194 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



transverse colon turns caudad the mesocolon is fused secondarily to the 

 mesogaster. 



The urinary bladder is a sac occupying the posterior end of the peritoneal 

 cavity, immediately internal to the body wall and ventral to the large intestine. 

 From the ventral surface of the bladder a mesentery, the median ligament of the 

 bladder (median umbilical fold of human anatomy) extends to the median ventral 

 line and here continues forward for some distance. Near the exit of the bladder 

 from the peritoneal cavity there is on each side a slightly developed ligament, 

 the lateral ligament of the bladder. 



The terminal portion of the descending colon is the rectum. Both the rectum 

 and the duct of the bladder pass to the exterior through the ring formed by the 

 pelvic girdle and vertebral column. They will be followed at a later time in 

 connection with the urogenital system. For the present it may be stated that 

 the rectum and urogenital ducts are completely separated from each other. A 

 cloaca is therefore absent. 



The small bodies which may have been noted in the mesentery, usually buried 

 in fat, are lymph glands, parts of the lymphatic system. Small portions of 

 lymphatic tissue called lymph nodules are also abundantly present in the wall 

 of the intestine. Aggregations of lymph nodules are known as Peyer's patches. 

 Peyer's patches are thickened oval spots on the surface of the intestine, of 

 slightly different color from the rest of the intestinal wall. Look for them. 

 They occur in the rabbit along the entire small intestine on the side opposite 

 that attached to the mesentery; there is a larger patch at the place of junction 

 of sacculus rotundus and caecum. The walls of the sacculus rotundus, caecum, 

 and vermiform appendix are composed almost entirely of lymph nodules. In 

 the cat, Peyer's patches are readily found as oval light-colored spots along the 

 colon. 



Slit open various parts of the digestive tract along the side opposite the attach- 

 ment of the mesentery. Wash out the interior. In the cat note the marked 

 ridges or rugae in the wall of the stomach; these are very slight in the rabbit. 

 Cut through the pylorus and note the thickened ridge or pyloric valve at 

 this place. In the wall of the small intestine observe the velvety appearance 

 due to the villi (finger-like projections of the mucous membrane). Find also 

 the depressions marking the positions of the lymph nodules. In the rabbit slit 

 open the sacculus rotundus, caecum, and appendix, and note the spotted appear- 

 ance of the interior owing to the lymph nodules composing the walls. In the 

 interior of the caecum is a spiral ridge. Cut through the junction of large and 

 small intestine and note in both animals an elevation, the ileocolic valve, projecting 

 into the ileum. 



H. THE COMPARATIVE ANATOMY OF THE COELOM AND THE MESENTERIES 



Since the development and comparative anatomy of the coelom and mesenteries are 

 extremely difficult and complicated subjects, it seems advisable at the risk of repetition to 



