The Pelvic Cavity

The pelvis is the posterior part of the trunk. It incloses the pelvic cavity (Cavum pelvis), which communicates in front with the abdominal cavity, the line of demarcation being the pelvic brim or terminal line.

The dorsal wall or roof is formed by the sacrum and first three coccygeal vertebræ. The lateral walls are formed by the parts of the ilia behind the ilio-pectineal lines and the sacro-sciatic ligaments. The ventral wall or floor is formed by the pubic and ischial bones. The boundary of the outlet is formed by the third coccygeal vertebra dorsally, the ischial arch ventrally, and the posterior edges of the sacro-sciatic ligaments and the semimembranosus muscles laterally. The outlet is closed by the perineal fascia; this consists of superficial and deep layers, which are attached around the margin of the outlet and centrally to the organs at the outlet—the anus and its muscles, the vulva (in the female), and the root of the penis (in the male).

The cavity contains the rectum, parts of the internal generative and urinary organs, some fœtal remnants, muscles, vessels, and nerves. It is lined by the fascia pelvis, and in part by the peritoneum.

The pelvic peritoneum is continuous in front with that of the abdomen. It lines the cavity as far back as the third or fourth sacral vertebra in the horse, where it is reflected on to the viscera, and from one organ to another. We may therefore distinguish an anterior, peritoneal, and a posterior, retroperitoneal part of the cavity. Along the mid-dorsal line it forms a continuation of the colic mesentery, the mesorectum, which attaches the first or peritoneal part of the rectum to the roof. In animals in fair condition a considerable quantity of subperitoneal and retroperitoneal fat is found on the walls and in the various interstices.

In the male the general disposition of the peritoneum here is as follows. If traced along the dorsal wall, it is reflected at the third or fourth sacral vertebra on to the rectum, forming the visceral peritoneum for the first part of that tube. Laterally it is reflected in a similar fashion. If the rectum be raised, it will be seen that the peritoneum passes from its ventral surface and forms a transverse fold which lies on the dorsal surface of the bladder (Fig. ). This is the urogenital fold (Plica urogenitalis). Its concave free edge passes on either side into the inguinal canal. The ventral layer of this fold is reflected on to the dorsal surface of the bladder near its neck. Thus there is formed a pouch between the rectum and bladder—the recto-vesical pouch (Excavatio recto-vesicalis), which is partially subdivided by the urogenital fold into recto-genital and vesico-genital cavities. The fold contains the vasa deferentia, part of the vesiculæ seminales, and the uterus masculinus (a fœtal remnant). The space on either side of the rectum is occupied by coils of the small colon and the pelvic flexure of the great colon usually. If the bladder is now raised, it is seen that the peritoneum passes from its ventral surface on to the pelvic floor, forming a median fold, the so-called middle ligament (Plica umbilicalis media). It also forms on each side a lateral fold, the lateral ligament (Plica umbilicalis lateralis), which contains in its edge the so-called round ligament (Ligamentum teres)—the partially occluded umbilical artery, which is a large vessel in the fœtus.

In the female the arrangement is modified by the presence of the uterus; the urogenital fold is very large, so as to inclose the uterus and a small part of the vagina. It forms two extensive folds, the broad ligaments of the uterus (Ligamenta lata uteri), which attach that organ to the sides of the pelvic cavity and the lumbar part of the abdominal wall (Fig. ). It thus divides the recto-vesical pouch completely into dorsal and ventral compartments—the recto-genital pouch (Excavatio recto-uterina), and the vesico-genital pouch (Excavatio vesico-uterina).

Further details will be given in the description of the pelvic viscera.