Muscles of the Pelvic Floor

The floor of the pelvic cavity is formed by three layers of muscles and fasciae that span the pelvic outlet and support the viscera (table 10.8). It is penetrated by the anal canal, urethra, and vagina, which open into a diamond-shaped region between the thighs called the perineum (PERR-ih-NEE-um). The perineum is bordered by four bony landmarks—the pubic symphysis anteriorly, the coccyx posteriorly, and the ischial tuberosities laterally. The anterior half of the perineum is the urogenital triangle and the posterior half is the anal triangle (fig. 10.20b). These are especially important landmarks in obstetrics.

The pelvic floor is divided into three layers or "compartments." The one just deep to the skin, called the superficial perineal space (fig. 10.20a, b), contains three muscles. The ischiocavernosus muscles converge like a V from the ischial tuberosities toward the penis or clitoris and assist in

Saladin: Anatomy & I 10. The Muscular System I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Chapter 10 The Muscular System 351

Superior nuchal line

Longissimus capitis-Splenius capitis

Serratus posterior superior

Splenius cervicis Erector spinae Iliocostalis Longissimus Spinalis

Serratus posterior inferior

Internal abdominal oblique

External abdominal oblique (cut)

Superior nuchal line

Longissimus capitis-Splenius capitis

Serratus posterior superior

Splenius cervicis Erector spinae Iliocostalis Longissimus Spinalis

Serratus posterior inferior

Internal abdominal oblique

External abdominal oblique (cut)

Iliocostalis
U Jnt. JM

Semispinalis capitis Semispinalis cervicis

Semispinalis thoracis

Multifidus

Quadratus lumborum

Figure 10.18 Muscles Acting on the Vertebral Column. Those on the right are deeper than those on the left.

Insight 10.3 Clinical Application erection. In males, the bulbospongiosus (bulbocavernosus)

forms a sheath around the base (bulb) of the penis; it expels semen during ejaculation. In females, it encloses the vagina like a pair of parentheses and tightens on the penis during intercourse. Voluntary contractions of this muscle in both sexes also help void the last few milliliters of urine. The superficial transverse perineus extends from the ischial tuberosities to a strong central tendon of the perineum.

In the middle compartment, the urogenital triangle is spanned by a thin triangular sheet called the urogenital diaphragm. This is composed of a fibrous membrane and two muscles—the deep transverse perineus and the external urethral sphincter (fig. 10.20c, d). The anal triangle contains the external anal sphincter. The deepest compartment, called the pelvic diaphragm, is similar in both sexes. It consists of two muscle pairs shown in figure 10.20e—the levator ani and coccygeus.

Hernias

A hernia is any condition in which the viscera protrude through a weak point in the muscular wall of the abdominopelvic cavity. The most common type to require treatment is an inguinal hernia. In the male fetus, each testis descends from the pelvic cavity into the scrotum by way of a passage called the inguinal canal through the muscles of the groin. This canal remains a weak point in the pelvic floor, especially in infants and children. When pressure rises in the abdominal cavity, it can force part of the intestine or bladder into this canal or even into the scrotum. This also sometimes occurs in men who hold their breath while lifting heavy weights. When the diaphragm and abdominal muscles contract, pressure in the abdominal cavity can soar to 1,500 pounds per square inch—more than 100 times the normal pressure and quite sufficient to produce an inguinal hernia, or "rupture." Inguinal hernias rarely occur in women.

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

352 Part Two Support and Movement

Trapezius -

Ribs

External intercostals

Erector spinae: Spinalis thoracis Iliocostalis thoracis Longissimus thoracis

Latissimus dorsi

Iliocostalis lumborum Thoracolumbar fascia

External intercostals

Latissimus dorsi

Iliocostalis lumborum Thoracolumbar fascia

Iliocostalis

Figure 10.19 Deep Back Muscles of the Cadaver.

Two other sites of hernia are the diaphragm and navel. A hiatal hernia is a condition in which part of the stomach protrudes through the diaphragm into the thoracic cavity. This is most common in overweight people over 40. It may cause heartburn due to the regurgitation of stomach acid into the esophagus, but most cases go undetected. In an umbilical hernia, abdominal viscera protrude through the navel.

Before You Go On

Answer the following questions to test your understanding of the preceding section:

  1. Which muscles are used more often, the external intercostals or internal intercostals? Explain.
  2. Explain how pulmonary ventilation affects abdominal pressure and vice versa.
  3. Name a major superficial muscle and two major deep muscles of the back.
  4. Define perineum, urogenital triangle, and anal triangle.
  5. Name one muscle in the superficial perineal space, one in the urogenital diaphragm, and one in the pelvic diaphragm. State the function of each.

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Responses

  • Paola
    What is the muscular floor of the pelvic cavity?
    6 years ago
  • costantino
    Which muscles support the pelvic cavity?
    6 years ago

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