Chapter 7. Anterior Abdominal Wall

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The abdomen typically is described topographically using two methods. The first method partitions the abdomen into four quadrants. The second method partitions the abdomen into nine regions.

Quadrant Partitions

The most direct method of partitioning the abdomen is through an imaginary transverse (transumbilical) plane that intersects with a sagittal midline plane through the umbilicus between the L3 and L4 vertebral levels (Figure 7-1A). The two intersecting planes divide the abdomen into four quadrants, described as right and left upper and lower quadrants. The four-quadrant system is straightforward when used to describe anatomic location. For example, the appendix is located in the lower right quadrant of the abdomen.

Regional Partitions

For a more precise description, the abdomen is partitioned into nine regions created by two imaginary vertical planes and two imaginary horizontal planes (Figure 7-1B).

• Vertical planes. Paired vertical planes correspond to the midclavicular lines, which descend to the midinguinal point.
• Subcostal (upper horizontal) plane. Transversely courses inferior to the costal margin, through the level of the L3 vertebra. The L3 vertebra serves as an important anatomic landmark in that it indicates the level of the inferior extent of the third part of the duodenum and the origin of the inferior mesenteric artery.
• Transtubercular (lower horizontal) plane. Transversely courses between the two tubercles of the iliac crest, through the level of the L5 vertebra.

Surface Landmarks

The following structures are helpful anatomic surface landmarks on the anterior abdominal wall (Figure 7-1C):

• Xiphoid process. The xiphoid process is the inferior projection of the sternum. It marks the dermatome level of T7.
• Umbilicus. The umbilicus lies at the vertebral level between the L3 and L4 vertebrae. However, the skin around the umbilicus is supplied by the thoracic spinal nerve T10 (T10 dermatome) A helpful mnemonic is “T10 for belly but-ten”.
• Inguinal ligament. The inferior border of the external oblique muscle and aponeurosis has an attachment between the anterior superior iliac spine and the pubic tubercle. This fascial attachment is known as the inguinal ligament and is evident superficially as a crease on the inferior extent of the anterior abdominal wall.

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Multiple layers of fascia and muscle form the anterior abdominal wall (Figure 7-1D). The layers, from superficial to deep, are skin, two layers of superficial fascia, three layers of muscles and their aponeuroses, transversalis fascia, extraperitoneal fat, and the parietal peritoneum.

Skin

The skin receives its vascular supply via the intercostal and lumbar vessels and its segmental innervation via the ventral rami of the intercostal and lumbar spinal nerves.

Superficial Fascia

The superficial fascia of the anterior abdominal wall consists of two layers: an external layer of adipose tissue (Camper's fascia) and an internal layer of dense collagenous connective tissue (Scarpa's fascia). Camper's fascia is absent in the perineum. In contrast, Scarpa's fascia continues into the perineum, but the nomenclature is changed relative to the region in which it is located. For example, Scarpa's fascia becomes Colles' fascia when surrounding the roots of the penis and clitoris; it becomes superficial penile (or clitoral) fascia when it surrounds the shaft of the penis (or clitoris); and it becomes dartos fascia in the scrotum.

Embedded in the adipose tissue of Camper's fascia are the superficial epigastric veins, which drain the anterior abdominal wall. These cutaneous veins drain into the femoral and paraumbilical veins.

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Five paired anterior abdominal wall muscles are deep to the superficial fascia. The external oblique, internal oblique, and transverse abdominis muscles, with their associated aponeuroses, course anterolaterally, whereas the rectus abdominis and tiny pyramidalis muscles course vertically in the anterior midline. Collectively, these muscles compress the abdominal contents, protect vital organs, and flex and rotate the vertebral column. Each muscle receives segmental motor innervation from the lumber spinal nerves.

External Oblique Muscle

The external oblique muscle is the most superficial of the anterolateral muscles and attaches to the outer surfaces of the lower ribs and iliac crest (Figure 7-2A). The external oblique muscle continues anteriorly as the external oblique aponeurosis, which courses anteriorly to the rectus abdominis muscle and inserts into the linea alba. The inferior border of the external oblique aponeurosis, between the anterior superior iliac spine and the pubic tubercle, is called the inguinal ligament.

Internal Oblique Muscle

The internal oblique muscle is the intermediate muscle of the anterolateral muscles (Figure 7-2A). The muscle attaches to the thoracolumbar fascia, iliac crest, inguinal ligament, and lower ribs. The internal oblique muscle continues anteriorly as the internal oblique aponeurosis, which splits around the rectus abdominis muscle to insert into the linea alba, with some inferior attachments to the pubic crest and pectineal line.

Transverse Abdominis Muscle

The transverse abdominis muscle is the deepest of the anterolateral muscles (Figure 7-2A). This muscle attaches to the thoracolumbar fascia, iliac crest, inguinal ligament, and the costal cartilages of the lower ribs. The transverse abdominis muscle continues anteriorly as the transverse abdominis aponeurosis, which courses deep to the rectus abdominis muscle and inserts into the linea alba, the pubic crest, and the pectineal line.

