MOVEMENTS OF THE ALIMENTARY CANAL 477 



segments; an instant later these constrictions disappear, and new 

 ones, midway between the first, are formed, by which the food is 

 again segmented, but in a shifted position. These rhythmic move- 

 ments may recur as often as thirty times a minute. Their effect is 

 to bring every particle of the contained food into intimate con- 

 tact with the intestinal walls, insuring thorough mixing with the 

 intestinal secretions, and also favoring absorption. 



The onward movement of the food is secured by peristaltic waves 

 which start at the pylorus and run rather slowly along the intes- 

 tine. They are normally gentle movements, which do not carry 

 the food bodily before them, but move it forward little by little. 

 During digestion the two sorts of movements alternate more or 

 less irregularly. After the segmentation has churned a food-mass 

 thoroughly in one section it dies away and a peristaltic wave de- 

 velops, which carries the food ahead of it into a fresh section; then 

 the peristalsis, in turn, subsides, and segmentation is resumed. 



The mechanism of these intestinal movements is not entirely 

 clear, the peristaltic waves, and possibly also the segmentations, 

 are special manifestations of the myenteric reflex described above, 

 but the conditions that govern their appearance and disappear- 

 ance, first in one part of the intestine and then in another, are 

 not known. 



Observations with the X-rays have shown that the rate of prog- 

 ress of the food through the human small intestine is about 4J^ 

 feet in the hour, so that the first food from any meal may appear at 

 the ileocolic valve about 4J^ hours after it begins to leave the 

 stomach. 



Extrinsic Control of Stomach and Intestinal Movements. It 

 has been shown that normal movements of both stomach and in- 

 testine may go on in animals in which the nerves leading to these 

 organs from the central nervous system are cut. To a certain ex- 

 tent, therefore, they, like the heart, contain within themselves the 

 essential requirements for normal activity. Like the heart, how- 

 ever, they are subject to reflex control through the central nervous 

 system. 



The vagus nerves carry cranial autonomic fibers which when 

 stimulated arouse the stomach and intestine to activity. The op- 

 posing thoracico-lumbar autonomies, which, as we have already 

 seen (p. 455), come by way of the splanchnics, are inhibitory. A 



