Surface Topography

Clinically, the abdominal wall is divided descriptively into quadrants or regions so that both the underlying visceral structures and the pain or pathology associated with these structures can be localized and topographically described. Common clinical descriptions use either quadrants or the nine descriptive regions, demarcated by two vertical midclavicular lines and two horizontal lines: the subcostal and intertubercular planes (Fig. 4-2 and Table 4-1).

Muscles

The muscles of the anterolateral abdominal wall include three flat layers that are continuations of the three layers in the thoracic wall (Fig. 4-3). These include two abdominal oblique muscles and the transversus abdominis muscle (Table 4-2). In the midregion a vertically oriented pair of rectus abdominis muscles lies within the rectus sheath and extends from the pubic symphysis and crest to the xiphoid process and costal cartilages 5 to 7 superiorly. The small pyramidalis muscle (Fig. 4-3, B) is inconsistent and clinically insignificant.

Rectus Sheath

The rectus sheath encloses the vertically running rectus abdominis muscle (and inconsistent pyramidalis), the superior and inferior epigastric vessels, the lymphatics, and the ventral rami of T7-L1 nerves, which enter the sheath along its lateral margins (Fig. 4-3, C). The superior three quarters of the rectus abdominis is completely enveloped within the rectus sheath, and the inferior one quarter is supported posteriorly only by the transversalis fascia, extraperitoneal fat, and peritoneum; the site of this transition is called the arcuate line (Fig. 4-4 and Table 4-3).

Innervation and Blood Supply

The segmental innervation of the anterolateral abdominal skin and muscles is by ventral rami of T7-L1. The blood supply includes the following arteries (Figs. 4-3, C, and 4-5):

Superficial and deeper veins accompany these arteries, but, as elsewhere in the body, they form extensive anastomoses with each other to facilitate venous return to the heart (Fig. 4-6 and Table 4-4).

Lymphatic drainage of the abdominal wall parallels the venous drainage, with the lymph ultimately coursing to the following lymph node collections:

Clinical Focus 4-1   Abdominal Wall Hernias

Abdominal wall hernias often are called ventral hernias to distinguish them from inguinal hernias. However, all are technically abdominal wall hernias. Other than inguinal hernias, which are discussed separately, the most common types of abdominal hernias include:

• Umbilical hernia: usually seen up to age 3 years and after 40.

• Linea alba hernia: often seen in the epigastric region and more common in males; rarely contains visceral structures (e.g., bowel).

• Linea semilunaris (spigelian) hernia: usually occurs in midlife and develops slowly.

• Incisional hernia: occurs at the site of a previous laparotomy scar.

Inguinal Canal

The gonads in both genders initially develop retroperitoneally from a mass of intermediate mesoderm called the urogenital ridge. As the gonads begin to descend toward the pelvis, a peritoneal pouch called the processus vaginalis extends through the various layers of the anterior abdominal wall and acquires a covering from each layer, except for the transversus abdominis muscle because the pouch passes beneath this muscle layer. The processus vaginalis and its coverings form the fetal inguinal canal, a tunnel or passageway through the anterior abdominal wall. In females the ovaries are attached to the gubernaculum, the other end of which terminates in the labioscrotal swellings (which will form the labia majora in females or the scrotum in males). The ovaries descend into the pelvis, where they remain, tethered between the lateral pelvic wall and the uterus medially (by the ovarian ligament, a derivative of the gubernaculum). The gubernaculum then reflects off the uterus as the round ligament of the uterus, passes through the inguinal canal, and ends as a fibrofatty mass in the future labia majora.

In males the testes descend into the pelvis but then continue their descent through the inguinal canal (formed by the processus vaginalis) and into the scrotum, which is the male homologue of the female labia majora (Fig. 4-8). This descent through the inguinal canal occurs around the 26th week of development, usually over several days. The gubernaculum terminates in the scrotum and anchors the testis to the floor of the scrotum. A small pouch of the processus vaginalis called the tunica vaginalis persists and partially envelops the testis. In both genders the processus vaginalis then normally seals itself and is obliterated. Sometimes this fusion does not occur or is incomplete, especially in males, probably caused by descent of the testes through the inguinal canal. Consequently, a weakness may persist in the abdominal wall that can lead to inguinal hernias.

