DIGESTION 183 



impaired and peristalsis fails to develop. After a variable period the neuro- 

 muscular mechanism develops a tonus like that given to it by the vagi, after 

 which the usual peristalsis returns. When once the peristalsis is well devel- 

 oped in digestion, division of the vagi has no effect. (Cannon.) 



The Nerve Mechanism of the Stomach. In preceding paragraphs 

 it was stated that during the period of gastric digestion the food is retained 

 in the stomach because of the closure of the cardia (the esophago-gastric 

 orifice) and of the pylorus (the gastro-duodenal orifice) both orifices being 

 tightly closed by the tonic contraction of sphincter muscles; that both sphinc- 

 ters relax from time to time, the one to permit the entrance of food into the 

 stomach for further digestion, the other to permit the exit of food into 

 the intestine after its more or less complete digestion, after which in both 

 instances the sphincters again contract and close the orifices; that the fundus 

 muscles are tonically contracted and steadily pressing' the food into and 



FIG. 76. GASTRIC CONTRACTIONS DURING DIGESTION. (Two hours after eating.) 

 The upper tracing shows respiratory excursions, the variation in the bases of which records the 

 changes in intragastric pressure. The heavier markings at the bases of the respiratory tracings 

 are heart-beats. Time marked in minutes. Analysis of the gastric contractions divides them into 

 long tonus waves of about 60 to 90 seconds duration, superposed upon which are shorter peristaltic 

 waves about 20 seconds in duration. 



The lower tracing shows gastric digestive contractions with heart-beats, taken while holding the 

 breath at the end of an expiration. The intervals represent periods of respiration during which the 

 pen was removed from the drum. Were the intervals connected, a regular sequence of gastric tonus 

 waves with peristaltic waves superposed would appear. 



through the cardiac region to the vestibule; that the pyloric or vestibular 

 muscles are vigorously active throughout the digestive period, triturating 

 the food, mixing it with gastric juice, and finally driving it through the 

 temporarily open pylorus into the intestine. 



These separate but related groups of muscle-fibers, by reason of their 

 physiologic endowments, and possibly by virtue of the presence of local nerve 

 mechanisms, exhibit activities which are independent of the central nerve 

 system. Thus the isolated stomach of the dog and of other animals as well, 

 if kept warm and moist, will exhibit rhythmic movements for a period of 

 time varying from an hour to an hour and a half. Though nerve-cells and 

 nerve-fibers (Auerbach's plexus) are present in the walls of the stomach be- 

 tween the layers of muscle-fibers, it is not believed that they are the immedi- 

 ate sources of the stimulus to the contraction, though they may act as a coordi- 

 nating mechanism. 