• Intercostal and lumbar nerves, arteries, and veins course along the anterolateral abdominal wall between the internal oblique and transverse abdominis muscles.

Rectus Abdominis Muscle

The rectus abdominis muscle is the anteriorly positioned vertical strap muscle (Figure 7-2A). The rectus abdominis muscle attaches inferiorly to the pubic bone and symphysis and superiorly to the xiphoid process and lower costal cartilages. The external oblique, internal oblique, and transverse abdominis aponeuroses envelope the rectus abdominis muscle in a fascial sleeve known as the rectus sheath. The linea alba is a vertical mid line of fascia that separates the paired rectus abdominis muscles and is formed by the fusion of the three pairs of anterolateral aponeuroses (Figure 7-2B and C). The rectus sheath completely encloses the superior three-fourths of the rectus abdominis muscle but only covers the anterior surface of the inferior one-fourth of the muscle. This demarcation region in the rectus sheath is known as the arcuate line. The arcuate line is located midway between the umbilicus and pubic bone and serves also as the site where the inferior epigastric vessels enter to the rectus sheath. Inferior to the arcuate line, the rectus abdominis muscle is in direct contact with the transversalis fascia because the rectus sheath only covers the anterior surface of the rectus abdominis muscle (Figure 7-2B).

Pyramidalis Muscle

The pyramidalis muscle is a small, pyramidally shaped muscle anterior to the rectus abdominis muscle (Figure 7-2A). The pyramidalis muscle attaches to the pubic bone and linea alba, and its only action is to tense the linea alba.

Deep Fascial Layers

In addition to the superficial layers of the abdominal fascia, there are three additional layers of abdominal fascia deep to the anterolateral muscle layers (Figure 7-2B,C).

• Transversalis fascia. A thin, aponeurotic membrane, deep to the transverse abdominis muscle.
• Extraperitoneal fat. A thin layer of connective tissue and fat lining the abdominal wall between the transversalis fascia and the parietal peritoneum. The extraperitoneal fat is more abundant in the posterior abdominal wall, especially around the kidneys and in the pelvic floor.
• Parietal peritoneum. Parietal peritoneum is a serous membrane lining the internal surface of the abdominal wall. The parietal peritoneum forms the mesentery that suspends the abdominal viscera and is continuous with the visceral peritoneum. The parietal peritoneum is innervated segmentally by the ventral rami of the spinal (somatic) nerves that course in the abdominal body wall.

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The neurovascular supply to the body wall courses between the second and third muscle layers. Intercostal, subcostal, lumbar, and epigastric arteries supply the skin of the anterolateral abdominal wall (Figure 7-3).

Intercostal and Lumbar Arteries

The inferior posterior intercostal arteries and the lumbar arteries from the descending aorta supply the lateral part of the anterior abdominal wall. These vessels course between the internal oblique and the transverse abdominis muscles and may anastomose with the inferior and superior epigastric arteries.

Superior Epigastric Artery

The superior epigastric artery arises from the internal thoracic artery, enters the rectus sheath, and descends on the deep surface of the rectus abdominis muscle. The superior epigastric artery anastomoses with the inferior epigastric artery within the rectus abdominis muscle.

Inferior Epigastric Artery

The inferior epigastric artery arises from the external iliac artery above the inguinal ligament, enters the rectus sheath at the arcuate line, and ascends between the rectus abdominis muscle and the posterior layer of the rectus sheath. The inferior epigastric artery anastomoses with the superior epigastric artery, providing collateral circulation between the subclavian and external iliac arteries.

Innervation of the Anterior Abdominal Wall

The nerves of the anterior abdominal wall are the ventral rami of the T6–L1 spinal nerves. These nerves course downward and anteriorly between the internal oblique and the transverse abdominis muscles. They segmentally supply cutaneous innervation to the skin and parietal peritoneum and are the motor supply to the anterolateral abdominal wall muscles. The lower intercostal nerves and the subcostal nerve pierce the deep layer of the rectus sheath and course through to the skin to become the anterior cutaneous nerves of the abdomen. The first lumbar nerve bifurcates into the iliohypogastric and ilioinguinal nerves, which do not enter the rectus sheath. Instead, the iliohypogastric nerve pierces the external oblique aponeurosis superior to the superficial inguinal ring, whereas the ilioinguinal nerve passes through the inguinal canal to emerge through the superficial inguinal ring.

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The inguinal canal is an oblique passage through the inferior region of the anterior abdominal wall. The inguinal canal is clinically more important in males because it is the passageway for structures to course to and from the testis to the abdomen. The presence of the spermatic cord in the male predisposes males to indirect inguinal harnias. In females, the inguinal canal is less important in that the only structure that traverses through the inguinal canal is the round ligament of the uterus. The thinness of the round ligament of the uterus, compared to the spermatic cord in males, is associated with lower frequency of indirect inguinal hernia in females. The inguinal canal is approximately 5-cm long and extends from the deep inguinal ring downward and medially to the superficial inguinal ring. The inguinal canal lies parallel to and immediately superior to the inguinal ligament. In males, the spermatic cord and scrotum consist of the same layers of muscle and fascia as does the anterior abdominal wall.

