THE SMALL INTESTINE. 399 



their free extremities. A continuation of the villous basement- 

 membrane forms the membrana propria of the crypts of Lie- 

 berkuhn. External to this we find the surrounding connective 

 tissue, which is disposed in reticula, containing many leuco- 

 cytes in its meshes. Hence it is also known as adenoid tissue. 



The blood-vessels enter and leave the intestine at the me- 

 senteric margin. The arteries, generally accompanied by one 

 or two veins, pierce the muscle-coat, giving off branches which 

 form networks in those layers, then enter the submucosa, where 

 they run parallel to the surface of the mucous membrane, and 

 finally send off vertical arfcerioles at the base of the villi. The 

 latter ascend on one side of the villus, and then suddenly 

 divide into a dense capillary network. This division takes 

 place near the middle, the capillaries then spreading out to the 

 apex and periphery. Here they become quite superficial, being 

 covered by the epithelial lining only. The venous rootlets of 

 the villus are generally two, or even three in number. About 

 the glands and follicles we encounter special networks with 

 variously shaped meshes. 



Lymphatics are found in all the layers of the intestinal 

 canal. Those of the serous coat empty into the large mesen- 

 teric trunks. In an inward direction we also find a network of 

 lymph-capillaries between the two layers of the muscle-coat. 

 The submucous layer contains the perifollicular lymph-sinuses 

 situated at the base of these bodies, and a reticulum of larger 

 channels, many of which are found provided with valves. The 

 lymphatics of the mucous membrane are present in the shape 

 of capillary networks surrounding the intestinal glands. 



In the villi we note, as already stated, one or more central 

 lacteals, communicating at the base of these structures with 

 the lymph-vascular networks situated around and between the 

 glands. 



The nerves of the intestine are known as the plexus of 

 Auerbach, and of Meissner. The former, situated between 

 the circular and longitudinal fibres of the musculosa, is com- 

 posed of flattened nerve-branches, made up of numerous ulti- 

 mate fibrils. Small nodules, containing characteristic gan- 

 glion-cells, are also found, while little twigs are given off from 

 the plexus myentericus, to be distributed to the layers of the 

 musculosa. 



The plexus of Meissner is situated in the submucous tis- 



