1 Introduction

The abdomen is the region between the thorax superiorly and the pelvis inferiorly. The abdomen is composed of the following:

In your study of the abdomen, first focus on the abdominal wall and note the continuation of the three muscle layers of the thorax (intercostal muscles) as they blend into the abdominal flank musculature.

Next, note the disposition of the abdominal organs. For example, you should know the region or quadrant of the abdominal cavity in which the organs reside; whether an organ is suspended in a mesentery or lies retroperitoneally (refer to embryology of abdominal viscera, i.e., foregut, midgut, or hindgut derivatives); the blood supply and autonomic innervation pattern to the organs; and features of the organs that will allow you to readily identify which organ or part of an organ you are viewing (particularly important in laparoscopic surgery). Also, you should understand the dual venous drainage of the abdomen by the caval and hepatic portal systems and the key anastomoses between these two systems that facilitate venous return to the heart.

Lastly, study the posterior abdominal wall musculature, and identify the components and distribution of the lumbar plexus of somatic nerves.

2 Surface Anatomy

Key Landmarks

Key surface anatomy features of the anterolateral abdominal wall include the following (Fig. 4-1):

• Rectus sheath: a fascial sheath containing the rectus abdominis muscle, which runs from the pubic symphysis and crests to the xiphoid process and fifth to seventh costal cartilages.

• Linea alba: literally the “white line”; a relatively avascular midline subcutaneous band of fibrous tissue where the fascial aponeuroses of the rectus sheath from each side interdigitate in the midline.

• Semilunar line: the lateral border of the rectus abdominis muscle in the rectus sheath.

• Tendinous intersections: transverse skin grooves that demarcate transverse fibrous attachment points of the rectus sheath to the underlying rectus abdominis muscle.

• Umbilicus: the site that marks the T10 dermatome, lying at the level of the intervertebral disc between L3 and L4; the former attachment site of the umbilical cord.

• Iliac crest: the rim of the ilium, which lies at about the level of the L4 vertebra.

• Inguinal ligament: a ligament composed of the aponeurotic fibers of the external abdominal oblique muscle, which lies deep to a skin crease that marks the division between the lower abdominal wall and thigh of the lower limb.

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).

3 Anterolateral Abdominal Wall


The layers of the abdominal wall include the following:

• Skin: epidermis and dermis

• Superficial fascia (subcutaneous tissue): a single, fatty connective tissue layer below the level of the umbilicus that divides into a more superficial fatty layer (Camper’s fascia) and a deeper membranous layer (Scarpa’s fascia; see Fig. 4-11).

• Investing fascia: tissue that covers the muscle layers.

• Abdominal muscles: three flat layers, similar to the thoracic wall musculature, except in the anterior midregion where the vertically oriented rectus abdominis muscle lies in the rectus sheath.

• Endoabdominal fascia: tissue that is unremarkable except for a thicker portion called the transversalis fascia, which usually lines the inner aspect of the transversus abdominis muscle; it is continuous with fascia on the underside of the diaphragm, fascia of the posterior abdominal muscles, and fascia of the pelvic muscles.

• Extraperitoneal (fascia) fat: connective tissue that is variable in thickness and contains a variable amount of fat.

• Peritoneum: thin serous membrane that lines the inner aspect of the abdominal wall (parietal peritoneum) and occasionally reflects off the walls as a mesentery to invest partially or completely various visceral structures (visceral peritoneum).


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).


Aponeuroses and Layers Forming Rectus Sheath*

Anterior lamina above arcuate line Formed by fused aponeuroses of external and internal abdominal oblique muscles
Posterior lamina above arcuate line Formed by fused aponeuroses of internal abdominal oblique and transversus abdominis muscles
Below arcuate line All three muscle aponeuroses fuse to form anterior lamina, with rectus abdominis in contact only with transversalis fascia posteriorly

*See Figure 4-4.

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:

4 Inguinal Region

The inguinal region, or groin, is the transition zone between the lower abdomen and the upper thigh. This region, especially in males, is characterized by a weakened area of the lower abdominal wall that renders this region particularly susceptible to inguinal hernias. Although occurring in either gender, inguinal hernias are much more common in males because of the descent of the testes into the scrotum, which occurs along this boundary region.

The inguinal region is demarcated by the inguinal ligament, the inferior border of the external abdominal oblique aponeurosis, which is folded under on itself and attaches to the anterior superior iliac spine and extends inferomedially to attach to the pubic tubercle (see Figs. 4-1 and 4-3, B). Medially, the inguinal ligament flares into the crescent-shaped lacunar ligament that attaches to the pecten pubis of the pubic bone (Fig. 4-7). Fibers from the lacunar ligament also course internally along the pelvic brim as the pectineal ligament (see Clinical Focus 4-2). A thickened inferior margin of the transversalis fascia, called the iliopubic tract, runs parallel to the inguinal ligament but deep to it and reinforces the medial portion of the inguinal canal.

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:

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.