As the testes descend, they bring their accompanying spermatic cord along with them and, as these structures pass through the inguinal canal, they too become ensheathed within the layers of the anterior abdominal wall (Fig. 4-9). The spermatic cord enters the inguinal canal at the deep inguinal ring (an outpouching in the transversalis fascia lateral to the inferior epigastric vessels) and exits the 4-cm-long canal via the superficial inguinal ring (superior to the pubic tubercle) before passing into the scrotum, where it suspends the testis. In females the only structure in the inguinal canal is the fibrofatty remnant of the round ligament of the uterus, which terminates in the labia majora. The contents in the spermatic cord include the following (Fig. 4-9):

Layers of the spermatic cord include the following (see Fig. 4-9):

The features of the inguinal canal include its anatomical boundaries, as shown in Figure 4-10 and summarized in Table 4-5. Note that the deep inguinal ring begins internally as an outpouching of the transversalis fascia lateral to the inferior epigastric vessels, and that the superficial inguinal ring is the opening in the aponeurosis of the external abdominal oblique muscle. Aponeurotic fibers at the superficial ring envelop the emerging spermatic cord medially (medial crus), over its top (intercrural fibers), and laterally (lateral crus) (Fig. 4-10).

Clinical Focus 4-2   Inguinal Hernias

The protrusion of peritoneal contents (mesentery, fat, and/or a portion of bowel) through the abdominal wall in the groin region is termed an inguinal hernia. Inguinal hernias are distinguished by their relationship to the inferior epigastric vessels. There are two types of inguinal hernia:

• Indirect (congenital) hernia: represents 75% of inguinal hernias; occurs lateral to the inferior epigastric vessels, passes through the deep inguinal ring and inguinal canal as a protrusion along the spermatic cord, and lies within the internal spermatic fascia.

• Direct (acquired) hernia: occurs medial to the inferior epigastric vessels, passes directly through the posterior wall of the inguinal canal, and is separate from the spermatic cord and its coverings derived from the abdominal wall.

Many indirect inguinal hernias arise from incomplete closure or weakness of the processus vaginalis. The herniated peritoneal contents may extend into the scrotum (or labia majora, but much less common in females) if the processus vaginalis is patent along its entire course.

Direct inguinal hernias pass through the inguinal (Hesselbach’s) triangle, demarcated internally by the inferior epigastric vessels laterally, the rectus abdominis muscle medially, and the inguinal ligament inferiorly. Often, direct hernias are more limited in the extent to which they can protrude through the inferomedial abdominal wall. They occur not because of a patent processus vaginalis but because of an “acquired” weakness in the lower abdominal wall. Direct inguinal hernias can exit at the superficial ring and acquire a layer of external spermatic fascia, with the rare potential to herniate into the scrotum.

Clinical Focus 4-3   Hydrocele and Varicocele

The most common cause of scrotal enlargement is hydrocele, an excessive accumulation of serous fluid within the tunica vaginalis (usually a potential space). This small sac of peritoneum is originally from the processus vaginalis that covers about two thirds of the testis. An infection in the testis or epididymis, trauma, or a tumor may lead to a hydrocele, or it may be idiopathic.

Varicocele is an abnormal dilation and tortuosity of the pampiniform venous plexus within the spermatic cord. Almost all varicoceles are on the left side, perhaps because the left testicular vein drains into the left renal vein rather than the larger inferior vena cava, as the right testicular vein does. A varicocele is evident at physical examination when a patient stands, but it often resolves when the patient is recumbent.

The distal end of the esophagus passes through the right crus of the abdominal diaphragm at about the level of the T10 vertebra and terminates in the cardiac portion of the stomach (Fig. 4-13).

