Chapter 37: The pelvic diaphragm and fascia
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 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).
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 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?
37-2 Which main structures pass between the right and left levatores ani?
37-3 List the diaphragms in the body.
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.