The superficial epigastric artery passes superiorly across the inguinal ligament and courses toward the umbilicus, supplying the superficial inguinal lymph nodes as well as the skin and the subcutaneous tissue of the medial, lower abdomen. The superficial circumflex iliac artery courses anterior to and parallel with the inguinal ligament (after piercing the fascia lata), providing blood to the upper thigh and lateral side of the abdomen. The external pudendal artery emerges through the fossa ovalis and travels medially across the spermatic cord or round ligament to supply the skin and subcutaneous tissue in the suprapubic region. One branch anastomoses with the dorsal artery of the penis or clitoris.

Venous Drainage of the Abdomen

The main collecting vessels of the abdomen are the inferior vena cava and the hepatic portal vein. The hepatic portal vein drains to the liver and it originates from smaller veins that drain the alimentary tract, its associated glands, and the spleen. Here we will focus on the inferior vena cava and its tributaries, starting with the superficial veins that drain the anterolateral abdominal wall. Please note that these veins accompany the arteries of the same name, mostly in duplicate (venae comitantes) on both sides of the artery, being enwrapped in the same sheath.

The external pudendal vein, aside from branches originating from the region above the symphysis pubis, receives the venous blood from the external genitalia (superficial dorsal vein of the penis or clitoris and the subcutaneous veins of the scrotum or labia majora), and joins, in many instances, the great saphenous vein or the femoral vein. The superficial epigastric and superficial circumflex iliac veins, draining the medial and lateral parts of the lower abdominal wall, respectively, pass superficial to the inguinal ligament and, piercing the cribriform fascia, enter the femoral vein (in other instances, the great saphenous vein). In the body's midaxillary line the superficial veins of the upper and lower halves of the trunk communicate through the thoracoepigastric veins, which unite in the axilla with the lateral thoracic veins, each a branch of an axillary vein. This system of anastomosis plays an important role in the event of an obstruction of the superior or inferior vena cava. The thoracoepigastric veins receive numerous tributaries from the surrounding superficial fascia as well as veins emerging from the lateral aspect of the mammary gland.

Another collateral venous circulation of clinical significance comes about through the superficial supraumbilical and infraumbilical veins, which, by means of five or six paraumbilical veins arising from the integument and the musculoaponeurotic structures or the abdominal wall, course within the ligamentum teres and enter the left branch of the portal vein. When portal venous pressure rises in liver cirrhosis, the paraumbilical veins establish collaterals with the superior and inferior epigastric and thoracoepigastric veins, and become enlarged and tortuous, assuming a radial pattern known as the caput medusae (head of Medusa).

The two deeper veins that drain the anterolateral abdominal wall are the inferior epigastric and deep circumflex iliac veins, both of which enter the external iliac vein (the continuation of the femoral vein) after having drained the same regions supplied by the corresponding arteries. This network of anastomoses, including the musculophrenic and superior epigastric veins, likewise conforms to the location of the arteries. The external iliac vein, beginning posterior to the inguinal ligaments, courses with its homonymous artery superiorly along the brim of the lesser pelvis to unite with the internal iliac vein anterior to the sacroiliac joint to form the common iliac vein.

The internal iliac vein collects the blood from all pelvic structures, except the upper part of the rectum and the sigmoid colon, which drain to the portal system via the inferior mesenteric vein, and the ovaries and testes, which reach the inferior vena cava directly via the gonadal veins. Starting near the superior part of the greater sciatic foramen and ascending over the piriform and psoas major muscles, the internal iliac vein receives the superior and inferior gluteal, internal pudendal, obturator, lateral sacral, middle rectal , and superior vesical veins. Many of these vessels have their origins in a rich venous plexus, such as the pudendal, urethrovesical, and uterovaginal plexuses.

The common iliac veins continue along the course of the external iliac veins in a median direction until the left vein meets the right vein, marking the starting point of the inferior vena cava. The left common iliac vein, often somewhat longer than its right counterpart, receives the middle sacral vein when this unpaired vessel does not enter (as it does frequently) the angle of the two iliac veins. Both common iliac veins receive the iliolumbar veins and, in some instances, the lateral sacral veins, if the latter have not entered the internal iliac vein or have not joined the fifth lumbar vein.

