120 



DIGESTION 



The serous coat is formed of the fold of the mesentery. 

 The muscular coat is in two layers, an outer one of longi- 

 tudinal fibers and an 

 inner of circular fibers. 

 The submucous coat of 

 coimective tissue binds 

 together the muscle 

 and mucous membrane 

 layers just as in the 

 stomach, and receives 

 the blood vessels which 

 enter in a fold of the 

 mesentery. The mu- 

 cous coat which lines 

 the whole intestine is 

 pink, soft, and filled 

 with minute vessels of 

 the blood and lym- 

 phatic system. In the 

 upper part of the in- 

 testine it is perma- 

 nently folded into 

 ridges known as the 



Frs. 40 -Wall of small intestine; a, 6, mucous i}alvulm COimiventeS, 



membrane with projecting villi ; c, eZ, connec- i • -i • 



tive tissue layers (submucosa) ; e, circular WUlCU increase tnC 



muscle fibers; eMongitudinal muscle fibers; digestive SUrfaCe and 

 /, serous membrane ; g, spaces between villi ; ° 



ft, opening of crypis or simple intestinal glands; delay the paSSage of 



i, blood vessels in villi. , , „ , , . , -^ 



the food particles. In 

 the lower part these ridges disappear, and to the eye it 

 appears smooth. Examined carefully with a lens the 

 entire inner surface of the intestine is seen to be raised 

 in minute projections called villi, like the pile of velvet. 



