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Abdomen and pelvis: Overview and surface anatomy
General Structure and Function of the Abdominopelvic Cavity
Although often considered separately, the abdomen and pelvis form the largest continuous visceral cavity in the body. Together, they provide multiple vital functions including: housing and protection of the digestive and urinary tracts and of the internal reproductive organs; a conduit for neurovascular communication between the thorax and lower limb; support and attachment for the external genitalia; access to and from the internal reproductive organs and urinary tract; assistance with physiological functions such as respiration, defecation and micturition; support for the vertebral column in weight-bearing, maintenance of posture and movement; and, in females, the ability to support gestation.
Musculoskeletal framework of the abdomen and pelvis
The walls of the abdominopelvic cavity consist of five lumbar vertebrae and their intervening intervertebral discs (lying in the posterior midline); the muscles of the anterior abdominal wall lying anteriorly (rectus abdominis) and anterolaterally (transversus abdominis, internal abdominal oblique and external abdominal oblique); the muscles of the posterior abdominal wall (psoas major, quadratus lumborum and the respiratory diaphragm); the bony ‘basin’ formed by the walls of the greater and lesser pelves; the muscles of the pelvic diaphragm and perineum lying inferiorly; and the respiratory diaphragm lying superiorly ( Fig. 59.1 ). In addition, the superior aspect of the abdominal cavity gains protection from the inferior six ribs and their cartilages, even though these structures are technically part of the thoracic wall.
The muscles of the abdominal wall are important for movement of the trunk (flexion, extension and rotation). The anterolateral muscles in particular provide assistance with rotation of the thorax in relation to the pelvis (or vice versa if the thorax is fixed).
The abdominal cavity is somewhat kidney-shaped in horizontal cross-section owing to the posterior indentation of the vertebral column. In standing, the linea terminalis lies at an angle of about 55° to the horizontal such that the anterior superior iliac spines and pubic tubercles are in approximately the same coronal plane. The mean angle between the superior border of the first segment of the sacrum and the horizontal plane is about 40–45° ( ).
Thoracoabdominal interface
The respiratory diaphragm (diaphragm, thoracic diaphragm) constitutes the interface between the thoracic and abdominal cavities ( Ch. 55 ). There are three principal pathways between the two cavities across the respiratory diaphragm: the caval foramen in the central tendon transmits the inferior vena cava and right phrenic nerve; the oesophageal hiatus, encircled by the right crus of the respiratory diaphragm, transmits the oesophagus, vagal trunks and vessels; and the aortic hiatus, posterior to the median arcuate ligament, transmits the aorta, thoracic duct and, usually, the azygos vein. The hemiazygos vein usually enters the thorax through the left crus of the respiratory diaphragm. Other lymphatics from the abdomen drain to the thorax alongside the inferior vena cava and via small vessels passing through and around the respiratory diaphragm. Thoracic splanchnic nerves reach the abdomen through the crura of the respiratory diaphragm and posterior to the medial arcuate ligaments, and the left phrenic nerve pierces the left side of the respiratory diaphragm. The subcostal vessels pass into the abdomen posterior to the lateral arcuate ligaments of the respiratory diaphragm. Anteriorly, the superior epigastric vessels pass between the costal and xiphoid process attachments, i.e. sternocostal foramen of the respiratory diaphragm. Neurovascular structures also cross between the thorax and abdomen within the subcutaneous tissues.
Pelvis–lower limb interface
The pelvis forms an integral part of the bony structure of both the abdominopelvic cavity and the lower limbs. It transmits the weight of the upright body and provides a stable platform for movement of the hip joint and bipedal locomotion. Its bony surfaces provide attachment sites for the muscles of the buttock and thigh ( Ch. 77 ), the pelvic diaphragm and perineal membrane, the abdominal wall and lower back. The pelvis also transmits the neurovascular structures that supply the lower limb. There are four principal pathways between the pelvis and lower limb: the interval deep to the inguinal ligament anterior to the superior ramus of the pubis and ilium, which transmits the femoral neurovascular structures and lymphatics; the greater and lesser sciatic foramina, which transmit the gluteal vessels and nerves, sciatic nerve and internal pudendal vessels and pudendal nerve; and the obturator foramen, which transmits the obturator nerve, vessels and lymphatics. Autonomic nerves travel with the arterial supply to the lower limb and with the branches of the sacral plexus. Neurovascular structures also cross between the lower limb and pelvis within the subcutaneous tissues.