The inferior vena cava commences at the right of L5, ascends along the aorta anterior to the vertebral column, and continues posterior to the liver in a groove between the bare area and the caudal lobe. Immediately after the inferior vena cava receives the three hepatic veins (draining the liver), the inferior vena cava leaves the abdomen through the diaphragm's caval hiatus in the central tendon. Because the caval hiatus lies superior to the aortic hiatus and the union of the two common iliac veins is inferior to the aortic bifurcation, the inferior vena cava in the abdomen is about 7 to 8 cm longer than the abdominal aorta. The first veins to enter the inferior vena cava are the lumbar veins. The lowest (fifth) lumbar vein empties to the iliolumbar vein, whereas the upper four lumbar veins, lying on the bodies of the vertebrae and accompanying the arteries, drain into the posterior wall of the inferior vena cava but may drain to the azygos or hemiazygos veins. The connections that the lumbar veins make with the renal, suprarenal, gonadal, deep circumflex, iliac, and other abdominal veins are manifold. The most important concerns the longitudinal anastomosis effected through the ascending lumbar veins. These veins, beginning in the pelvis as a continuation of the lateral sacral veins, ascend deep in the sulcus between the tendinous origins of the psoas major muscle and the bodies and transverse processes of the vertebrae; after receiving branches from the lumbar veins, the right ascending lumbar vein drains into the azygos and the left into the hemiazygos, or sometimes into the left renal vein. Posteriorly, the ascending lumbar veins make numerous connections with the valveless veins of the vertebral venous system and thus bring the caval system into relationship with the veins of the spine, spinal cord, dura mater, vertebrae, and brain. These relationships provide an explanation for the spread of infections, tumors, and thrombi from the pelvis, abdomen, or thorax into the central nervous system, or bones of the skull and spine.

The right gonadal (testicular or ovarian) vein enters the inferior vena cava superior to the lumbar veins, whereas the left gonadal vein usually merges with the left renal vein, or possibly the suprarenal vein, or one of the lumbar veins. The testicular veins, starting from the pampiniform plexus in the spermatic cord, ascend along the ductus deferens, pass through the inguinal canal, and, following the artery, course superiorly on the psoas major muscle. The ovarian veins, derived from the uterovaginal and ovarian plexuses, take a similar course.

The large renal veins lie anterior to the corresponding arteries and show much less variation than the renal arteries. The right renal vein rarely receives tributaries, whereas on the left side, supernumerary veins such as the left gonadal and suprarenal veins typically join the vessel. The right suprarenal vein usually terminates with a direct connection with the inferior vena cava and, occasionally, right renal vein. The left suprarenal vein typically drains into the left renal or inferior phrenic vein.

Superior to the hepatic veins are the uppermost tributaries of the inferior vena cava, the inferior phrenic veins, which generally follow the course of the homonymous arteries. The left one may join the left renal vein separately or via a common trunk with the left suprarenal vein (5%).

Lymph Drainage of the Abdomen

The major lymphatic channels of the posterior abdominal wall are essentially located along the large blood vessels. Thus the external iliac lymph vessels, interrupted by nodes of the same name, course with the external iliac arteries and veins. Entering the pelvis posterior to the inguinal ligament about midway between the anterior superior spine of the ilium and symphysis pubis, these vessels receive lymph from the deep (and thereby also superficial ) inguinal lymph nodes, through which pass the lymphatic drainage of the lower extremities, the inferior parts of the anterolateral abdominal wall, and the perineum (including the external genitalia and anal region). The internal iliac lymph vessels run, interrupted by the internal iliac nodes, with the artery and vein of the same name and drain the larger part of the organs and wall of the true pelvis, whereas the remaining part of this region releases lymph through the presacral lymphatics. The external and internal iliac lymphatics join to form common iliac lymph vessels and nodes of the same name. Common iliac lymph vessels also receive input from the presacral lymphatics with their lateral and middle sacral nodes. The latter are situated in the retrorectal connective tissue over the anterior surface of the sacrum. In the region of the aortic bifurcation, the common iliac lymph vessels proceed superiorly along the lateral walls of the aorta to become the right and left lumbar trunks. These trunks and the interposed lateral aortic lymph nodes receive afferents from the kidney and the visceral (preaortic) lymph nodes. The extremely large area of drainage that the lumbar trunks serve includes, thus, the walls and organs of the lower abdomen as well as of the lower extremities.