The stomach is a dilated, saclike portion of the GI tract that exhibits significant variation in size and configuration, terminating at the thick, smooth muscle sphincter (pyloric sphincter) by joining the first portion of the duodenum. The stomach is tethered superiorly by the lesser omentum (gastrohepatic ligament portion; Table 4-6) extending from its lesser curvature and is attached along its greater curvature to the greater omentum and the gastrosplenic ligament (see Figs. 4-12 and 4-13). Generally, the J-shaped stomach is divided into the following regions (Fig. 4-13 and Table 4-7):

The interior of the unstretched stomach is lined with prominent longitudinal mucosal gastric folds called rugae, which become more evident as they approach the pyloric region. As an embryonic foregut derivative, the stomach’s blood supply comes from the celiac trunk and its major branches (see Embryology).

Small Intestine

The small intestine measures about 6 meters in length (somewhat shorter in the fixed cadaver) and is divided into the following three parts:

The duodenum is the first portion of the small intestine and descriptively is divided into four parts (Table 4-8). Most of the C-shaped duodenum is retroperitoneal and ends at the duodenojejunal flexure, where it is tethered by a musculoperitoneal fold called the suspensory ligament of the duodenum (ligament of Treitz) (Fig. 4-14).

The jejunum and ileum are both suspended in an elaborate mesentery. The jejunum is recognizable from the ileum because the jejunum (Fig. 4-15):

The small intestine ends at the ileocecal junction, where a sphincter called the ileocecal valve controls the passage of ileal contents into the cecum (Fig. 4-16). The valve is actually two internal mucosal folds that cover a thickened smooth muscle sphincter.

The small intestine is a derivative of the embryonic midgut and receives its arterial supply from the superior mesenteric artery and its branches. An exception to this generalization is the first part of the duodenum, and sometimes the second part, which receives arterial blood from the gastroduodenal branch (from the common hepatic artery of the celiac trunk). This overlap reflects the embryonic transition from the foregut and midgut derivatives (stomach to first portions of duodenum).

Large Intestine

The large intestine is about 1.5 meters long, extending from the cecum to the anal canal, and includes the following segments (Figs. 4-16 and 4-17):

Lateral to the ascending and the descending colon lie the right and the left paracolic gutters, respectively. These depressions provide conduits for abdominal fluids to pass from region to region, largely dependent on gravity. Functionally the colon (ascending colon through the sigmoid part) absorbs water and important ions from the feces. It then compacts the feces for delivery to the rectum. Features of the large intestine include the following (Fig. 4-17):

The arterial supply to the cecum, ascending colon, appendix, and most of the transverse colon is provided by branches of the superior mesenteric artery; these portions of the large intestine are derived from the embryonic midgut. The embryonic hindgut gives rise to the distal transverse colon, descending colon, sigmoid colon, rectum, and anal canal. These are supplied by branches of the inferior mesenteric artery and, in the case of the distal rectum and anal canal, rectal branches from the internal iliac and internal pudendal arteries (see pages 197-200).

Clinical Focus 4-4   Acute Appendicitis

Appendicitis is a fairly common inflammation of the appendix, often caused by bacterial infection. Initially the patient feels diffuse pain in the periumbilical region. However, as the appendix becomes more inflamed and irritates the parietal peritoneum, the pain becomes well localized to the right lower quadrant (circumscribed tenderness to palpation). Surgical resection is the treatment of choice to prevent life-threatening complications such as abscess and peritonitis.

Clinical Focus 4-5   Gastroesophageal Reflux Disease (GERD)

The terminal end of the esophagus possesses a lower esophageal sphincter (specialized smooth muscle that is pharmacologically different from the smooth muscle lining the lower esophagus). It prevents the reflux of gastric contents into the lower esophagus. However, it can become compromised, usually by a loss of muscle tone or a sliding hiatal hernia, leading to GERD and inflammation of the esophageal lining. GERD often presents with upper abdominal pain, dyspepsia, gas, heartburn, dysphagia, bronchospasm, or asthma.

Clinical Focus 4-6   Hiatal Hernia

Herniation of the diaphragm that involves the stomach is referred to as a hiatal hernia. A widening of the space between the muscular right crus forming the esophageal hiatus allows protrusion of part of the stomach superiorly into the posterior mediastinum of the thorax. The two anatomical types are as follows:

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