General Arrangement of Abdominopelvic Autonomic Nerves
What follows is a brief overview of the autonomic nervous system in the abdominopelvic region; descriptions of the neurovascular supply to individual organs are given in the relevant chapters. The autonomic supply to the abdominal and pelvic viscera is via the abdominopelvic part of the sympathetic trunk and the greater, lesser and least thoracic splanchnic nerves (sympathetic), and the vagus and pelvic splanchnic nerves (parasympathetic). There are numerous interconnections between plexuses and ganglia, particularly in the major plexuses around the abdominal aorta; hence, the descriptions tend to be simplifications based on the ‘main’ supply to each organ ( Fig. 59.2 ). Details of the terminations of these fibres are given in the description of the general microstructure of the wall of the hollow viscera.
As a general rule, sympathetic neurones from the abdominopelvic autonomic plexuses inhibit visceral smooth muscle motility and glandular secretions, induce sphincter contraction and cause vasoconstriction. Parasympathetic stimulation leads to opposing effects. Visceral afferents also pass through these autonomic plexuses.
Sympathetic innervation
The cell bodies of neurones of the sympathetic supply of the abdomen and pelvis lie in the intermediolateral grey matter of the first to the twelfth thoracic and the first two lumbar spinal segments. Myelinated axons from these neurones travel in the ventral ramus of the spinal nerve of the same segmental level, leaving it via a white ramus communicans to enter a thoracic or lumbar ganglion. Visceral branches can exit at the same level or ascend or descend several levels in the sympathetic trunk before exiting; they leave the ganglia without synapsing and pass medially, giving rise to the paired greater, lesser and least thoracic splanchnic nerves and to the lumbar and sacral splanchnic nerves. Axons destined to supply somatic structures synapse in the sympathetic ganglion of the same level, and postganglionic, unmyelinated axons leave the ganglion as one or more grey rami communicantes to enter the spinal nerve of the same segmental level.
Greater thoracic splanchnic nerve
On each side, the greater thoracic splanchnic nerve is derived from the medial branches of the fifth to ninth thoracic ganglia. It gives off branches to the descending thoracic aorta and enters the abdomen through the fibres of the ipsilateral crus of the respiratory diaphragm, on which it descends anteroinferiorly. The main trunk of the nerve enters the superior aspect of the coeliac ganglion, where most of the preganglionic fibres synapse (but not those destined for the medulla of the suprarenal gland).
Lesser thoracic splanchnic nerve
On each side, the lesser thoracic splanchnic nerve is derived from the medial branches of the tenth and eleventh (or ninth and tenth) thoracic ganglia. It enters the abdomen running through the most inferior fibres of the ipsilateral crus of the respiratory diaphragm or under the medial arcuate ligament, and then lies on the crus as it runs anteroinferiorly. The trunk of the nerve joins the aorticorenal ganglion and may give branches to the lateral aspect of the coeliac ganglion. It occasionally joins the greater and least thoracic splanchnic nerves as a single splanchnic nerve.
Least thoracic splanchnic nerve
On each side, the least thoracic splanchnic nerve is derived from the medial branches of the eleventh and/or twelfth thoracic ganglion. It enters the abdomen medial to the sympathetic trunk under the medial arcuate ligament of the respiratory diaphragm and runs inferiorly to join the renal plexus. The trunk of the nerve enters the aorticorenal ganglion and may give branches to the lateral aspect of the coeliac ganglion. It is sometimes part of the lesser thoracic splanchnic nerve, when it forms a twig that enters the renal plexus just below the aorticorenal ganglion.
The thoracic splanchnic nerves show considerable individual variation in their origin and distribution ( ). For example, the greater thoracic splanchnic nerve can receive a contribution from the fourth or tenth thoracic ganglion of the sympathetic trunk or originate only from the sixth to ninth ganglia.
Lumbar sympathetic system
The lumbar part of each sympathetic trunk usually contains four ganglia lying on the anterolateral aspects of the lumbar vertebrae along the medial margin of psoas major (see Figs 59.2 – 59.4 , 61.15 ) ( ). Superiorly, it is continuous with the thoracic part of the sympathetic trunk posterior to the medial arcuate ligament. Inferiorly, it passes posterior to the common iliac vessels and is continuous with the sacral part of the sympathetic trunk. On the right side, it lies posterior to the inferior vena cava; on the left, it is posterior to the lateral aortic nodes. It is anterior to most of the lumbar vessels but may pass posterior to some lumbar veins.