Both lumbar trunks unite in the region of the aortic hiatus, anterior to the vertebral column (in the majority of cases), at the level of the upper third of L1 and the intervertebral disc between vertebrae T12 and L1, to form the beginning of the thoracic duct. In about 50% of individuals, the thoracic duct starts with a distinctive, elongated, saccular dilatation (≈ 1 to 1.5 cm in diameter and 5 to 7 cm in length), the cisterna chyli. Its three main roots are the single intestinal trunk and the two lumbar trunks; however, two smaller tributaries coming from a cranial direction and descending through the aortic hiatus of the diaphragm also join the cisterna chyli.

The thoracic duct passes first to the right across the posterior surface of the aorta, through the aortic hiatus of the diaphragm into the mediastinum, ascending between the aorta and the azygos vein, anterior to the lower thoracic vertebrae and right intercostal arteries. On reaching the level of the fifth thoracic vertebra, it courses posterior to the esophagus to the left side of the spinal column, where it runs for a short distance to the right of the aorta and then crosses posterior to the aortic arch to continue its ascent. Opposite the third thoracic vertebra, the thoracic duct draws away from the spinal column in an anterior direction and proceeds between the left common carotid artery and the left subclavian artery, through the superior thoracic aperture, and into the left supraclavicular fossa. Here it arches superior to the subclavian artery and opens either into the angle at which the left jugular and left subclavian veins join to form the left brachiocephalic vein or, less often, into one of the two veins forming this angle. At its point of entry into the veins, the thoracic duct does not always form a single entity but sometimes divides into a triangular structure composed of two or more branches. During its passage through the thorax, the thoracic duct is joined by vessels connecting with the posterior parietal, tracheobronchial, and posterior mediastinal lymph nodes, as well as smaller lymph vessels draining the thoracic wall and thoracic organs.

Innervation of Abdomen and Perineum

The segmentally arranged nerves are attached to the sides of the spinal cord by a series of anterior (ventral) and posterior (dorsal) roots. An anterior and posterior root at each spinal segment unite to form the spinal nerve, which emerges through the corresponding intervertebral foramen. The anterior roots contain axons from the motor nerve cells in the anterior horn of the spinal cord and the posterior roots contain the axons projecting from the pseudounipolar sensory cells located in the posterior (dorsal) root ganglia (spinal sensory ganglia).

The spinal nerve only exists for a short span before dividing into anterior and posterior rami, each of which carries both motor and sensory axons to their target tissues. Before splitting into rami, each of the spinal nerves gives off a small recurrent meningeal branch that is sensory to the nearby spinal dura mater and intervertebral disc. After emerging from the intervertebral foramen, each spinal nerve receives a branch or branches (gray rami communicantes) from an adjacent ganglion of the sympathetic trunk, which contains postganglionic sympathetic axons originating from the cells of that ganglion. Of the first thoracic through the first two or, occasionally, three lumbar anterior rami, each contributes a branch or branches (white rami communicantes), which contain preganglionic sympathetic fibers to the corresponding sympathetic ganglia.

In general, the posterior rami are smaller than the anterior rami and do not unite to form plexuses. They divide into medial and lateral branches that supply the muscles and skin of the back. The anterior rami supply the anterolateral aspects of the trunk as well as the limbs. In the cervical, lumbar, sacral, and coccygeal regions, the anterior rami converge to form plexuses, but in the thoracic region, they maintain their segmental character and each runs separately and independently to the site or structure it innervates.

