INTESTINAL DIGESTION 205 



principle is, nevertheless, the same in the two places. 

 The low rate in the small intestine is due partly to slower 

 muscular action but also to the fact that it is a dis- 

 continuous movement; a given collection of food is often 

 at rest for a while. 



A peristaltic wave in the small intestine as in the 

 esophagus seems to have two phases, a region of less- 

 ened tone running ahead of a ring of contraction. 

 The nervous mechanism of the intestinal wall is capable 

 of actuating peristalsis without the aid of the central 

 nervous system. It may be said of the small intestine 

 as of the stomach that inhibition is more commonly 

 exercised by the central gray matter than augmentation, 

 although both are possible. Where there is an accumu- 

 lation of food and secretions a peristaltic wave is likely 

 to make its appearance and to push the contents along 

 for some 'distance, but soon the energy of the propulsion 

 seems to flag and there is a fresh period of rest. The 

 distention of the tube by the food and the degree of tone 

 prevailing in its wall at the moment determine whether 

 or not a peristalsis shall be developed. 



Rhythmic Segmentation. This name is given to a type 

 of movement often seen in the small intestine which 

 does not definitely urge on the contents. A series of 

 tight contractions will appear upon a loop with small, 

 slack pouches between. After a moment there will be 

 relaxation where at first there was contraction and con- 

 traction in the intermediate zones which were previously 

 flaccid. The alternation continues for some time and 

 is a relatively brisk action for smooth muscle. Its effect 

 is to slip small quantities of the food back and forth 

 within a short space, mixing them with the juices and 

 shifting their contact with the absorbing cells. The 

 massaging effect upon the blood-vessels and lymph spaces 

 of the intestinal wall may be an important feature of 

 this reaction. 



The Fluid Exchanges of the Intestine. During diges- 

 tion the secretions enter the canal and the water of which 