On each side, the first, second and sometimes the third lumbar ventral rami are connected to the lumbar part of the sympathetic trunk by white rami communicantes. All lumbar ventral rami are joined near their origins by long, slender, grey rami communicantes from the four lumbar ganglia. Their arrangement is irregular: one ganglion can give rami to two or three lumbar ventral rami, one lumbar ventral ramus can receive rami from two ganglia, or grey rami can leave the sympathetic trunk between ganglia ( ).
Somatic and vascular branches
Sympathetic nerve branches accompany the lumbar arteries round the sides of the vertebral bodies, medial to the fibrous arches to which psoas major is attached, to provide sympathetic innervation to the segmental distribution of the lumbar arteries. Branches from the lumbar ganglia innervate the abdominal aorta and common, external and internal iliac arteries, forming delicate nerve plexuses that extend along the vessels. Other postganglionic sympathetic nerves to vessels and skin travel with somatic nerves. Thus, the femoral nerve carries vasoconstrictor sympathetic nerves to the femoral artery and its branches in the thigh, as well as sympathetic fibres in its cutaneous branches. Postganglionic fibres travelling with the obturator nerve supply the obturator artery and a variable patch of skin of the medial thigh. Sympathetic denervation of vessels in the lower limb can occur by removing or ablating the proximal three lumbar ganglia and intervening parts of the sympathetic trunk; this procedure can be useful for treating some varieties of vascular insufficiency of the lower limb.
Lumbar splanchnic nerves
Four lumbar splanchnic nerves pass as medial branches from the ganglia to join the coeliac, inferior mesenteric and superior hypogastric plexuses (see Fig. 59.4 ). The first lumbar splanchnic nerve, from the first ganglion of the lumbar part of the sympathetic trunk, gives branches to the coeliac, renal and inferior mesenteric plexuses. The second lumbar splanchnic nerve joins the inferior part of the intermesenteric or inferior mesenteric plexus. The third lumbar splanchnic nerve arises from the third or fourth ganglion and joins the superior hypogastric plexus. The fourth lumbar splanchnic nerve from the lowest ganglion joins the distal part of the superior hypogastric plexus, or the hypogastric nerve. It is important to note that lumbar splanchnic nerves contribute to the superior and inferior hypogastric plexuses and therefore to the innervation of the neck of the urinary bladder, ductus deferens and prostate, among other structures. Injury to these nerves, e.g. during aortoiliac surgery, can result in sexual dysfunction.
Pelvic sympathetic system
The sacral part of the sympathetic trunk usually consists of four or five ganglia located medial or anterior to the anterior sacral foramina posterior to the presacral fascia ( ). Occasionally, one or two coccygeal ganglia are present. The sacral part of the sympathetic trunk is continuous above with the lumbar part of the sympathetic trunk, and preganglionic fibres descend from the distal lumbar spinal cord segments via this route. The first sacral ganglion is the largest. Ganglia become progressively smaller in the caudal direction ( ). The sacral sympathetic trunk is often asymmetric, with absent or fused ganglia, and cross-communications between sides are frequent. Each ganglion sends at least one grey ramus communicans to its adjacent spinal nerve but up to 11 such branches from a single ganglion have been reported ( ).
The sacral part of the sympathetic trunk converges caudally to form a solitary retroperitoneal structure, the ganglion impar (ganglion of Walther), which lies at a variable level between the sacrococcygeal joint and the apex of the coccyx; it is occasionally paired, unilateral or absent ( ). It conveys sympathetic fibres to and nociceptive afferents from the perineum and terminal urogenital regions. Anaesthetic blockade of the ganglion impar can be used to treat intractable perineal pain carried by visceral afferent fibres in patients with pelvic cancers ( ).