The thoracic anterior rami, the intercostal nerves, are distributed chiefly to the anterolateral walls of the thorax and abdomen. They are 12 in number on each side, but only 11 are truly intercostal. The 12th pair lie below the last ribs and are termed subcostal nerves. The upper six pairs of intercostal nerves are limited in their supply to the thoracic body wall, although the first and second intercostal nerves also contribute to the brachial plexus, the innervation of the upper limbs. The fourth nerve innervates the skin at the level of the nipple. The lower five pairs of intercostal nerves and the subcostal nerves supply the thoracic and abdominal body wall and also contribute fibers to the diaphragm.

Typically, the 7th to 11th intercostal nerves course anteriorly along the thoracic wall below the corresponding rib and intercostal vessels. Posteriorly, the nerve lies between the pleura and the posterior intercostal membrane and passes between the internal and innermost intercostal muscles. Each nerve gives off a collateral branch and a lateral cutaneous branch. The former, separating from the primary ramus only a few centimeters away from the vertebrae, inclines inferiorly from the parent nerve, runs along the lower border of the intercostal space, and ends anteriorly as a small cutaneous nerve. The lateral cutaneous branch accompanies the main intercostal nerve as far as the midaxillary line before piercing the intercostal muscles and dividing into anterior and posterior branches, which are mainly cutaneous in distribution. The intercostal nerves supply intercostal, subcostal, and transverse thoracic muscles. The lower five or six intercostal nerves also supply sensory axons to the peripheral parts of the diaphragm.

The lower five intercostal nerves and the subcostal nerves pass posterior to the costal cartilages and enter the abdominal wall to supply the external and internal abdominal oblique, transversus abdominis, and rectus abdominis muscles and end as anterior abdominal cutaneous branches. The 10th nerve serves the dermatome at the level of the umbilicus. The lateral cutaneous branch of the subcostal nerve (T12) pierces the internal and external oblique abdominal muscles and descends over the iliac crest to assist in supplying the skin over the upper lateral part of the thigh.

The anterior rami of the lower spinal nerves (five lumbar, five sacral, and one coccygeal) divide and reunite in a plexiform fashion to form the lumbar, sacral, and coccygeal plexuses. They are interconnected as described above with the sympathetic trunks via rami communicantes.

The lumbar plexus is formed by the anterior rami of the first three lumbar nerves and the greater part of the fourth lumbar nerve, along with a contribution from the subcostal nerve. It is situated anterior to the lumbar vertebral transverse processes and is embedded in the posterior part of the psoas major muscle, which needs to be dissected to make the plexus accessible. The most common course and distribution of the components of the plexus and its relationship are described and illustrated here, but it should be kept in mind that variations of the lumbar plexus are frequent.

The first lumbar nerve, after receiving a twig from the subcostal nerve, splits into an upper branch and a smaller lower branch. The former divides into the iliohypogastric and ilioinguinal nerves, and the latter unites with a twig of the second lumbar nerve to form the genitofemoral nerve. The rest of the second lumbar nerve, the third, and that part of the fourth which contributes to this plexus, each divide also into anterior and posterior sections, which combine to constitute the obturator and femoral nerves, respectively. The accessory obturator nerve, when present, is formed by branches from the anterior divisions of the third and fourth nerves, whereas the lateral femoral cutaneous nerve evolves by the fusion of small offshoots from the posterior divisions of the second and third lumbar nerves. Muscular branches from the subcostal and upper four lumbar nerves supply the quadratus lumborum muscle, and those of the first and second reach the psoas major and psoas minor muscles. The psoas major muscles are further innervated by branches from the third and, sometimes, fourth lumbar nerves, which also supply the iliacus muscles.

The iliohypogastric and ilioinguinal nerves resemble the thoracic nerves in their course and distribution, being analogous, respectively, to the main trunk and the collateral branch of an intercostal nerve. The former nerve gives off a lateral branch, which crosses the iliac crest a short distance posterior to the corresponding branch of the subcostal nerve, both nerves supplying skin of the superior lateral part of the thigh. Continuing anteriorly, the anterior branch of the iliohypogastric nerve sends filaments to the transverse and oblique abdominal muscles, pierces the external oblique aponeurosis about 3 cm superior to the superficial inguinal ring, and terminates innervating the skin just superior to the pubis.

