THE DIGESTIVE APPARATUS AND NUTRITION 225 



intestine, each separate valvula reaching about one half 

 or two thirds the way round. Their function is to fur- 

 nish a large amount of secreting and absorbing surface. 



318. The Villi. Still further to increase this surface, 

 the valvulse are covered with villi. These are minute 

 projections from one fiftieth to one eighth of an inch 



Fig. 112. Diagrammatic representation of a small area of the mucous 

 membrane of the small intestine. 



1 cellular structure of the epithelium, 



or outer layer. 



2 a vein. 



8 fibrous layer. 



4 villi covered with epithelium. 



5 a villus in section, showing its 



covering of epithelium, and its 

 blood vessels and lymphatics. 



6 a villus partly uncovered. 



7 a villus stripped of its epithelium. 



8 lymphatics or lacteals. 



9 orifices of the glands opening be- 



tween the villi. 

 10, 11, 12 glands. 

 13 capillaries surrounding the orifice 



of a gland. 



in length, and are found only in the small intestine (Fig. 

 112). They give to its mucous lining a peculiar velvety 

 appearance. Each vittus is covered with columnar epithe- 

 lium resting on a fine membrane. Within are blood vessels 