Somatic and vascular branches
Grey rami communicantes containing postganglionic sympathetic nerves pass from the pelvic sympathetic ganglia to the sacral and coccygeal spinal nerves. There are no white rami communicantes at this level. The postganglionic fibres are distributed via the sacral and coccygeal plexuses ( ). Thus, sympathetic fibres in the tibial nerve are conveyed to the popliteal artery and its branches in the leg and foot, whilst those in the pudendal and superior and inferior gluteal nerves accompany these arteries to the perineum and buttocks, respectively. Small branches also travel with the median and lateral sacral arteries.
Sacral splanchnic nerves
Sacral splanchnic nerves pass directly from the ganglia of the sacral part of the sympathetic trunk to the inferior hypogastric plexus and, from there, to pelvic viscera; they usually arise from the first two sacral ganglia.
Parasympathetic innervation
The parasympathetic neurones innervating the abdomen and pelvis lie either in the posterior nucleus of the vagus nerve or in the intermediolateral grey matter of the second, third and fourth sacral spinal cord segments. Traditionally, the vagus nerves have been said to supply parasympathetic innervation to the abdominal viscera as far as the distal transverse colon, i.e. they supply derivatives of the foregut and midgut. Derivatives of the hindgut are supplied by parasympathetic fibres travelling via the pelvic splanchnic nerves (see below); the overlap between these two supplies is variable. The vagal trunks are derived from the oesophageal plexus and enter the abdomen via the oesophageal hiatus, closely related to the anterior and posterior walls of the abdominal part of the oesophagus (see Fig. 56.6 ). The anterior vagal trunk is mostly derived from the left vagus nerve and the posterior vagal trunk from the right vagus nerve. The nerves supply the abdominal part of the oesophagus and stomach directly. The anterior vagal trunk gives off a hepatic branch, which innervates the liver and its vasculature, including the gallbladder and bile ducts, and the other structures in the free edge of the lesser omentum (hepatoduodenal ligament). The posterior vagal trunk supplies branches to the coeliac plexus; these fibres frequently constitute the largest portion of the fibres contributing to the plexus. They arise directly from the posterior vagal trunk and from its gastric branch, and run posterior to the peritoneum, deep to the posterior wall of the superior part of the omental bursa, to reach the coeliac plexus. Their synaptic relays with postganglionic neurones are situated in the myenteric (Auerbach’s) and submucosal (Meissner’s) plexuses in the wall of the hollow viscera (see below).
Pelvic splanchnic nerves
Pelvic splanchnic nerves to the pelvic viscera travel in the ventral rami of the second, third and fourth sacral spinal nerves. They leave the nerves as they exit the anterior sacral foramina and pass in the presacral tissue as a fine network of branches that are distributed to three principal destinations. Most pass anterolaterally into the network of nerves that form the inferior hypogastric plexus; from there, they pass to the pelvic viscera. Some join directly with the hypogastric nerves and ascend out of the pelvis as far as the superior hypogastric plexus; from there they are distributed with branches of the inferior mesenteric artery ( Ch. 65 ). A few run superolaterally in the presacral fascia, over the linea terminalis anterior to the left iliac vessels, and pass directly into the tissue of the retroperitoneum toward the sigmoid and descending colons. The pelvic splanchnic nerves are motor to the smooth muscle of the derivatives of the hindgut and wall of the urinary bladder, supply vasodilator fibres to the erectile tissues of the penis and clitoris, and are secretomotor to the derivatives of the hindgut.
An experimental study of the differential expression of transcription factors in embryonic mice argued that thoracic and sacral pools of preganglionic neurones share a common sympathetic identity and that the sacral autonomic outflow should therefore be regarded as sympathetic ( ). This controversial view has been challenged ( ).
Abdominopelvic autonomic plexuses and ganglia
The abdominopelvic autonomic plexuses are somewhat variable and often fuse or are closely interrelated. The following descriptions encompass their main features ( Figs 59.3 – 59.4 ).
Coeliac plexus
The coeliac plexus is located at the level of the twelfth thoracic and first lumbar vertebrae. It is a dense network that unites the coeliac ganglia, and surrounds the coeliac trunk and the proximal superior mesenteric artery. It is posterior to the stomach and omental bursa and anterior to the crura of the respiratory diaphragm and the beginning of the abdominal aorta, and lies medial to the suprarenal glands. The plexus and ganglia receive the greater and lesser thoracic splanchnic nerves and branches from the vagal trunks. The plexus is in continuity with small branches along adjacent arteries and is connected to the phrenic, splenic, hepatic, superior mesenteric, suprarenal, renal and ovarian/testicular plexuses. Visceral afferent fibres in the coeliac plexus convey pain and other sensations from proximal abdominal viscera. Anaesthesia or ablation of these nerves (coeliac plexus blockade) is sometimes undertaken to treat medically intractable pain from pancreatic disorders.