The ilioinguinal nerve supplies filaments to the adjacent muscles and, after piercing the same muscles as the iliohypogastric nerve, enters the inguinal canal, runs deep to the spermatic cord, and emerges through the superficial inguinal ring to supply the superior medial side of the thigh, the root of the penis, and the anterior part of the scrotum in the male, and the mons pubis and labium majora in the female.

The genitofemoral nerve, after emerging from the lumbar plexus, passes through the psoas major muscle and descends on its anterior surface, deep to the peritoneum, to divide into the genital and femoral branches at about the level of the fifth lumbar vertebra. The former branch enters the inguinal canal through the deep inguinal ring, innervates the cremaster muscle, and contributes some twigs to the skin of the scrotum, or the labium majora of the female. The femoral branch runs lateral to the external iliac and femoral arteries, passes posterior to the inguinal ligament, and, after piercing the anterior layer of the femoral sheath and the fascia lata, ramifies in the superficial tissues and skin over the femoral triangle. The genitofemoral nerve and its branches carry many of the efferent and afferent fibers to and from the common iliac, external iliac, and femoral arteries.

Other branches of the lumbar plexus (e.g., the femoral nerve ), except for muscular rami to the quadratus lumborum, psoas major, and iliacus muscles, are distributed to the lower limb and, consequently, are not discussed in this volume.

The anterior rami of the sacral and coccygeal nerves, which, in contrast to the lumbar nerves, diminish in size as they progress inferiorly, divide and reunite to contribute to the sacral and coccygeal plexuses. These lie on the posterior wall of the pelvis, posterior to the ureters, internal iliac vessels, and intestinal coils, and anterior to the piriformis and coccygeus muscles. The inferior and smaller part of the fourth lumbar nerve unites with the anterior ramus of the fifth lumbar nerve as the lumbosacral trunk, which, together with the anterior rami of the first three and the upper part of the fourth sacral nerves, constitutes the sacral plexus. The lower part of the fourth sacral joins the fifth sacral and coccygeal nerves to form the small coccygeal plexus.

Each nerve entering into the composition of these two plexuses receives postganglionic sympathetic fibers by way of one or more gray rami communicantes from an adjacent ganglion of the sympathetic trunk. Preganglionic parasympathetic fibers originate in the second to fourth sacral levels of the spinal cord; they emerge with the second, third, and fourth sacral nerves and leave thereafter as pelvic splanchnic nerves.

The sacral plexus, by convergence and fusion of its roots, develops into a flattened band, from which many branches arise, before the large sciatic nerve passes through the greater sciatic foramen inferior to the piriformis muscle. This large nerve consists of a tibial section and a common fibular section, which usually remain fused until about the lower third of the thigh, but which may occasionally be separated at their points of origin or may divide before the nerve leaves the pelvis. The nerve of the sacral plexus splits into anterior and posterior divisions, which, in some individuals, unite again to produce the nerves. Most branches of the sacral plexus supply the lower limb and will be discussed in the volume addressing the musculoskeletal system. Others are distributed in the pelvic and perineal regions.

The nerves to the piriformis, levator ani, and coccygeus muscles pierce the anterior or pelvic surfaces of these muscles. The nerve to the obturator internus muscle (not to be confused with the obturator nerve) leaves the pelvis through the greater sciatic foramen inferior to the piriformis muscle, crosses the ischial spine lateral to the pudendal nerve and internal pudendal vessels, reenters the pelvis through the lesser sciatic foramen, and sinks into the pelvic surface of the obturator internus muscle.

The pudendal nerve passes between the piriformis and coccygeus muscles, leaves the pelvis through the greater sciatic foramen, alongside the sciatic nerve, crosses posteroinferior to the ischial spine (medial to the internal pudendal artery), and accompanies that vessel through the lesser sciatic foramen into the pudendal canal on the obturator internus fascia. As the nerve enters the canal, it gives off the inferior rectal nerve and shortly thereafter terminates by splitting into the perineal nerve and the dorsal nerve of the penis or clitoris, respectively.

