ALIMENTARY CANAL AND ITS APPENDAGES 



409 



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 (^V to ^j inch) in length; some are conical and 

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

 ameter (Fig. 132). 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 



FIG. 132. Villi of the small intestine; magnified about 80 diameters. In the 

 right-hand figure the lacteals, a, b, c, are filled with white injection; d, blood- 

 vessels. In the left-hand figure the lacteals alone are represented, filled with a 

 dark injection. The epithelium covering the villi, and their muscular fibers, are 

 omitted. 



incomplete layer of plain muscular tissue, continuous below with a 

 muscular stratum forming the deepest layer of the mucous mem- 

 brane and named the muscularis mucosce. In the center is an off- 



