DIGESTION. 179 



steadily contract and gradually force its contents into the tubular portion. 

 This continues until the fundus is completely emptied. The changes in 

 shape which the cardiac portion undergoes during digestion are represented in 

 Fig. 77. The fundus acts as a reservoir for the food and forces out its 

 contents a little at a time as the antral mechanism is ready to receive them. 

 Since peristaltic movements are absent from the cardiac portion the food is 

 not mixed with gastric juice, and therefore salivary digestion can continue 

 for a considerable period. There is no evidence of a circulation of food 

 in the stomach as sometimes described. On the contrary, the movement 

 through the pre-antral tube and antrum is in general a progressive though 

 an oscillating one. As the constriction waves rapidly pass over the food it is 

 advanced toward the pyloric opening, but as this is closed the food is forced 

 backward through the advancing constricted ring for a variable distance. 



The effect of the constriction waves is to mix the food with the gastric 

 juice, triturate and soften it. So soon as this is effected, the pylorus relaxes, 

 when the advancing constriction wave expels it into the intestine. With its 

 expulsion room is afforded for an additional quantity of 'food, and hence 

 there is a general advance of the food mass toward the pylorus. 



Though these observations were made on the cat, evidence is accumu- 

 lating which goes to show that in human beings the walls of the stomach 

 exhibit constriction waves which are similar in all respects to those above 

 described. 



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 

 pyloric or antral 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 

 endowments, and possibly by virtue of the presence of local nerve mechan- 

 isms, 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 vary- 

 ing 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 between 

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

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

 ing mechanism. The stimulus in all probability develops in the muscle- 

 fiber itself and is therefore myogenic. 



The sphincter cardia muscle surrounding the esophago-gastric orifice 

 is always, under normal conditions, tonically contracted and the orifice 

 closed. This contraction is partly due to inherent causes as shown by the 

 fact that it persists for from 24 hours to several days after division of all 