The inferior rectal nerve perforates the medial wall of the pudendal canal, crosses the ischioanal fossa obliquely with the inferior rectal vessels, and divides into branches that are the main supply of the external anal sphincter, the lining of the lower part of the anal canal, and the skin around the anus. Its branches communicate with the perineal branches of the posterior femoral cutaneous, fourth sacral, and perforating cutaneous nerves and the perineal nerve , which is the larger terminal branch of the pudendal nerve. This latter nerve runs anteriorly in the pudendal canal inferior to the internal pudendal artery, projecting toward the posterior border of the urogenital diaphragm, near which it divides into superficial and deep branches. The superficial one divides into medial and lateral posterior scrotal (or labial) nerves, which spread over the skin of the scrotum or labia majora, communicating with the perineal branch of the posterior femoral cutaneous nerve. The deep branches supply the anterior parts of the external anal sphincter, the superficial and deep transverse perineal, bulbospongiosus, and ischiocavernosus muscles, as well as the sphincter urethrae (and, in a subsidiary fashion, the levator ani). A twig, termed the nerve of the bulb, arises from the branch to the bulbospongiosus muscle and is distributed to the erectile tissue of the corpus spongiosum and the mucous membrane of the urethra.

The dorsal nerve of the penis accompanies the internal pudendal artery in its course through the deep transversal perineal muscle and passes anterior to the pubic arch under cover of the ischiocavernosus muscle and corpus cavernosum penis. Passing through a gap between the inferior fascia and the apex of the urogenital diaphragm, the nerve comes to lie alongside the dorsal artery of the penis and continues as far as the glans and the prepuce. In the female the dorsal nerve of the clitoris is smaller, but its distribution is similar.

The posterior femoral cutaneous nerve, besides innervating the skin of the posterior thigh, gives off a gluteal branch, the inferior cluneal nerve, supplying the skin area over the lower part of the gluteus maximus and, in the same region, a perineal branch that curves anteriorly and medially inferior to the ischial tuberosity to the skin and fasciae of the perineum, scrotum, and root of the penis. The distribution is similar in the female, to the perineum, labia majora, and root of the clitoris. Its terminal twigs communicate with the inferior rectal and perineal branches of the pudendal and terminal filaments of the ilioinguinal nerves. The perforating cutaneous nerve pierces the sacrotuberous ligament and turns around the lower margin of the gluteus maximus to become cutaneous a short distance lateral to the coccyx. Its origin and distribution, however, are not constant. It may be joined or replaced by branches from the pudendal nerve, posterior femoral cutaneous nerve, or perineal branch of the fourth sacral nerve, arising from a loop between the third and fourth sacral nerves. This branch reaches the posterior angle of the ischioanal fossa by perforating the coccygeus muscle and then divides into some twigs that run anteriorly to assist the innervation of the external anal sphincter and others that ramify in the overlying skin and fascia.

The coccygeal plexus is formed by the union of the inferior part of the anterior ramus of the fourth sacral nerve with those of the fifth sacral and coccygeal nerves. The plexus is small and really consists of two loops on the pelvic surface of the coccygeus and the levator ani muscles. It gives off fine twigs to the parts adjacent to both these structures, as well as the delicate anococcygeal nerves that pierce the sacrotuberous ligament and supply the skin in the vicinity of the coccyx.

Having discussed the nerves supplying the wall of the abdominal cavity, the lumbar, sacral, and coccygeal plexuses, and the nerves they release to innervate part of the abdominal viscera and floor (pelvis as well as perineum) of the abdominal cavity, it remains to consider the innervation of the diaphragm, which forms the roof of the abdominal cavity. The diaphragm is supplied by the phrenic and lower intercostal nerves. Each phrenic nerve contains both motor and sensory fibers; the latter convey afferent impulses from the pleura, pericardium, peritoneum, and other structures. The motor fibers are the axons of the phrenic nucleus in the third, fourth, and fifth cervical cord segments. If one phrenic nerve is destroyed, complete muscular atrophy occurs in the corresponding half of the diaphragm, so it is presumed the intercostal nerve supply must be sensory.

