DISSECTION OF THE PELVIS. 353 



and it is considerably larger in the mare than the horse. In form it is 

 nearly circular. 



The outlet of the pelvis is circumscribed by the first one or two 

 coccygeal bones above, by the posterior edges of the ischial bones below, 

 and by the posterior edge of the sacro-sciatic ligament on each side. In 

 outline it is ovoid, with the broad end below ; and it looks backwards 

 and upwards, being nearly parallel to the inlet. 



The Cavity of the Pelvis is the irregularly tubular passage between 

 the inlet and the outlet. Its transverse section approaches the circular 

 in front, but changes gradually to the oval as it is taken more posteriorly. 

 For convenience of description, however, it may be said to have a roof 

 a floor, and two lateral walls. The roof is formed by the inferior surface 

 of the sacrum and first one or two coccygeal bones. The floor is formed 

 by the pubic and ischial bones. Each lateral wall is formed for a short 

 space in front by the pelvic surface of the shaft of the ilium, and for the 

 rest of its extent by the sacro-sciatic ligament. 



Contents of the Cavity. — These vary with the sex. In both sexes 

 it contains the rectum, the urinary bladder, the termination of the 

 ureters, and numerous important vessels and nerves. In the male it 

 lodges, besides these, the vasa deferentia (in part), the seminal vesicles, 

 the prostate, Gowper's glands, the ejaculatory ducts, and the prostatic 

 and membranous portions of the urethra. In the female it lodges the 

 posterior part of the uterus, the vagina, and the vulva. 



The Peritoneum. The serous lining of the abdominal cavity is con- 

 tinued into the pelvis, whose walls and contents it in part covers. Thus, 

 if it be followed backwards along the roof of the cavity, it will be seen 

 to cover the lower face of the sacrum about as far as its 4th segment, 

 but at that point it is reflected on to the rectum. Again, if the peri- 

 toneum be traced over the pelvic brim at the pubes, it will be found to 

 cover the floor of the pelvis for a short distance, and then to become 

 reflected on to the bladder. In the same way, along a curved line on the 

 side of the pelvis between these two points, the peritoneum leaves the 

 pelvic wall and passes on to the viscera. Since this reflection, however, 

 takes place anterior to the posterior extremity, or outlet, of the cavity, 

 it results that the pelvic viscera get at most only a partial covering of 

 peritoneum. Thus, the rectum for a length of from four to six inches 

 in front of the anus, the posterior extremity of the vesiculge seminales, and 

 (in the collapsed state) nearly the half of the upper face of the bladder 

 and three-fourths of its lower face are without a serous covering. 



In the mare, in the same manner, the posterior part of the vagina and 

 the whole of the vulva are without a serous covering. 



The peritoneum in passing on to the viscera forms certain folds, or 

 ligaments. Thus, it forms below and on each side of the urinary bladder 

 a double fold, the inferior and lateral ligaments of the organ (Plate 44). 



2a 



