DISSECTION OF THE PELVIS. 359 



placed between the rectum and the posterior part of the upper face of 

 the bladder. Each is a small ovoid sac, like a miniature bladder. The 

 anterior end of the sac is rounded and free ; the posterior end contracts, 

 and unites with the vas deferens to form the ejaculatory duct. Only the 

 anterior half of the vesicula is covered by peritoneum, which in passing 

 between the two bodies forms a small triangular serous fold. 



The Common Ejaculatory Ducts. Each of these is a short tube 

 formed under cover of the prostate, by the union, at a very acute angle, 

 of the neck of the vesicula seminalis with the vas deferens. Its opening 

 into the roof of the urethra will be presently exposed. 



Directions. — Carefully raise the fundus of the bladder, and cut its 

 peritoneal and connective-tissue adhesions to the sides and floor of the 

 pelvis. Free, in the same way, the membranous urethra at the ischial 

 arch ; and cut the crus penis and its erector muscle from the tuber 

 ischii. This will enable the dissector to remove from the pelvis the 

 organs just described, while maintaining their mutual relations. Lay 

 the bladder on a table with its upper or rectal aspect downwards, and 

 open it by a mesial incision on its lower face. Carry the incision 

 backwards into the urethra, so as to open the whole extent of its 

 prostatic and membranous portions. Care must be taken that the 

 incision in both bladder and urethra is on the inferior face. 



Structure of the Bladder. This comprises four coats : — 



1. The Serous or Peritoneal Coat. This, as already seen, is an incom- 

 plete investment. 



2. The Muscular Coat is composed of bundles of non-striped fibres 

 arranged in all directions. Compared with its condition in many other 

 animals, this coat is very thin ; and its fasciculi in the distended 

 bladder seem hardly to form a continuous layer. At the neck of the 

 bladder some of the fibres have a circular disposition, forming the 

 sphincter vesical. 



3. The Submucous Coat is composed of vascular areolar connective- 

 tissue, and it loosely unites the muscular and mucous coats. 



4. The Mucous Coat. This forms a complete internal lining for the 

 bladder, and in the empty viscus it is thrown into folds, or rugce. 

 Observe the slit-like orifices of the ureters, near one another and a little 

 anterior to the urethral orifice (Fig. 50). Pass a probe or bristle into 

 one of them, and notice that the ureter perforates the wall very 

 obliquely — an arrangement which has a valvular action in preventing 

 the regurgitation of urine from the distended bladder. Between the 

 uretral and urethral orifices in the human subject is a triangular area 

 — the trigone — over which the mucous membrane is smooth even in the 

 contracted bladder. In the horse, however, this area is wrinkled like 

 the rest of the surface. The epithelium of the mucous membrane is 

 stratified and transitional. 



