Chapter 37: The pelvic diaphragm and fascia

Pelvic diaphragm

The pelvic diaphragm is a muscular partition formed by the levatores ani and coccygei, with which may be included the parietal pelvic fascia on their upper and lower aspects. It separates the pelvic cavity above from the perineal region below.

The right and left levatores ani (figs. 37-1 and 37-2) lie almost horizontally in the floor of the pelvis, separated by a narrow gap that transmits the urethra, vagina, and anal canal. The levator ani is usually considered in three parts: pubococcygeus, puborectalis, and iliococcygeus. The pubococcygeus, the main part of the levator, runs backward from the body of the pubis toward the coccyx and may be damaged during parturition. Some fibers are inserted into the prostate, urethra, and vagina. The right and left puborectales unite behind the anorectal junction to form a muscular sling (see fig. 37-1). Some regard them as a part of the sphincter ani externus. The iliococcygeus, the most posterior part of the levator ani, is often poorly developed.

The coccygeus, situated behind the levator ani and frequently tendinous as much as muscular, extends from the ischial spine to the lateral margin of the sacrum and coccyx.

Innervation and function.

The pelvic diaphragm is supplied chiefly by the ventral rami of S.N. 3,4. The diaphragm helps to support the pelvic viscera, resists increases in intra-abdominal pressure, and aids in micturition.

The fascia of the pelvic diaphragm, as part of the parietal pelvic fascia, is usually described in two layers (see figs. 38-2 and 38-3). The superior fascia covers the pelvic surface of the muscles and presents a tendinous arch that forms the medial puboprostatic ligament. The inferior fascia covers the lower surface of the muscles and forms the medial wall of the ischiorectal fossa (see fig. 38-3).

Pelvic fascia

The pelvic fascia (see figs. 38-2 and 38-3) has parietal and visceral divisions.

The parietal pelvic fascia is a part of the general lining of the abdominal and pelvic walls. It contributes to the floor of the pelvis as the superior and inferior fasciae of the pelvic diaphragm, and it lines the lateral pelvic wall as the obturator fascia. The obturator fascia lines the obturator internus, and, below the origin of the levator ani, it forms the lateral wall ofthe ischiorectal fossa (see fig. 38-3). In this wall, a fascial tunnel, the pudendal canal, houses the internal pudendal vessels and the pudendal nerve.

The visceral pelvic fascia is the extraperitoneal tissue that ensheathes the pelvic organs and vessels.

The membranous partition between the rectum and the bladder and prostate is termed the rectovesical septum. It provides a cleavage plane during surgery. The existence of a rectovaginal septum is disputed.


37-1 What is the pelvic diaphragm?

37-1 The pelvic diaphragm comprises the levatores ani and coccygei and their covering fasciae. The pelvic diaphragm supports the weight of the pelvic viscera in the erect position. The levatores ani, which have been described as a dilator as well as a sphincter, probably serve for the "fixation and prevention of prolapse of the pelvic viscera passing through the levator hiatus" (S. F. Ayoub, J. Anat., 128:571, 1979).

37-2 Which main structures pass between the right and left levatores ani?

37-2 The pelvic diaphragm has a hiatus for the passage of the urethra, vagina, and anal canal (see fig. 37-1).

37-3 List the diaphragms in the body.

37-3 Five structures in the body receive the name diaphragm (Gk, a partition): (1) the (respiratory) diaphragm, (2) the diaphragma oris, (3) the pelvic diaphragm, (4) the urogenital diaphragm, and (5) the diaphragma sellae. All are muscular or musculotendinous except the last, which is composed of dura mater.

Figure legends

Figure 37-1 Muscles of the pelvic diaphragm in the female, from below. (After Milligan and Morgan.)

Figure 37-2 Muscles of the pelvic diaphragm, pelvic aspect, showing different parts of the levator ani. The pubococcygeus has several portions, sphincter vaginae, puborectalis, and pubococcygeus proper, depending upon the direction and insertion of the fibers. Some fibers of the puborectalis pass toward the sphincter ani externus and can elevate the anus.

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