THE ALIMENTARY CANAL AND ITS APPENDAGES 453 



less conspicuous; and they finally disappear altogether about the 

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

 face of the mucous membrane both for absorption and secretion, 

 and they also delay the food somewhat in its passage, 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 every- 

 where (both over the valvulae conniventes and between them) 

 with closely packed minute processes, standing up somewhat like 

 the "pile" on velvet, and known as the villi. Each villus is from 

 0.5 to 0.7 millimeter (^ to ^ inch) in length; some are conical 



FIG. 130. A portion of the small intestine opened to show the valvulce conniventes. 



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

 diameter (Fig. 131). In structure a villus is somewhat complex. 

 Covering it is a single layer of columnar epithelial cells, the ex- 

 posed ends of the majority having a peculiar bright striated border 

 and being probably of great importance in absorption. 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 discharged 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 be regarded as made up of a framework of connective tissue, 

 supporting the more essential constituents. Near the surface is 

 an incomplete layer of plain muscular tissue, continuous below 

 with a muscular stratum forming the deepest layer of the mucous 

 membrane and named the muscularis mucosce. In the center is an 



