122 BIOLOGY OF THE LABORATORY MOUSE 



by cuboidal epithelium with spherical nuclei and pale staining cytoplasm. 

 Their excretory ducts open at the bases of the intestinal glands. In the 

 submucosa near the entrance of the pancreatic duct small groups of pan- 

 creatic acini are often present. An inner well-developed circular and a 

 thin outer longitudinal smooth muscle coat covered by the serosa complete 

 the wall. 



Solitary lymph nodules occur in the lamina propria of the small intestine. 

 Aggregations of lymph nodules known as Peyer's patches also occur. 

 Where these are present the villi are absent or shortened. These aggre- 

 gated nodules extend into the submucosa and are covered only by the thin 

 muscle coat and serosa. They cause a bulging of the outer surface which is 

 visible grossly. Our observations are in accord with those of Hummel (54) 

 in rats, who found that variations exist in the number and location of the 

 patches, although in position the nodules are usually opposite the attach- 

 ment of the mesentery. 



Each villus usually contains a central, endothelial-lined, lymphatic 

 vessel, the lacteal, which drains the absorbed fat or the white lymph (chyle). 

 The lacteals anastomose with lymph vessels of the muscularis mucosae. 



The large intestine. — The large intestine consists of the caecum, the 

 colon and the rectum. 



The caecum. — The caecum is a curved, blindly ending sac which com- 

 municates with the ileum and the colon. At its inner curvature it has 

 several transverse folds, while most of its surface is smooth. Villi are not 

 present. The epithelial cells of the lumen and the glands are like those in 

 other parts of the large intestine. The lamina propria of the proximal part 

 contains few lymphocytes. The muscularis mucosae is well developed and 

 takes part in the formation of the transverse folds. The distal, blind end 

 contains an aggregation of lymphatic tissue between the surface epithelium 

 and muscularis externa. The inner circular, smooth muscle layer of the 

 muscularis externa is well developed, while the outer longitudinal layer is 

 thin. The serosa is like that of the small intestine. The caecum does not 

 end in a typical vermiform process. 



The colon and the rectum. — The colon and the rectum are devoid of 

 villi. Except that the tubules are straighter and slightly longer, the glands 

 of the large intestine are similar in structure to those of the small intestine. 

 Goblet cells are present in large numbers, while the cells of Paneth are 

 absent. The free surface between glands is covered by simple columnar 

 epithelial cells with striated borders. The lamina propria contains the 

 same elements described in the small intestine. The muscularis mucosae is 



