196 NORMAL HISTOLOGY AND ORGANOGRAPHY. 



inches long, about one-third inch broad and one- 

 eighth inch thick. The inner surface of the small in- 

 testine is thus thrown up in a series of shelves. This 

 mechanism has an analogue in the typhlosole of the 

 earthworm and the spiral valve of some fishes. 



(6) Villi. These are tongue-like elevations of the 

 mucosa one-thirtieth to one-fortieth inch in height, 

 and barely visible to the naked eye. They are 

 found on both sides of the valvulae conniventes 

 and on the general surface of the mucosa. Collec- 

 tively they give the surface a velvety appearance. 

 The villi are most numerous in the upper part of the 

 intestine, where they number fifty to eighty to the 

 square inch. They are longer but more slender and 

 less numerous in the ileum, where they number 

 forty to sixty to the square inch. Their total num- 



Fig. 145. a t Cross section of a villus; b, cross section of crypt of Lie- 

 berkiihn. 



ber in the small intestine is estimated at 4,000,000. 

 Each villus has a lining of simple columnar 

 epithelium which covers a connective-tissue core. 

 A few smooth muscle fibers enter this core from 

 the muscularis mucosa. In addition there is a 

 rich blood supply and a central lymphatic duct. 

 The latter is a part of the lymphatic system of 

 the intestine known as lacteals because of the milky 



