A special aponeurosis covers the muscles.
Behind the symphysis is the bladder, and the peritoneum that covers it, held by two lateral fibrous cords; these are the anterior ligaments of the bladder.
The anterior surface of sacrum and coccyx is covered by the rectum, which, above, in front of the left sacro-iliac symphysis, reaches the median line towards the middle of the sacrum. It adheres to these parts by a loose peritoneal fold, the meso-rectum. Reaching the end of the coccyx, it is carried forward, and, for a distance of seven to eight inches, crosses the muscles to an orifice which forms the anus.
The lateral parts of the sacrum, externally to the greater sciatic notch, and the anterior sacral canals, are occupied by the pyramidalis muscle. It stretches over the osseous spaces which separate the anterior sacral canals, across the greater sciatic notch, to the internal surface of the great trochanter. The anterior branches of the sacral nerves, which form the sacral plexus, the hypogastric artery and vein, cross this muscle.
A true apoueurosis covers the pyramidalis muscle.
Finally, the aponeurotic and peritoneal lamina cover those portions of the cavity which are not covered by either muscles or viscera.
Floor of the Pelvis.
We shall borrow the greater part of our anatomical description from Dubois and Pajot, for in their work this intricate part is most clearly explained.
The floor of the pelvis, inferior wall of the pelvis, perineal floor, perineum, is formed by a muscular aponeurotic plane, pierced by three openings, the anus, the vulva and the orifice of the urethra, which open on its surface. It forms an elastic couch, which plays an important part in delivery.
Two very distinct parts can be distinguished: an aponeurosis, formed of layers which unite and make distinct spaces, and muscles, which fill these spaces and are accompanied by important vessels and nerves.
Superior Pelvic Aponeurosis. (Figs. 24 and 25.) Proceeding from within outwards, the first aponeurosis is the superior pelvic aponeurosis. Attached to the anterior surface of the sacrum and coccyx; inside to the sacral canals, and in front on the internal sur face of the body of the pubes, near the symphysis, it extends along the entire wall of the pelvis, excepting the superior half of the anterior semi circumference, On a level with the edge of the abdominal strait, it blends in front with the aponeurosis of the abdominal walls; laterally with the iliac muscle; behind with the lumbo-iliac aponeurosis. Below it covers
the entire floor of the pelvis like a kind of inferior diaphragm.
On a level with the superior border of the great sciatic notch, it divides into two lamina) which, twisting at almost a right angle, form a trans verse partition that divides this aponeurotic cavity into two parts, the anterior, which is the larger, and the posterior. These two laminae, joined by their superior edge, turn aside at their inferior part, thus form ing a triangle, base downward.
Their internal portion meets the rectum and vagina, with the walls of which they blend, their external portion reaching the soft parts, which occupy the ischiatic canal. The perineal portion is crossed by the rectum behind, the vagina in front, and finally, still further forward, by the bladder, the lower end of which projects slightly below the level of the aponeurosis. As Pajot says, it is not a true perforation that the aponeu rosis undergoes, for the edges of these openings turn up from below, and prolong into fibrous expansions on the walls of each of the organs which they traverse. This is particularly noticeable in the bladder and vagina.
The parietal portion covers the sacrum, the coccyx, the anterior branches of the sacral nerves, the pyramidalis muscle and its aponeurosis, the superior part of the obturator and its aponeurosis, and lastly the superior half of the body and symphysis of the pubes. Therefore, it oc cupies at the same time, the cavity and the inferior strait.
The other aponeuroses occupy only the anterior half of the inferior strait. They are three in number and are called the perineal aponeuroses.