Chapter 6. Overview of the Abdomen, Pelvis, and Perineum

Big Picture

In the adult, the pelvis (os coxae) is formed by the fusion of three bones: ilium, ischium, and pubis (Figure 6-1A and B). The union of these three bones occurs at the acetabulum. The paired os coxae articulate posteriorly with the sacrum and anteriorly with the pubic symphysis.

Pelvic Bone

The following structures are formed within the fused os coxa (Figure 6-1A–C):

• Acetabulum. A cup-shaped socket into which the ball-shaped head of the femur fits firmly.
• Obturator foramen. Covered by a flat sheet of connective tissue called the obturator membrane. A small opening located at the top of the membrane provides a route through which the obturator nerve, artery, and vein course.
• Greater sciatic notch. Located between the posterior inferior iliac spine and the ischial spine. The sacrospinous ligament converts the notch into the greater sciatic foramen, where the piriformis muscle, sciatic nerve, and pudendal neurovascular structures course.
• Lesser sciatic notch. Located between the ischial spine and the ischial tuberosity. The sacrotuberous ligament converts the notch into the lesser sciatic foramen.
• Pubic symphysis. Fibrocartilage connecting the two pubic bones in the anterior midline of the pelvis.
• Pelvic inlet. The superior aperture of the pelvis. The pelvic inlet is oval shaped and bounded by the ala of the sacrum, arcuate line, pubic bone, and symphysis pubis. The pelvic inlet is traversed by structures in the abdominal and pelvic cavities.
• Pelvic outlet. The inferior aperture of the pelvis. The pelvic outlet is a diamond-shaped opening formed by the pubic symphysis and sacrotuberous ligaments. Terminal parts of the vagina and the urinary and gastrointestinal tracts traverse the pelvic outlet. The perineum is inferior to the pelvic outlet.

The pelvic bone is formed by the fusion of three bones: ilium, ischium, and pubis.

Ilium

• Iliac crest. Thickened superior rim.
• Iliac fossa. Concave surface on the anteromedial surface.
• Anterior superior iliac spine. Anterior termination of the iliac crest. Serves as an attachment site for the sartorius and tensor fascia lata muscles.
• Anterior inferior iliac spine. Serves as an attachment site for the rectus femoris muscle.
• Posterior superior iliac spine. Posterior termination of the iliac crest.
• Posterior inferior iliac spine. Forms the posterior border of the ala of the sacrum.

Ischium

• Ischial tuberosity. A large protuberance on the inferior aspect of the ischium for attachment of the hamstring muscles and for supporting the body when sitting.
• Ischial spine. A pointed projection that separates the greater and lesser sciatic notches.
• Ischial ramus. A bony projection that joins with the inferior pubic ramus to form the ischiopubic ramus (conjoint ramus).

Pubis

• Pubic tubercle. A rounded projection on the superior ramus of the pubis.
• Superior pubic ramus. A bony projection that forms a bridge from the acetabulum to the ischiopubic ramus, and thus the ischium. The crest on the superior aspect of the superior pubic ramus is the pectineal line, which serves as part of the border for the pelvic inlet and as an attachment site for muscles.
• Inferior pubic ramus. A bony projection that forms a bridge from the superior pubic ramus to the ischial ramus. The inferior pubic ramus serves as an attachment site for muscles of the lower limb.

Sex Differences in the Pelvis

The female pelvis differs from the male pelvis because of its importance in childbirth.

• Pelvic inlet. A typical female pelvic inlet is usually more circular-shaped compared to the typically heart-shaped male pelvic inlet.
• Pelvic outlet. A typical female pelvic outlet is wider and has shorter and straighter ischial spines compared to the typical male pelvis. In addition, the ischial spines project more medially in males than in females.
• Pubic arch. The pubic arch is the angle between adjacent ischiopubic rami. A typical female pubic arch is usually larger (85 degrees) than the male pubic arch (60 degrees). The angle formed by the female pubic arch can be estimated by the angle between the thumb and the forefinger; in contrast, the male pubic arch is estimated by the angle between the index and the middle fingers (Figure 6-1C and D).

Big Picture

The gut tube is subdivided into segments based on structural (lumen diameter) and functional (vascular supply) characteristics.

• Structural subdivision. The diameter of the lumen in the small intestines is smaller than that in the large intestines.
• Functional subdivision. The gut tube is classified by its vascular supply. The foregut, midgut, and hindgut all receive their own vascular supply.

Both methods of classifying regions of the gut tube are used in this text book, as well as in clinical medicine.

Structural Subdivision of the Gut Tube

The gut tube is divided segmentally into the small intestine and the large intestine (Figure 6-2A).

• Small intestine. The small intestine functions mainly in the chemical breakdown of food and its subsequent absorption into the blood stream. The veins of the small intestine transport the absorbed nutrients to the liver for processing and ultimately to all other parts of the body. The small intestine consists of three parts: duodenum, jejunum, and ileum.
• Large intestine (colon). Receives its name because of its large luminal diameter. The colon absorbs water and vitamins and houses numerous bacteria. The blood supply of the colon overlaps with that of the midgut and hindgut.

Functional Subdivision of the Gut Tube

The gut tube is divided into the following three regions based on their primary arterial supply (Figure 6-2B):

• Foregut. Supplied primarily by the celiac trunk. This region of the gut tube extends from the distal end of the esophagus to the proximal half of the duodenum.
• Midgut. Supplied primarily by the superior mesenteric artery. This region of the gut tube extends from the distal half of the duodenum to the splenic flexure of the colon.
• Hindgut. Supplied primarily by the inferior mesenteric artery. This region of the gut tube extends from the splenic flexure of the colon to the rectum.

Abdominal Venous Drainage

Blood in the abdomen drains back to the heart via two routes: caval drainage and portal drainage (Figure 6-2C).

Caval Drainage

Venous blood that is returned to the heart from the anterior and posterior abdominal walls and the retroperitoneal organs via the superior or inferior vena cava.

• Inferior epigastric veins. Return blood to the heart via the inferior vena cava.
• Intercostal veins. Return blood to the heart via the superior vena cava.
• Lumbar veins. Return blood to the heart directly via the inferior vena cava or indirectly via the superior vena cava (lumbar veins may drain into the ascending lumbar veins to the azygos system of veins to the superior vena cava).

Portal Drainage

Venous blood from the gut tube and its derivatives returns to the heart via the hepatic portal vein to the liver. In other words, venous blood from the gut tube reaches the inferior vena cava after coursing through the liver.

• Foregut. Branches from the gastric and splenic veins to the portal vein.
• Midgut. Branches from the superior mesenteric vein to the portal vein.
• Hindgut. Branches from the inferior mesenteric vein to the portal vein.

Abdominal Lymphatics

Lymphatics generally follow neurovascular bundles throughout the body. Clusters of lymph nodes, which are important in monitoring the immune system, are found along the course of the lymphatics. The central lymph nodes in the abdomen are named according to their associated artery. For example, the lymph nodes clustered at the origin of the celiac trunk are called celiac lymph nodes.

Innervation of the Gut Tube

The regions of the gut tube receive the following autonomic innervation:

• Foregut. Sympathetics from the greater splanchnic nerves (T5–T9). Parasympathetics from the vagus nerves.
• Midgut. Sympathetics from the lesser splanchnic nerves (T10–T11). Parasympathetics from the vagus nerves.
• Hindgut. Sympathetics from the lumbar splanchnic nerves. Parasympathetics from the pelvic splanchnics (S2–S4 spinal cord levels).