MANUAL OF OATTLE-EEEDHSTG. 67 



lip into tlie circulation by tlie resorbent vessels wliicli line 

 the stomach and intestines. 



The Epithelium. — In all vertebrate animals, the whole 

 surface of the intestines, from end to end, is covered with 

 so-called epithelial cells, which are remarkably similar in 

 all animals. These cells are roughly cylindrical, and are 

 thickly crowded together, leaving no spaces between tliem. 

 They are separated from each other by a cell wall, but are 

 open toward the interior of the intestines, and also, ac- 

 cording to some authorities, communicate on the other 

 side with the lacteals. 



The cells contain a soft mass of protoplasm, w^hich, wheA 

 resorption is not going on, bears on its intestinal surface 

 minute upright fibres, which give the surface of the intes- 

 tines a velvety appearance. During resorption, however, 

 these fibres nearly disappear into the main part of tlie cell 

 contents. 



The YiBi, — In the higher animals the extent of resorb- 

 ing surface in the intestines is greatly increased by various 

 folds and projections of its surface, of which the most 

 important are the villL These are little conical, round, or 

 club-shaped protuberances of the inner surface of the in- 

 testines. They are covered, like all parts of the intestinal 

 surface, with the epithelial cells just described, and under- 

 neath these thei^e is said to be a fine membrane. Beneatli 

 this membrane there are found numerous minute capiUaiy 

 blood-vessels, a layer of smooth (involuntary) muscular 

 fibres, and a net-work of nerves. All three layers fol- 

 low the epithelium of the intestines in all its folds and 

 projections, and thus in the villi take somewhat the 

 shape of a glove-finger. In the centre of each of the 

 villi ends a vessel called a lacteal, belongij% to the 

 lymphatic system. 



