98 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION 



length of the mucous membrane over that of the gut proper. 

 They are not effaced by the passage of food or by other cir- 

 cumstances, for the two surfaces of the fold which are in ap- 

 position are bound together by loose connective tissue. The 

 fold as a whole, however, is freely movable upward or down- 

 ward in the intestine and has no tendency to obstruct the 

 canal. The only function of the valvulae conniventes is to 

 furnish a greater secreting surface and, by somewhat re- 



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



intestine. 



a, villi; b, Lieberkuhn's glands; c, tunica muscularis mucosae, 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; f, longitudinal muscle fibers; g, serous coat. (Yeo.) 



tarding 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 

 numbers from the pylorus to the ileo-cecal valve, covering 

 the valvulse conniventes as well as the general surface of the 

 mucous membrane. The largest are about ^o in. long and ^o 

 in. in diameter at their base. They are only elevations of the 

 mucous membrane containing a central tube, the lacteal, 

 which is nothing but an intestinal lymphatic. The structure 