5 Abdominal Viscera

Peritoneal Cavity

The abdominal viscera are contained within a serous membrane–lined recess called the abdominopelvic cavity (sometimes just “abdominal” or “peritoneal” cavity) or lie in a retroperitoneal position adjacent to this cavity, often with only their anterior surface covered by peritoneum (e.g., the kidneys and ureters). The abdominopelvic cavity extends from the abdominal diaphragm inferiorly to the floor of the pelvis (Fig. 4-11).

The walls of the abdominopelvic cavity are lined by parietal peritoneum, which can reflect off the abdominal walls in a double layer called a mesentery, which embraces and suspends a visceral structure. As the mesentery wraps around the viscera, it becomes visceral peritoneum. Viscera suspended by a mesentery are considered intraperitoneal, whereas viscera covered on only one side by peritoneum are considered retroperitoneal.

The parietal peritoneum lines the inner aspect of the abdominal wall and thus is innervated by somatic afferent fibers of the ventral rami of the spinal nerves innervating the abdominal musculature. Inflammation or trauma to the parietal peritoneum therefore presents as well-localized pain. The visceral peritoneum, on the other hand, is innervated by visceral afferent fibers carried in the sympathetic and parasympathetic nerves. Pain associated with visceral peritoneum thus is more poorly localized, giving rise to referred pain (see Table 4-12).

Anatomists refer to the peritoneal cavity as a “potential space” because it normally contains only a small amount of serous fluid that lubricates its surface. If excessive fluid collects in this space because of edema (ascites) or hemorrhage, it becomes a “real space.” Many clinicians, however, view the cavity only as a real space because it does contain serous fluid, although they qualify this distinction further when ascites or hemorrhage occurs.

The abdominopelvic cavity is further subdivided into the following (Figs. 4-11 and 4-12):

In addition to the mesenteries that suspend the bowel, the peritoneal cavity contains a variety of double-layered folds of peritoneum, including the omenta (attached to the stomach) and peritoneal ligaments. These are not “ligaments” in the traditional sense but rather short, distinct mesenteries that connect structures (for which they are named) together or to the abdominal wall (Table 4-6). Some of these structures are shown in Figures 4-11 and 4-12, and we will encounter the others later in the chapter as we describe the abdominal contents.


Mesenteries, Omenta, and Peritoneal Ligaments

Greater omentum “Apron” of peritoneum hanging from greater curvature of stomach, folding back on itself and attaching to transverse colon
Lesser omentum Double layer of peritoneum extending from lesser curvature of stomach and proximal duodenum to inferior surface of liver
Mesenteries Double fold of peritoneum suspending parts of bowel and conveying vessels, lymphatics, and nerves of bowel (meso-appendix, transverse mesocolon, sigmoid mesocolon)
Peritoneal ligaments Double layer of peritoneum attaching viscera to walls or to other viscera
Gastrocolic ligament Portion of greater omentum that extends from greater curvature of stomach to transverse colon
Gastrosplenic ligament Left part of greater omentum that extends from hilum of spleen to greater curvature of stomach
Splenorenal ligament Connects spleen and left kidney
Gastrophrenic ligament Portion of greater omentum that extends from fundus to diaphragm
Phrenocolic ligament Extends from left colic flexure to diaphragm
Hepatorenal ligament Connects liver to right kidney
Hepatogastric ligament Portion of lesser omentum that extends from liver to lesser curvature of stomach
Hepatoduodenal ligament Portion of lesser omentum that extends from liver to 1st part of duodenum
Falciform ligament Extends from liver to anterior abdominal wall
Ligamentum teres hepatis Obliterated left umbilical vein in free margin of falciform ligament
Coronary ligaments Reflections of peritoneum from superior aspect of liver to diaphragm
Ligamentum venosum Fibrous remnant of obliterated ductus venosus
Suspensory ligament of ovary Extends from lateral pelvic wall to ovary
Ovarian ligament Connects ovary to uterus (part of gubernaculum)
Round ligament of uterus Extends from uterus to deep inguinal ring (part of gubernaculum)

Abdominal Organs

Abdominal Esophagus and Stomach

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):

• Cecum: a pouch that is connected to the ascending colon and the ileum; it extends below the ileocecal junction, although it is not suspended by a mesentery.

• Appendix: a narrow tube of variable length (usually 7-10 cm) that contains numerous lymphoid nodules and is suspended by mesentery called the meso-appendix.

• Ascending colon: is retroperitoneal and ascends on the right flank to reach the liver, where it bends into the right colic (hepatic) flexure.

• Transverse colon: is suspended by a mesentery, the transverse mesocolon, and runs transversely from the right hypochondrium to the left, where it bends to form the left colic (splenic) flexure.

• Descending colon: is retroperitoneal and descends along the left flank to join the sigmoid colon in the left groin region.

• Sigmoid colon: is suspended by a mesentery, the sigmoid mesocolon, and forms a variable loop of bowel that runs medially to join the midline rectum in the pelvis.

• Rectum and anal canal: are retroperitoneal and extend from the middle sacrum to the anus (see Chapter 5).

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).

Jun 16, 2016 | Posted by in ANATOMY | Comments Off on Abdomen

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