PELVIC DIAPHRAGM I—FROM BELOW
Removing the superﬁcial muscles and fasciae of the pelvic ﬂoor, the pelvic diaphragm, viewed from below, forms a hammock of muscle from the pelvic brim, investing the urethra, vagina, and rectum and attaching posteriorly to the sacrum and coccyx. The principal muscles of this group are the levatores ani, consisting of both medial and lateral components on each side and supplied by the pudendal nerve. The larger medial component, the pubococcygeus, arises from the posterior surface of the superior ramus of the pubis adjacent to the symphysis, whence the ﬁbers pass downward and backward around the lateral walls of the vagina, with some ﬁbers reaching the coccyx, some terminating in the fascia forming the central tendinous point of the perineum, and others blending with the longitudinal muscle coats of the rectum. The pubococcygei are separated medially by the interlevator cleft through which pass the dorsal vein of the clitoris, the urethra, vagina, and rectum. These organs are supported by musculofascial extensions from the pubococcygei, their inferior fascia being continuous with the superior fascia of the urogenital diaphragm.
The lateral component of the levatores ani, the iliococcygeus, arises from the ischial spine and from the tendinous arch, a condensation of the parietal pelvic fascia covering the inner surface of the obturator internus muscle, which extends from the posterior surface of the pubis to the spine of the ischium. The iliococcygeus inserts in the last two segments of the coccyx, but some elements cross the midline anterior to the coccyx to unite with those from the opposite side in a raphe, where they are joined at a more superﬁcial level by ﬁbers from the sphincter ani externus and the transverse perineal muscles.
Posteriorly, the main pelvic diaphragm is nearly completed by the triangular coccygeus muscle. The apex of the coccygeus is attached to the spine of the ischium and the sacrospinous ligament, which it directly overlies; the base is attached to the lower portion of the lateral sacrum and the coccyx. This is best seen in the lateral view. In addition to supporting the pelvic viscera, the muscles of the pelvic diaphragm aid in the constriction of the vagina during coitus, in parturition, micturition, and in defecation. The obturator internus and piriformis muscles round out the posterior pelvis before passing through the lesser and greater sciatic foramina, respectively, to insert on the femur. These muscles lie close to the lateral walls of the pelvis.
The obturator internus arises from the circumference of the obturator fossa by ﬁbrous attachments directly to the bone and, to a lesser extent, from the obturator membrane, the tendinous arch, and the obturator fascia, which covers the inner surface of the muscle. The ﬁbers pass downward and backward, forming tendinous bands as they near the lesser sciatic notch and then, passing through this notch, they insert outside the pelvis on the medial surface of the greater trochanter of the femur.
The piriformis, best seen in the lateral view, arises from the lower portion of the sacrum and the sacrotuberous ligament, with its ﬁbers covering a large part of the greater sciatic notch, through which it passes out of the pelvis to attach in the superior portion of the greater trochanter of the femur. The piriformis is supplied by sacral nerves 1 and 2; the obturator internus by sacral nerves 1, 2, and 3. They aid in external rotation and abduction of the hip and are not directly concerned with support of the pelvic ﬂoor. However, the fascia covering these muscles is continuous with the pelvic diaphragm and with the endopelvic fascia.