Muscles

The anterolateral abdominal wall houses three broad thin sheets of muscle that are most pronounced laterally and that become aponeurotic anteriorly. From exterior (superficial) to interior (deep), they are the external oblique, internal oblique, and transversus abdominis (Fig. 6.9). These are the abdominal equivalents of the intercostal muscles. Additionally, the paired rectus abdominis muscles form wide vertical straps on either side of the anterior midline (Fig. 6.10). As the aponeuroses of the three sheets pass forward, they enclose the rectus abdominis to form the rectus sheath. The lower part of the rectus sheath might contain a small muscle called the pyramidalis. Details of the attachments, innervation, and actions of the anterolateral wall muscles are summarized in Table 6.1.

Figure 6.10 Anterior view of the rectus abdominis muscle and the rectus sheath. Left. The anterior wall of the sheath has been partly removed, revealing the rectus muscle with its tendinous intersections. Right. The posterior wall of the rectus sheath is shown. The edge of the arcuate line is shown at the level of the anterior superior iliac spine.

External Oblique

The external oblique muscle is a broad, thin, muscular sheet, with most of its fibers inserting by means of a wide aponeurosis (see Figs. 6.9 and 6.10). Note that the most posterior fibers passing down to the iliac crest form a posterior free border.

A triangular-shaped defect in the external oblique aponeurosis lies immediately above and medial to the pubic tubercle. This is the superficial inguinal ring. The spermatic cord (or round ligament of the uterus) passes through this opening and carries the external spermatic fascia (or the external covering of the round ligament of the uterus) from the margins of the ring (Figs. 6.11 and 6.12).

Figure 6.11 A. Continuity of the different layers of the anterior abdominal wall with coverings of the spermatic cord. B. The skin and superficial fascia of the abdominal wall and scrotum have been included, and the tunica vaginalis is shown.

Figure 6.12 Scrotum dissected from in front. Note the spermatic cord and its coverings.

The lower border of the aponeurosis is folded backward on itself between the anterior superior iliac spine and the pubic tubercle, forming the inguinal ligament (Figs. 6.13 and 6.14; also see Fig. 6.9). The lacunar ligament extends backward and upward from the medial end of the inguinal ligament to the pectineal line on the superior ramus of the pubis (see Figs. 6.13 and 6.14). Its sharp, free crescentic edge forms the medial margin of the femoral ring (see Chapter 11). On reaching the pectineal line, the lacunar ligament becomes continuous with a thickening of the periosteum called the pectineal ligament.

The lateral part of the posterior edge of the inguinal ligament gives origin to part of the internal oblique and transversus abdominis muscles (see Figs. 6.9, 6.10, and 6.14). The deep fascia of the thigh, the fascia lata, is attached to the inferior rounded border of the inguinal ligament (see Fig. 6.8A).

Internal Oblique

The internal oblique muscle is also a broad, thin, muscular sheet that lies deep to the external oblique. Most of its fibers run at right angles to those of the external oblique and radiate as they pass upward and forward (see Fig. 6.9). The internal oblique has a lower free border that arches over the spermatic cord (or round ligament of the uterus) and then descends behind it to attach to the pubic crest and the pectineal line (Fig. 6.15; also see Fig. 6.14). Near their insertion, the lowest tendinous fibers are joined by similar fibers from the transversus abdominis to form the conjoint tendon (see Figs. 6.14 and 6.15C). The conjoint tendon attaches medially to the linea alba, but it has a lateral free border.

Figure 6.15 Inguinal canal showing the arrangement of the external oblique muscle (A), the internal oblique muscle (B), the transversus abdominis muscle (C), and the transversalis fascia (D). Note that the external oblique and the internal oblique form the anterior wall of the canal and the transversalis fascia and the conjoint tendon form the posterior wall. The deep inguinal ring lies lateral to the inferior epigastric artery.

As the spermatic cord passes under the lower border of the internal oblique, it carries some muscle fibers that are derived from the internal oblique. These fibers that surround the spermatic cord constitute the cremaster muscle (see Figs. 6.14 and 6.15B). The cremasteric fascia is the cremaster muscle plus its fascia. The cremaster exerts tension on the spermatic cord and acts to raise or lower the testis.

Transversus Abdominis

The transversus abdominis is a thin sheet of muscle that lies deep to the internal oblique. Its fibers run horizontally forward (see Fig. 6.9). The lowest tendinous fibers join similar fibers from the internal oblique to form the conjoint tendon, which is fixed to the pubic crest and the pectineal line (see Figs. 6.14 and 6.15C). Note that the posterior border of the external oblique muscle is free, whereas the posterior borders of the internal oblique and the transversus muscles are attached to the lumbar vertebrae by the lumbar fascia (see Fig. 6.9).

Rectus Abdominis

The rectus abdominis is a long strap muscle that extends along the whole length of the anterior abdominal wall (Fig. 6.16; also see Fig. 6.10). It is broader above and lies close to the midline, being separated from its fellow by the linea alba.

When the rectus contracts, its lateral margin forms a curved ridge, the linea semilunaris, that can be palpated and often seen. This extends from the tip of the ninth costal cartilage to the pubic tubercle.