The phrenic nerves are distributed mainly on the inferior surface of the diaphragm. The right pierces the central tendon just lateral to the caval hiatus and divides into anterior and posterior branches that supply all the muscle fibers on the same side, including the crural fibers on the right side of the esophagus and those arising from the arcuate ligaments. The left nerve pierces the diaphragm about 3 cm anterior to the central tendon and thereafter supplies the left half of the muscle, including the fibers of the right crus lying to the left of the esophageal hiatus. The phrenic branches communicate with autonomic fibers from the celiac plexus accompanying the inferior phrenic arteries. On the right side a small ganglion marks one of these interconnections.

Overview of the Digestive System

The digestive system is by far the largest and most complex of the internal organ systems. The interplay of its multiple organs, its intrinsic hormonal and neural systems, and its intricate and interacting physiologic functions are among the most fascinating aspects of human physiology. Although the primary function of each organ is to interact effectively with other organs to provide nutrition to the rest of the body, several organs also have distinct metabolic functions that are of vital importance.

Appreciating the role of each organ begins with asking how the four essential functions of each are regulated and how immune and other defense mechanisms are protecting that organ. The wall of each luminal organ is composed of three layers of distinctly functioning muscle groups responsible for moving nutrients and fluids from the mouth until they are discharged from the anus. The electromechanical coupling mechanisms responsible for motility by which this occurs are surprisingly distinct for each organ. An electrical syncytium regulates these contractions with rhythmic depolarizations called slow waves and contraction-inducing depolarizations resulting in action potentials. Action potentials are similar throughout the luminal organs, but slow wave activities in the stomach, duodenum, and colon vary in frequency.

All luminal and solid gastrointestinal organs are involved with secretions that facilitate digestion and mucosal protection, leading to nutrient absorption. In contrast, the esophagus has the least secretion and no absorption and the liver and pancreas are involved with secretion only and not motility or absorption.

The liver is the most important and largest metabolic organ. Metabolic functions are also provided by proteins synthesized by the small bowel and by hormones secreted by the stomach, pancreas, and small bowel.

Regulation of each organ is achieved with a complex interaction between the extrinsic autonomic nervous system, intrinsic or enteric nervous system, and hormones secreted both within and outside the digestive system. The hormones of the digestive system were the first endocrine substances to be discovered. The small intestine is clearly the largest of all endocrine organs. Each of the neurotransmitters identified in the enteric nervous system of the digestive system is also found in the brain. Again, many were first discovered in the gut.

The lumen of each of the digestive organs is filled with potentially lethal chemicals and microorganisms. Distinct, organ-specific, highly effective defense mechanisms exist in each organ to prevent disease. The gut's microbiome develops shortly after birth and grows to a point of containing 10 trillion organisms, or nearly 10 times as many cells as the rest of the body! Other important defense mechanisms with unique actions in each organ include motility, intrinsic secretion, lubrication with fluid and mucus, and frequent cell turnover.

When these protective systems break down or become impaired, disease begins. Disorders of the digestive tract are the second most common reason, after upper respiratory tract disorders, that patients seek help from a primary care physician or are absent from work or school. In a typical year, approximately 60% of individuals experience some digestive system dysfunction, whether acute or chronic. Common digestive disorders affecting 15% to 20% of the U.S. population include functional bowel disturbances, gastroesophageal reflux disease, peptic ulcer disease, hepatitis, gallbladder stones, and infectious diseases of the stomach and intestines. These and other gastrointestinal disorders account for 25% of all hospitalizations. Of all primary neoplasms leading to death, one third originate in digestive system organs. Cancers of the digestive system continue to be the most common cause of all cancer deaths. Lung cancer is overall the most common type of cancer, but cancers of the colon, pancreas, liver, stomach, and esophagus are among the 10 commonest cancers.

One of the most fascinating aspects of the pathophysiology of the digestive system is the marked difference in the prevalence of disorders in men and women. Eosinophilic esophagitis, esophageal adenocarcinoma, and hepatocellular carcinoma are much more common in men, and irritable bowel syndrome and gallstone disease are much more common in women.