128 



THE HUMAN MECHANISM 



which have thus far escaped digestion ; in addition there are 

 certain constituents of the digestive juices which are not 

 absorbed and some (for example, certain constituents of the 

 bile) which are distinctly excretory products. 



Special provision seems to be made to insure the approxi- 

 mately complete digestion and absorption of proteins, carbo- 

 hydrates, and fats before the food enters the large intestine. 

 The opening from the small into the large intestine is guarded 



by a circular muscle, the ileo- 

 colic sphincter, which ordinarily 

 prevents the passage of food out 

 of the small intestine much as 

 the passage of food from the 

 stomach is regulated at the 

 pylorus (p. 113). In this man- 

 ner considerable accumulations 

 of material may occur at the 

 end of the small intestine and 

 remain there for two hours or 

 more while the combined ac- 

 tion of enzymes and segmenting 

 movements completes the diges- 

 tion and absorption of the nu- 

 trients. Recent work indicates 

 that this material is discharged 

 periodically into the large intestine by a relaxation of the 

 ileocolic sphincter and a vigorous peristalsis in the terminal 

 portion of the small intestine. It would also seem that this 

 discharge is especially apt to occur when food is taken into 

 the stomach, as if there is a reflex to this discharging 

 mechanism. Obviously the end attained is the more complete 

 digestion of the food in the small intestine. 



Reference to Fig. 154 will show that the large intestine 

 consists of four parts, the ascending, transverse, and de- 

 scending colons and the rectum, there being an S-like bend 



FIG. 63. Longitudinal section of 

 the large intestine 



Note the absence of villi 