Coeliac and aorticorenal ganglia
The coeliac ganglia are irregular neural masses, measuring approximately 2 cm across, located between the origin of the coeliac trunk and superior mesenteric artery, medial to the suprarenal glands and anterior to the crura of the respiratory diaphragm ( ). They vary in number, size, shape and precise location; there are often two, one on each side. The right ganglion is frequently posterior to the inferior vena cava, and the left ganglion often lies posterior to the origin of the splenic artery. The ipsilateral greater thoracic splanchnic nerve joins the superior part of each ganglion and the lesser thoracic splanchnic nerve joins the inferior part. The most inferior part of each ganglion usually forms a distinct subdivision, the aorticorenal ganglion, which receives the ipsilateral lesser splanchnic nerve and gives origin to most of the renal plexus.
Phrenic plexus
The phrenic plexus is a periarterial extension of the coeliac plexus around the right inferior phrenic artery, adjacent to the right crus of the respiratory diaphragm ( ). It contains one or two phrenic ganglia and a definable nerve trunk that connects the coeliac plexus and phrenic nerve: the phrenic ganglion is probably sympathetic ( ). The phrenic plexus supplies branches to the suprarenal glands and the respiratory diaphragm.
Superior mesenteric plexus and ganglion
The superior mesenteric plexus lies in the pre-aortic connective tissue posterior to the pancreas, around the origin of the superior mesenteric artery. It is an inferior continuation of the coeliac plexus and includes branches from the posterior vagal trunk and coeliac plexus. Its branches accompany the superior mesenteric artery and its branches. The superior mesenteric ganglion lies superiorly in the plexus, usually above the origin of the superior mesenteric artery.
Intermesenteric plexus
Like other parts of the abdominal aortic autonomic plexus, the intermesenteric plexus is not a discrete structure but is part of a continuous periarterial nerve plexus connected to the ovarian/testicular, inferior mesenteric, iliac and superior hypogastric plexuses. It lies on the lateral and anterior aspects of the aorta, between the origins of the superior and inferior mesenteric arteries, and consists of numerous fine, interconnected nerve fibres and a few ganglia continuous superiorly with the superior mesenteric plexus and inferiorly with the superior hypogastric plexus. It is not well characterized but receives parasympathetic and sympathetic branches from the coeliac plexus and additional sympathetic contributions from the first and second lumbar splanchnic nerves.
Inferior mesenteric plexus
The inferior mesenteric plexus lies around the origin of the inferior mesenteric artery and is distributed along its branches. It is formed predominantly from the aortic plexus, supplemented by sympathetic fibres from the first and second lumbar splanchnic nerves and ascending pelvic parasympathetic fibres from the inferior hypogastric plexus (via the hypogastric nerves and superior hypogastric plexus). Disruption of the inferior mesenteric plexus alone rarely causes clinically significant disturbances of autonomic function.
Superior hypogastric plexus
The superior hypogastric plexus lies anterior to the aortic bifurcation, the left common iliac vein, median sacral vessels, fifth lumbar vertebral body and promontory of the sacrum, and between the common iliac arteries. It lies within extraperitoneal connective tissue in the midline, extending a little to the left. The breadth of the plexus and its constituent nerves varies; appearances range from a reticular-like arrangement to a band-like structure or one or two distinct nerve trunks ( ). The medial attachment of the sigmoid mesocolon and superior limit of the mesorectum lie anterior to the distal part of the plexus, separated from it by a thin layer of loose connective tissue. The plexus is formed by branches from three main sources: the abdominal aortic plexus (sympathetic and parasympathetic), lumbar splanchnic nerves (sympathetic) and pelvic splanchnic nerves (parasympathetic), which ascend from the inferior hypogastric plexus via the right and left hypogastric nerves ( ). Visceral sensory fibres also pass through the plexus. The hypogastric nerves lie in loose connective tissue just posterolateral to the proximal mesorectum and pass over the linea terminalis medial to the internal iliac vessels. The superior hypogastric plexus conveys branches to the inferior mesenteric plexus and to the ureteric, ovarian/testicular and common iliac nerve plexuses; additional small branches turn abruptly forwards into the proximal mesorectum to travel with the superior anorectal artery.
