90 PHYSIOLOGY. 



Structure of the Small Intestine. 



Like the stomach, the intestine has four coats : ( 1 ) the external 

 serous, (2) the muscular, (3) the submucous, and (4) the mucous 

 coat. The serous coat is furnished by the peritoneum. The mus- 

 cular coat is composed of two layers of pale, unstriped fibers, the ex- 

 ternal layer of longitudinal fibers, and the internal layer of circular 

 fibers. The submucous coat is thinner than that in the stomach, but 

 is also extensible. 



The mucous coat is thinner and redder than that of the stomach, 

 and, like it, has a columnar epithelium. It has folds of mucous and 

 submucous tissue, running in a transverse direction and in the shape 

 of a crescent, which are called the valvulae conniventes. These valv- 

 ulae are more abundant in the upper part of the small intestine, 



Fig. 20. Portion of the Wall of the Small Intestine, Laid Open to 

 Show the Valvulae Conniventes. (BBINTON, RAYMOND.) 



where they overlap the edges. As you go down the small intestine 

 you find the number of the valvulse gradually lessen, and in the ileum 

 they disappear. These folds are permanent. The minute elevations 

 called villi beset the mucous membrane of the small intestine and 

 even the valvulse conniventes. They give a velvety appearance to 

 the surface of the small intestine. In the upper part of the small in- 

 testine the villi appear as fine folds, but farther down the intestine 

 they appear as flattened, conical projections. The villi are 1 / 40 inch 

 in height, and in structure are appendages of the intestinal mucous 

 membrane. 



Villi.* 



Upon the surface of the villi you find an epithelium of regular 

 cylindrical cells. The border cells of the epithelium of the villi have 

 a broad, finely striated border which spreads over their ends like a 

 cuticle or mosaic. The other end of the cell often ends in a point 



* Szymonowics's Histology has been drawn upon in the description of 

 the villi, 



