Abdominal Wall Anatomy

Anterolateral Abdominal Wall

Before describing the abdominal parietes (wall), it is important to define the anatomic limits of the abdomen. For the purpose of a specific description, the abdomen is the area of the body that limits with the diaphragm superiorly and the pelvic cavity inferiorly. The parietes of the abdomen can be divided into four general parts: anterolateral abdominal wall; posterior abdominal wall; diaphragm (roof of the abdominal cavity); and parietes of the pelvis (floor of the abdominal cavity).

The anterolateral abdominal wall fills in the bony cartilaginous frame between the costal margin above and the hipbones below ( Fig. 44.1 ). This wall can contract and relax, which helps accommodate the size of the abdominopelvic cavity to changes in the volume of contained viscera, regulating the intraabdominal pressure. Surgical approach to the abdominopelvic cavity is usually made through this wall.

General layers of the anterolateral abdominal wall from the outside in are skin, subcutaneous tissue, superficial fascia, muscles with their related fascia, transversalis fascia, extraperitoneal fascia, and parietal peritoneum.

The abdominal skin is of an average thickness, usually thicker dorsally, and loosely attached to the underlying layers except in the umbilical area. The superficial fascia (tela subcutanea) is soft and mobile and contains a variable amount and distribution of fat, depending on the patient's nutritional status. At the area inferior to the level of the umbilicus, the superficial fascia is classically described as having a superficial fatty layer (Camper fascia) and a deep membranous layer (Scarpa fascia). Camper fascia is continuous with the fatty layer of surrounding areas, as seen in the fatty layer of the thigh. Scarpa fascia merges with the fascia lata in a parallel line to and below the inguinal ligament and is adherent to the linea alba in the midline. Medial to the pubic tubercle, both layers continue into the urogenital region. In males, the two layers merge into the scrotum and blend into a single smooth muscle-containing layer; at this point, the layers begin to form the scrotum. Cephalad to the symphysis pubis, additional closely set strong bands of Scarpa fascia form the fundiform ligament of the penis, extending down into the dorsum and the sides of the penis.

The outer investing layer of the deep fascia is not readily distinguishable from the muscular fascia on the external surface of the external abdominal oblique muscle and its aponeurosis. It is easily demonstrated over the fleshy portion of the muscle; however, separating this from the aponeurotic portion of the muscle can be difficult. This layer is attached to the inguinal ligament and merges with the fascia emerging from the ligament to form the fascia lata. In addition, it joins the fascia on the inner surface of the external oblique at the subcutaneous inguinal ring to form the external spermatic fascia. External to the lower end of the linea alba, the outer investing layer thickens into the suspensory ligament of the penis, anchoring the penis to the symphysis and the arcuate ligament of the pubis. It is also continuous with the deep investing fascia.

The nerve supply of the external abdominal oblique muscle is derived from the anterior primary division of the 6th to 12th thoracic (T6–T12) spinal nerves. T7 to T11 are intercostal nerves that continue from the intercostal spaces into the anterolateral abdominal wall, to lie in the plane between the internal oblique and transversus muscles. T12 is the subcostal nerve and follows a similar course. The iliohypogastric nerve, from T12 and first lumbar (L1), also contributes to the nerve supply.

The cremaster muscle, which is variably developed only in males, represents an extension of the lower border of the internal oblique, and possibly the transversus abdominus, over the testis and the spermatic cord. Laterally, the cremaster is thicker and fleshier and attaches to the middle of the turned-under edge of the external oblique aponeurosis and to the inferior edge of the internal oblique. From here, the scattered fibers, interspersed with connective tissue (cremasteric fascia), spread in loops over the spermatic cord and testis to end at the pubic tubercle and the anterior layer of the rectus sheath. The cremaster's nerve supply is from the genital (external spermatic) branch of the genitofemoral nerve and, generally, a branch from the ileo-inguinal nerve. The action of the cremaster muscle is to lift the testis toward the external inguinal ring.

The rectus abdominis muscle generally acts in conjunction with the external oblique, internal oblique, and transversus abdominis muscles, but it is specifically involved in producing forced expiration and flexion of the vertebral column. With a fixed pelvis and thoracic cavity, the rectus abdominus serves to constrict the contents of the abdominal cavity and increase intraabdominal pressure to aid in things such as defecation and vomiting.