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

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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)

Splenius Cervicis
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.

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