340 THE HUMAN BODY. 



They are first found about two inches from the pylorus, and 

 are most thickly set and largest in the upper half of the 

 jejunum, in the lower half of which they become gradually 

 less conspicuous; and they finally disappear altogether about 

 the middle of the ileum. The folds serve greatly to increase 

 the surface of the mucous membrane both for absorption and 

 secretion, and they also delay the food somewhat in its pas- 

 sage, since it must collect in the hollows between them, and 

 so be longer exposed to the action of the digestive liquids. 

 Examined closely with the eye or, better, with aid of a lens, 

 the mucous membrane of the small intestine is seen to be not 

 smooth but shaggy, being covered everywhere (both over the 

 valvulae conniventes and between them) with closely packed 

 minute processes, standing up somewhat like the 4 ' pile "on 

 velvet, and known as the villi. Each villas is from 0.5 to 0.7 

 millimeter (-fa to fa inch) in length; some are conical and 

 rounded, but the majority are compressed at the base in one 

 diameter (Fig. 115). In structure a villus is somewhat com- 

 plex. Covering it is a single layer of columnar epithelial cells, 

 the exposed ends of the majority having a peculiar bright 

 striated border and being probably of great importance in ab- 

 sorption. Mixed with these cells are others in which most of 

 the cell has become filled with a clear mass which does not 

 stain readily with reagents; the deep narrow end of the cell 

 stains easily and contains the nucleus. From time to time the 

 clear substance (mucigen) is converted into mucus and dis- 

 charged into the intestine, leaving behind only the nucleus and 

 the protoplasm around it. These reconstruct the cell and form 

 more mucigen. These mucus-forming cells are named goblet* 

 cells, from their shape. Beneath the epithelium the villus may 



FIG. 114. A portion of the small intestine opened to show the valvulae, conniventes. 



be regarded as made up of a framework of connective tissue, 

 mainly of the adenoid variety (Chap. XXIII), supporting the 



