THE MECHANISMS OF DIGESTION 499 



of them a rhythmic to-and-fro shifting of the masses of food along the 

 lumen of the gut, rapidly repeated for many minutes. 



When the intestines are floated out in a warm bath of saline solution, 

 it is seen that the rhythmic segmentation is caused by narrow rings of 

 contraction. Under such conditions also it is often noted that the 

 loops of intestine sway from side to side. The balloon method also re- 

 veals the presence of slight waves of contraction that pass rapidly along 

 the gut, and follow each other at the rate of twelve to thirteen per minute. 

 Both of the muscular coats of the intestine are involved, and it is believed 

 that the contractions are responsible not only for the pendular move- 

 ments but for the rhythmic segmentation observed by the x-ray method. 

 According to this view these movements are constantly passing along 

 the intestine, and become exaggerated by the mechanical stimulus which 

 is offered by the masses of food to such an extent that they divide the 

 masses into portions. The evidence for this belief rests on the fact that 



Fig. 161. Diagrammatic representation of the process of segmentation in the intestine. An 

 unbroken shadow is shown in / and its segmentation in 2. The dotted lines across each mass 

 show the position of division and in j is shown how new masses are formed by the split portions 

 coming together. (.From Cannon.) 



when the contraction is studied by the balloon method, it becomes marked 

 over the middle of the balloon, where the greatest tension exists. 



Several functions can be assigned to these movements. They cause 

 intimate mixture of the food with the digestive juices, and by bringing 

 ever new portions of food in contact with the mucosa, they encourage 

 absorption. They also have an important massaging influence on the 

 blood and lymph in the vessels of the intestinal walls. Indeed, the pas- 

 sage of lymph from the lacteals into the mesenteric lymphatics seems to 

 depend very largely upon these movements. 



The investigations of Alvarez 20 and his coworkers show that the rhyth- 

 mic intestinal contractions are not of uniform rate throughout the entire 

 length of the small bowel, but vary in frequency inversely with the dis- 

 tance from the pylorus. For example, the contractions of a segment 

 of the duodenum proceed at a more rapid rate (17-21 per minute) than 

 do the contractions of an ileal segment (10-12 per minute) under the 