Inferior hypogastric plexus
The inferior hypogastric plexus lies in the thin extraperitoneal connective tissue on the pelvic side wall anterolateral to the mesorectum. The internal iliac vessels and the attachments of levator ani and obturator internus lie laterally, and the superior vesical and occluded part of the umbilical arteries superiorly. In males the inferior hypogastric plexus lies posterolaterally on either side of the seminal glands, prostate and the fundus of the urinary bladder ( ): the point where the ductus deferens crosses the ureter provides an approximate guide to the superior limit of the plexus ( ). The proximal border of the plexus lies beneath the peritoneum of the rectovesical pouch and is in contact with the lateral aspect of the fundus of the urinary bladder. The anterior border reaches the posterior aspect of the prostate; at its inferior limit, the cavernous nerves of the penis pass forwards to reach the posterolateral aspect of the prostate (see Fig. 73.8 ). In females, each plexus lies lateral to the cervix of the uterus, vaginal fornix and the fundus of the urinary bladder, often extending into the broad ligament of the uterus: the proximal limit of the plexus corresponds approximately to the level where the uterine artery crosses the ureter ( ).
The inferior hypogastric plexus is formed mainly from pelvic splanchnic (parasympathetic) and sacral splanchnic (sympathetic) branches; a smaller contribution is derived from sympathetic fibres (from the inferior lumbar ganglia) that descend into the plexus from the superior hypogastric plexus via the hypogastric nerves. It gives origin to a complex network of small pelvic branches that supply the pelvic viscera either directly or indirectly via periarterial plexuses. The branches of the inferior hypogastric plexus supply the ductus deferens, seminal glands, prostate, bulbo-urethral glands and penis in males; the ovary, uterine tubes, uterus, cervix of the uterus and vagina in females; and the urinary bladder and distal ureter in both sexes. The plexus plays a key role in continence and sexual function.
Hypogastric nerves
The hypogastric nerves are usually paired nerve bundles but can consist instead of multiple filaments. They contain sympathetic fibres (mostly descending from the superior hypogastric plexus) and parasympathetic fibres (ascending from the inferior hypogastric plexus). The nerves run between the superior and inferior hypogastric plexuses on each side posterior to the presacral fascia medial to the internal iliac vessels and lateral to the anterior sacral foramina.
Other autonomic plexuses and ganglia
Additional plexuses are described for abdominopelvic viscera, such as the hepatic and ovarian/testicular plexuses. Each plexus tends to lie around the main arterial supply to the named organ, receive both sympathetic and parasympathetic fibres from one or more of the major autonomic plexuses, and contain at least one ganglion. Thus, autonomic ganglia supplying the testis are found around the origin of the testicular arteries from the abdominal aorta and have connections with the renal and intermesenteric plexuses and the first and second lumbar splanchnic nerves ( ).
Para-aortic bodies
The para-aortic bodies (also known as paraganglia or, collectively, as the organ of Zuckerkandl) are collections of neural crest-derived chromaffin tissue found in close relation to the abdominal aortic plexus. They are relatively large in the fetus, reach a maximum size at around 3 years of age, and have usually regressed by adulthood. They are most commonly found as a pair of bodies lying anterolateral to the abdominal aorta in the region of the inferior mesenteric and superior hypogastric plexuses, but multiple smaller collections may be present. Occasionally they are found as high as the coeliac plexus or as low as the inferior hypogastric plexus in the pelvis, or can be close to the ganglia of the lumbar part of the sympathetic trunk. Rarely, scattered cells that persist into adulthood can be the sites of paraganglioma ( ).
General arrangement of abdominopelvic vascular supply
The major vessels that occupy the peritoneal cavity and pelvis not only supply the viscera, retroperitoneal structures and much of the bony and soft tissue walls, but also course through the cavities en route to supplying the lower limbs. From the caval foramen and aortic hiatus in the respiratory diaphragm, the inferior vena cava and abdominal aorta follow the general parabolic shape of the lumbar spine ( Figs 59.5 – 59.6 ). The individual parts of the aortoiliac and iliocaval systems are described in the relevant chapters. FLOAT NOT FOUND
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