THE PANCREAS 181 



The wall has a inucosa with Lieberkiihn glands. The eharaeteristic 

 feature is the ahiuuiaiice of lyni])h()i(l tissue which is j)resent, not 

 only as nodules in the nuicosa and submucosa, hut also as diffuse 

 tissue throughout the tiuiica propria. The circular coat of the 

 muscularis is well represented, hut the outer longitudinal coat is 

 thin. In some cases the glands may lie obliterated by lymphoid 

 tissue and the lumen filled with lymphocytes that penetrate 

 through the nnicosa. 



The Rectum and Anus of Mammals. — The rectum has long and 

 large tubular glands and solitary lymph nodules are common 

 in its mucosa and sulnnucosa. Longitudinal folds of the mucosa 

 appear in the lower part of the rectum, and with them occurs 

 a change in the character of the mucosa. The columnar epithe- 

 lium and simple tubular glands of the rectum change abruptly to 

 stratified squamous epitheliiun of the anus, and this epithelium 

 is continuous with the skin in much the same fashion as already 

 noted in the case of the lip at the other end of the digestive tract. 

 The muscularis of the rectum has two complete layers, and near the 

 anus striated muscle rejilaces the smooth muscle. 



The Cloaca. — In vertel)rates below mammals the rectum and uro- 

 genital ducts open into a common chamber, the cloaca. In addi- 

 tion to being a mere receptacle it shows modifications for transmis- 

 sion of genital products and there is considerable variation in its 

 structure. In fishes it is lined with stratified squamous ejiithelium 

 supported by dense fibroelastic connective tissue and some muscle. 

 In Amphibia it may be a sac lined with columnar cells, among which 

 are munerous goblet cells, or it may have glandular linings associated 

 with reproductive activities. Among the reptiles and birds, the 

 \'entral wall of the cloaca may be developed into organs for copula- 

 tion and portions of the wall may be glandular. 



The anus or cloacal opening is lined with stratified epithelium 

 similar to that of the skin and is supported by loose fibroelastic 

 connecti\'e tissue joining with the muscles underlying the skin of 

 this region. 



THE PANCREAS. 



The pancreas has been called the salivary gland of the abdomen, 

 and at first glance resembles a parotid (see Parotid) in the serous 

 appearance of its secretory cells. It is a compound tubulo-alveolar 

 gland lying behind the stomach and attaching to the duodenal wall. 

 No distinct capsule covers it, and its lobules are separated by a 



