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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.

The transpyloric plane is an imaginary horizontal line through the L1 vertebra, a line that is important when performing radiographic imaging studies. The pylorus of the stomach, the first part of the duodenum, the fundus of the gallbladder, the neck of the pancreas, the origin of the superior mesenteric artery, the hepatic portal vein, and the splenic vein are all located along the level of the transpyloric plane.

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.

McBurney’s point is the name given to a point on the lower right quadrant of the abdomen, approximately one-third the distance along an imaginary line from the anterior superior iliac spine to the umbilicus. McBurney’s point roughly corresponds to the skin overlying the most common attachment of the appendix to the cecum.

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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.

A patient diagnosed with cirrhosis (fibrotic scarring) of the liver may present with portal hypertension. Blood pressure within the portal vein increases because of the inability of blood to filter through the diseased (cirrhotic) liver. In an attempt to return blood to the heart, small collateral (paraumbilical veins) veins expand at and around the obliterated umbilical vein to bypass the hepatic portal system. These paraumbilical veins form tributaries with the veins of the anterior abdominal wall, forming a portal–caval anastomosis, and drain into the femoral or axillary veins. In patients with chronic cirrhosis, the paraumbilical veins on the anterior abdominal wall may swell and distend as they radiate from the umbilicus and are termed caput medusae because the veins appear similar to the head of the Medusa from Greek mythology.

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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.