At each side of the abdomen there are three flat muscles: the external oblique, the internal oblique, and the transversus abdominis. There is one vertical muscle, the rectus abdominis.
EXTERNAL OBLIQUE MUSCLE
The external oblique muscle arises from the lower eight ribs to insert at the outer lip of the anterior half of the iliac crest. The fibers of the muscle continue anteriorly as a thin but dense aponeurosis to decussate with those of the opposite side at the linea alba (Fig. 1). The linea alba represents the insertion of the muscles of the anterior abdominal wall at the midline. Below, it attaches with the pubic tubercle medially and the anterior superior iliac spine laterally, with the free lower border of the aponeurosis folding inward to become the inguinal ligament. Short of the pubic tubercle, the fibers form the triangular lacunar ligament, whereas those fibers that extend along the pectineal line form the pectineal ligament of Cooper. Fibers attaching to the pubic tubercle constitute the lateral crus of the superficial inguinal ring, whereas the fibers extending to the front of the pubis form its medial crus.
Fascial and muscular layers of the anterior abdominal wall and the inguinal and subinguinal regions.
INTERNAL OBLIQUE MUSCLE
The internal abdominal oblique arises from the anterior two thirds of the intermediate lip of the iliac crest and the lateral two thirds of the inguinal ligament. Most of the fibers are directed upward and forward at a right angle to those of the external oblique muscle and continue as the aponeurosis. Posterior fibers ascend vertically to insert in the lower four fibs, whereas the lowest fibers run horizontally and inferiorly.
The linea arcuata is a tendinous band in the posterior rectus sheath located at the approximate midpoint between the umbilicus and the pubic symphysis. Above the arcuate line, the aponeurosis of the internal oblique splits at the lateral border of the rectus muscle (linea semilunares) into anterior and posterior layers. These participate in the formation of the rectus sheath and decussate with the opposite muscle at the linea alba. Below the arcuate line, the aponeurosis does not split but joins totally the anterior lamina of the rectus sheath.
TRANSVERSUS ABDOMINUS MUSCLE
The transversus abdominus muscle arises from the thoracolumbar fascia, the inner surface of the lower six costal cartilages, the inner lip of the anterior two thirds of the iliac crest, and the lateral one third of the inguinal ligament. Its fibers course transversely forward and continue as the aponeurosis, joining the posterior lamina of the rectus sheath above the arcuate line and the anterior lamina below the arcuate line.
The fascia transversalis is thin and is part of the endoabdominal fascia and forms the posterior wall of the rectus sheath below the arcuate line. When fat is absent, the fascia transversalis adheres to the peritoneum as a single layer.
The thoracolumbar fascia consists of three layers arising from the vertebral spines, the tips of the transverse processes, and the anterior portion of the transverse processes, respectively. Layers join laterally to provide origin for the internal oblique and transversus muscles.
Fig. 1 . The rectus abdominis muscle. A, The anterior abdominal wall is made up on each side of three layers of flattened muscle and a pair of elongated vertical muscles, each known as rectus abdominis. The rectus muscle has two or three transverse whitish bands, the tendinous inscriptions, between the level of the umbilicus and the xiphoid. In cross-section A-A, just inferior to the umbilicus, the external oblique aponeurosis lies anterior to the rectus; the aponeurosis of the internal oblique muscle split passes both in front of and behind the rectus; and the aponeurosis of the transversus abdominus muscle passes posterior to the rectus. In cross-section B-B, all three of these aponeurotic layers lie anterior to the rectus; only the transversalis fascia and peritoneum lie posterior. B, A portion of the rectus muscle has been removed to reveal the arrangement of aponeuroses derived from the more lateral abdominal muscles. The gently curved aponeurotic edge, the arcuate line, represents the point at which all the aponeurotic layers first He anterior to the rectus. Viewed from the interior, the arcuate line marks the point at which the inferior epigastric vessels enter and lie in the transversalis fascia posterior to the rectus. The small pyramidalis muscle lies anterior to the lower end of the rectus abdominis muscle.
The rectus sheath contains the rectus abdominus muscle, which arises by a tendon from the pubic bone medial to the tubercle. The muscle inserts into the fifth and seventh costal cartilages and the xiphoid process. The pyramidalis muscle is present in front of the lower part of the rectus muscle, rising from the pubis below the origin of the rectus, and inserts into the linea alba. In addition to these two muscular structures, the rectus sheath contains the anterior cutaneous branches of the seventh through twelfth thoracic nerves and the superior and inferior epigastric vessels, which lie posterior to the muscle bed. The superior epigastric artery is a terminal branch of the internal thoracic, whereas the inferior is a branch of the external iliac artery. As described previously, the rectus sheath is formed by the aponeuroses of the external oblique, internal oblique, and transversus muscles. Fibers of the right and left sheaths decussate at the midline to form the linea alba. The anterior lamina of the sheath (see Fig. 2-3) is composed of all the aponeuroses below the level of the arcuate line but only of the external oblique and anterior layer of the internal oblique above the arcuate line. The posterior lamina above the arcuate line is formed by the aponeurosis of the transversus muscle and the posterior lamina of the internal oblique aponeurosis.
Group actions of the abdominal wall muscles include constriction of the cavity, assisting in the noble function of parturition and the more servile functions of defecation, urination, and emesis. They also assist in flexion of the thorax or pelvis as well as in rotation of the trunk. They can also assist in expiration.