98 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION 



third of the ileum and greatly increase the length of the mucous 

 membrane over that of the gut proper. They are not effaced 

 by the passage of food or by other circumstances, for the two 

 surfaces of the fold which are in apposition are bound together 

 by loose connective tissue. The fold as a whole, however, is 

 freely movable upward or downward in the intestine and has no 

 tendency to obstruct the canal. The only function of the val- 

 vulae conniventes is to furnish a greater secreting surface and, 



FIG. 39. Diagram of a longitudinal section of the wall of the small intestine. 



a, villi; b, Lieberkuhn's glands; c, tunica muscularis mucosse, below which lies 

 Meissner's nerve plexus; d, connective tissue in which many blood and lymph vessels 

 lie; e, circular muscle fibers cut across with Auerbach's nerve plexus, below it;/, 

 longitudinal muscle fibers; g, serous coat. (Yeo.) 



by somewhat retarding the passage of the alimentary mass, to 

 subject it for a longer time to the digestive fluids. 



2. The villi are especially important in connection with 

 absorption, and their description properly belongs under that 

 head. They are conical elevations responsible for the velvety 

 character of the mucous membrane. They exist in great num- 

 bers from the pylorus to the ileo-cecal valve, covering the valvulae 

 conniventes as well as the general surface of the mucous mem- 

 brane. The largest are about -fa in. long and T ^ m - m diameter 

 at their base. They are only elevations of the mucous membrane 

 containing a central tube, the lacteal, which is nothing but an 



