THE ALIMENTARY CANAL AND ITS APPENDAGES. 341 



more essential constituents. Near the surface is an incomplete 

 layer of plain muscular tissue, continuous below with a mus- 

 cular stratum forming the deepest layer of the mucous mem- 

 brane and named the muscularis mucosce. In the centre is an 

 offshoot of the lymphatic system; sometimes in the form of a 

 single vessel with a closed dilated end, and sometimes as a net- 

 work formed by two main vessels with cross-branches. During 

 digestion these lymphatics are filled with a milky-white liquid 

 absorbed from the intestines, and they are accordingly called 

 the lacteals. They communicate with larger branches in the 

 submucous coat, which end in trunks that pass out through 

 the mesentery to join the main lymphatic system. Finally, 

 in each villus, outside the lacteals and beneath the muscular 

 layer of the villus, is a close network of blood-vessels. 



Opening on the surface of the small intestine, between 

 the bases of the villi, are small glands, the crypts of Lieber- 

 kiilin. Each is a simple unbranched tube lined by a layer of 

 columnar cells some of which have a striated free border, 

 though less marked than that on the corresponding cells of 



FIG. 115. Villi of the small intestine: magnified about 80 diameters. In the 

 right-hand figure the lacteals, a, 6, c, are filled with white injection ; d, blood-ves- 

 sels. In the left-hand figure the lacteals alone are represented, filled with a dark 

 injection. The epithelium covering the villi, and their muscular fibres, are omitted. 



the villi, and others are goblet-cells. The crypts of Lieber- 

 kdhn are closely packed, side by side, like the glands of the 

 stomach. In the duodenum are found other minute glands, 

 the glands of B runner. They lie in the submucous coat 



