CHAPTER X 



THE SMALL INTESTINE: ITS MOVEMENTS, 

 SECRETIONS, AND DIGESTIVE PROCESSES 



THE intestinal content is propelled on its winding way 

 by peristaltic movements. These are similar in their 

 mechanical principle to the waves of muscular contraction 

 passing down the esophagus in the act of swallowing. 

 They are, however, much slower and gentler in character. 

 As a rule they do not run an uninterrupted course from the 

 pylorus to the ileocecal valve, though such a phenomenon 

 is an occasional possibility. More commonly a wave will 

 travel a limited distance and then fade out, leaving the 

 material which it was moving to rest for a time in some 

 depending loop. If this is true, the progress of the food is 

 intermittent; x-ray observations upon human subjects 

 have led to the estimate that it takes four or five hours for 

 the passage through the whole length of the small intestine. 

 This time may be assumed to vary widely with the indi- 

 vidual and the diet. Reduced to an average rate the data 

 quoted above give us about one inch per minute. 



Intestinal peristalsis, like the movements of the stom- 

 ach, is governed mainly by local mechanisms. Yet in this 

 case as in the other the central nervous system may exert 

 an influence tending to accelerate or to suppress the ac- 

 tivity of the muscular coats. The second or inhibitory 

 action is the more marked. A question much discussed 

 is whether the direction of peristalsis in the small intestine 

 is at all subject to reversal. The sum of the evidence at 

 present supports the view that such a reversal (antiperis- 

 talsis) is possible, but only under conditions which are 

 clearly abnormal. Ordinarily it is plain that the intestine 

 is distinctly specialized to act in one way rather than the 

 other. 