Inguinal Canal Structure

The inguinal canal is much like a rectangular tube in that it consists of four walls with openings at both ends, described as follows (Figure 7-4):

• Anterior wall. Formed by the external oblique aponeurosis.
• Posterior wall. Formed by the conjoint tendon of the internal oblique and the transverse abdominis muscles and the transversalis fascia.
• Roof. Formed by arching fibers of the internal oblique and the transverse abdominis muscles.
• Floor. The medial half of the inguinal ligament forms the inferior wall of the inguinal canal. This rolled-under, free margin of the external oblique aponeurosis forms a gutter or trough on which the contents of the inguinal canal are positioned. The lacunar ligament reinforces most of the medial part of the floor.
• Deep inguinal ring. Formed by an opening in the transversalis fascia. The deep inguinal ring is located superior to the inguinal ligament, lateral to the inferior epigastric vessels, and halfway between the pubic bone and the anterior superior iliac spine.
• Superficial inguinal ring. Formed by an opening in the external oblique aponeurosis superior and medial to the pubic tubercle.

The contents of the inguinal canal include the genital branch of the genitofemoral nerve (L1–L2), the spermatic cord in males, and the round ligament of the uterus in females. Additionally, in both males and females, the ilioinguinal nerve (L1) passes through part of the canal. The ilioinguinal nerve courses between the internal oblique and the transverse abdominis muscles and enters in the middle of the inguinal canal in both males and females. The ilioinguinal nerve exits the inguinal canal through the superficial inguinal ring with other contents that course through the inguinal canal.

Hernias are classified as direct or indirect, with the inferior epigastric vessels serving as the differentiating landmark.

• Direct hernia. Results when the small intestine protrudes into the canal medial to the inferior epigastric vessels.
• Indirect hernia. Results when the small intestine protrudes into the canal lateral to the inferior epigastric vessels into the inguinal canal.
  

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The primary sex organs of the males are the testes because they produce sperm. The testes are housed in a layered sac of muscle and fascia called the scrotum, which is connected to the anterior abdominal wall via the spermatic cord. Unlike the spinal cord, which is protected by the vertebral column, bones do not protect the testes. A temperature of 34°C is required for the testes to produce sperm, a temperature that is 3°C lower than core body temperature; therefore, the testes must be housed outside the body. During embryonic development, the testes begin development in the region of the kidneys and descend throughout development until they traverse the inguinal canal, protruding through the inferior portion of the anterior abdominal wall. This developmental migration of the testes through the anterior abdominal wall is the basis of formation of the spermatic cord and scrotum from the muscle and fascial layers of the anterior abdominal wall. This developmental migration also is the basis of congenital (indirect) inguinal hernia in newborn male infants.

Scrotum

The layers of the scrotal sac are as follows (it may be helpful to review the muscle and fascial layers of the anterior abdominal wall) (Figure 7-4):

• Skin.
• Dartos fascia. A continuation of Scarpa's fascia. Consists of a thin layer of loose connective tissue with some smooth muscle (dartos muscle). The smooth muscle contracts and thus wrinkles the skin of the scrotum, enhancing the radiation of heat when the temperature of the testes increases excessively (e.g., a hot bath).
• External spermatic fascia. Extension of the external oblique aponeurosis.
• Cremasteric muscle. Extension of the internal oblique muscle. If the temperature of the testes drops, contraction of the cremasteric muscle moves the testes closer to the body wall, thus helping to maintain appropriate temperature.

• Internal spermatic fascia. Extension of the transversalis fascia (the transverse abdominis aponeurosis has no contributions to the scrotal sac).
• Tunica vaginalis. Extensions of the parietal peritoneum. However, the tunica vaginalis is only found surrounding the testes and does not have a counterpart in the spermatic cord.

Spermatic Cord

Similar to the scrotal sac, the spermatic cord contains the same muscular and fascial layers as the anterior abdominal wall because the testis descends through the abdominal wall to the scrotum during embryonic development. The spermatic cord contains the following structures:

• Ductus deferens. During ejaculation, functions to transport sperm from the testes, through the spermatic cord and inguinal canal to the ejaculatory duct in the prostate. Much of the ductus deferens is composed of smooth muscle and, as a result, feels rigid and hard to the touch. Therefore, the ductus deferens is easy to palpate in the spermatic cord.

• Testicular (gonadal) artery. The paired gonadal arteries are bilaterally symmetrical. Each gonadal artery originates on the abdominal aorta, inferior to the renal arteries. Each artery traverses the deep inguinal ring and courses through the inguinal canal to provide the vascular supply to the testes and ductus deferens on the corresponding side of the body.
• Testicular (gonadal) vein. The paired gonadal veins are not bilaterally symmetrical. Each vein drains blood from the testis and courses from the scrotal sac, through the spermatic cord, and traverses the superficial inguinal ring through the inguinal canal before entering the abdominal cavity. The right gonadal vein drains into the inferior vena cava, whereas the left gonadal vein drains into the left renal vein.
• Autonomic neurons. Sympathetic and visceral sensory neurons.