182 TEXT-BOOK OF PHYSIOLOGY 



This explanation of the opening of the pylorus is believed by its advocates 

 to supersede a former view, yet widely held, however, that the pylorus re- 

 mains open until closed reflexly from the duodenum by the passage into it 

 of acid, coarse food particles or other irritants from the stomach. When 

 these irritants are neutralized or removed, the pylorus relaxes, to be closed 

 again by another duodenal reflex. This intermittent opening and closing 

 continues until the stomach empties. Recent investigations apparently 

 suggest this interpretation of the mechanism to be the one that obtains for 

 the human pylorus. In these experiments the stomach was found to empty 

 more quickly when its contents were of a weak alkaline reaction than 

 when acid was present in the gastric contents. 



If the pylorus is more or less open during the early stages of digestion it 

 is easier to understand how the intestinal juices can be regurgitated into the 

 stomach, as appears to be experimentally proven by the investigations of 

 Boldyreff, Spencer and his co-workers with the result of neutralizing the 

 superfluous acidity of the gastric juice. The initial high acidity of this fluid, 

 viz. : 0.32 to 0.48 causes, on its passage into the duodenum, a secretion of 

 pancreatic juice and bile and the development of an anti-peristaltic wave or 

 rhythmic pulsations which drives these alkaline fluids into the stomach until 

 the acidity is brought down to that point where the juice is no longer 

 irritating to the duodenal mucous membrane. 



3. The Movements of the Cardiac Portion. As digestion proceeds, the pre- 

 vestibular or cardiac portion of the stomach elongates and assumes the shape 

 of a tube, which becomes the seat also of peristaltic constriction waves. As 

 a result, some of the food is gradually forced into the pyloric region to succeed 

 that which has been prepared and ejected into the duodenum. As the pre- 

 vestibular tube is emptied of its contents the longitudinal and circular fibers 

 of the fundus 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. 76. The fundus acts as a reservoir for the food and 

 forces out its contents a little at a time as the vestibular mechanism is ready 

 to receive them. Since peristaltic movements are absent from the fundus 

 portion, the food is not mixed with gastric juice, and therefore salivary diges- 

 tion can continue for a considerable period. There is no evidence of a circu- 

 lation of food in the stomach as sometimes described. On the contrary, the 

 movement through the elongated tube is in general a progressive though an 

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

 advanced toward the pyloric orifice, 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 pyloric orifice 

 opens, 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 Jhat in human beings the walls of the stomach 

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

 described. Evidence of this character has been furnished by experiments 

 conducted in the author's laboratory by Spencer and Meyer. 