The rectus abdominis muscle is divided into distinct segments by three transverse tendinous intersections: one at the level of the xiphoid process, one at the level of the umbilicus, and one halfway between these two. These intersections are strongly attached to the anterior wall of the rectus sheath (see below). When the rectus is well developed, the tendinous intersections produce the classic “six-pack abs” appearance (see Fig. 6.16).

The rectus abdominis is enclosed between the aponeuroses of the external oblique, internal oblique, and transversus abdominis, which form the rectus sheath.

Pyramidalis

The pyramidalis is a small muscle that lies in front of the lower part of the rectus abdominis (see Fig. 6.10). However, it is often absent.

Rectus Sheath

The rectus sheath is a long fibrous envelope that encloses the rectus abdominis and pyramidalis (if present) muscles. Thus, the sheath has two walls, anterior and posterior. It also contains the anterior rami of the lower six thoracic nerves and the superior and inferior epigastric vessels and lymph vessels. It is formed mainly by the aponeuroses of the three lateral abdominal muscles (Fig. 6.17; also see Fig. 6.10). The composition of the walls of the rectus sheath changes at different levels. For ease of description, the rectus sheath is generally considered at three levels (Fig. 6.18).

Above the costal margin, the anterior wall is formed by the aponeurosis of the external oblique. The posterior wall is formed by the thoracic wall (i.e., the fifth, sixth, and seventh costal cartilages and the intercostal spaces).

Between the costal margin and the arcuate line (at about the level of the anterior superior iliac spine), the aponeurosis of the internal oblique splits to enclose the rectus muscle. The external oblique aponeurosis is directed in front of the muscle, and the transversus aponeurosis is directed behind the muscle.

Between the level of the arcuate line (at about the anterior superior iliac spine) and the pubis, the aponeuroses of all three muscles form the anterior wall. The posterior wall is absent, and the rectus muscle lies in contact with the transversalis fascia.

Note that where the aponeuroses forming the posterior wall pass in front of the rectus at the level of the anterior superior iliac spine, the posterior wall has a free, curved lower border called the arcuate line (see Figs. 6.10 and 6.17). At this site, the inferior epigastric vessels enter the rectus sheath and pass upward to anastomose with the superior epigastric vessels.

The rectus sheath is separated from its fellow on the opposite side by a fibrous band called the linea alba. This extends from the xiphoid process down to the symphysis pubis and is formed by the fusion of the aponeuroses of the lateral muscles of the two sides. It is wider above the umbilicus and narrows down below the umbilicus to be attached to the symphysis pubis.

The posterior wall of the rectus sheath is not attached to the rectus abdominis muscle. However, the anterior wall is firmly attached to it by the muscle’s tendinous intersections.

Rectus Sheath Hematoma

Hematoma of the rectus sheath is uncommon but important, because it is often overlooked. It occurs most often below the level of the umbilicus. The source of the bleeding is the inferior epigastric vein or, more rarely, the inferior epigastric artery. These vessels may be stretched during a severe bout of coughing or in the later months of pregnancy, which may predispose to the condition. The cause is usually blunt trauma to the abdominal wall, such as a fall or a kick. The symptoms that follow the trauma include midline abdominal pain. An acutely tender mass confined to one rectus sheath is diagnostic.

Function

The oblique muscles laterally flex and rotate the trunk (Fig. 6.19). The rectus abdominis flexes the trunk and stabilizes the pelvis, and the pyramidalis keeps the linea alba taut during the process.

The muscles of the anterior and lateral abdominal walls assist the diaphragm during inspiration by relaxing as the diaphragm descends so that the abdominal viscera can be accommodated.

The muscles assist in the act of forced expiration that occurs during coughing and sneezing by pulling down the ribs and sternum. Their tone plays an important part in supporting and protecting the abdominal viscera. By contracting simultaneously with the diaphragm, with the glottis of the larynx closed, they increase the intra-abdominal pressure and help in micturition, defecation, vomiting, and parturition.

Nerve Supply

The lower six thoracic nerves and the iliohypogastric and ilioinguinal nerves (L1) supply the oblique and transversus abdominis muscles (Fig. 6.20; also see Fig. 6.26). The lower six thoracic nerves supply the rectus abdominis. The 12th thoracic nerve supplies the pyramidalis.

Abdominal Muscles, Abdominothoracic Rhythm, and Visceroptosis

The abdominal muscles contract and relax with respiration, and the abdominal wall conforms to the volume of the abdominal viscera. Normally, during inspiration, when the sternum moves forward and the chest expands, the anterior abdominal wall also moves forward—the abdominothoracic rhythm. If the anterior abdominal wall remains stationary or contracts inward when the chest expands, the parietal peritoneum is probably inflamed and has caused a reflex contraction of the abdominal muscles.

The shape of the anterolateral abdominal wall depends on the tone of its muscles. A middle-aged woman with poor abdominal muscles who has had multiple pregnancies is often incapable of supporting her abdominal viscera. The lower part of the anterior abdominal wall protrudes forward in a condition known as visceroptosis. This should not be confused with an abdominal tumor such as an ovarian cyst or with the excessive accumulation of fat in the fatty layer of the superficial fascia.