The muscles of the pelvic floor extend from the sacrum and coccyx to the ischium and pubis. These muscles (1) support the organs of the pelvic cavity, (2) flex the sacrum and coccyx, and (3) control the movement of materials through the urethra and anus (Figure 11-13 and Table 11-10).

The boundaries of the perineum, the muscular sheet that forms the pelvic floor, are established by the inferior margins of the pelvis. If you draw a line between the ischial tuberosities, you will divide the perineum into two triangles—an anterior urogenital triangle and a posterior anal triangle. The superficial muscles of the urogenital triangle are the muscles of the external genitalia. They cover deeper muscles that strengthen the pelvic floor and encircle the urethra. These muscles constitute the urogenital diaphragm, a deep muscular layer that extends between the pubic bones.

An even more extensive muscular sheet, the pelvic diaphragm, forms the muscular foundation of the anal triangle. This layer, covered by the urogenital diaphragm, extends as far as the pubic symphysis.

The urogenital and pelvic diaphragms do not completely close the pelvic outlet, for the urethra, vagina, and anus pass through them to open on the external surface. Muscular sphincters surround their openings and permit voluntary control of urination and defecation. Muscles, nerves, and blood vessels also pass through the pelvic outlet as they travel to or from the lower limbs.

Damage to the external intercostal muscles would interfere with what important process?

If someone hit you in your rectus abdominis muscle, how would your body position change?

After spending an afternoon carrying heavy boxes from his basement to his attic, Joe complains that the muscles in his back hurt. Which muscle(s) is (are) most likely sore?

Hernias When your abdominal muscles contract forcefully, pressures in your abdominopelvic cavity can skyrocket, and those pressures are applied to internal organs. If you exhale at the same time, the pressure is relieved, because your diaphragm can move upward as your lungs collapse. But during vigorous isometric exercises or when you lift a weight while holding your breath, pressure in the abdominopelvic cavity can rise high enough to cause a variety of problems, among them a hernia. A hernia develops when an organ protrudes through an abnormal opening. The most common hernias are inguinal and diaphragmatic hernias. Inguinal hernias typically occur in males, at the inguinal canal, the site where blood vessels, nerves, and reproductive ducts pass through the abdominal wall to reach the testes. Elevated abdominal pressure can force open the inguinal canal and push a portion of the intestine into the pocket created. Diaphragmatic hernias develop when visceral organs, such as a portion of the stomach, are forced into the left pleural cavity. If herniated structures become trapped or twisted, surgery may be required to prevent serious complications, such as intestinal blockage or tissue degeneration due to the interruption of blood flow. Hernias

FIGURE 11-13 Muscles of the Pelvic Floor.