DIGESTION 181 



empty, they do not entirely divide the cavity. After the pyloric region has 

 lengthened, a wave takes about thirty-six seconds to move from the middle of 

 the stomach to the pyloric orifice. At all periods of digestion the waves 

 recur at intervals of almost exactly ten seconds. It results from this rhythm 

 that when one wave is just beginning several others are already running in 

 order before it. Between the rings of constriction the stomach is bulged 

 out, as shown in the various outlines in Fig. 74. 



2. The Movements of the Sphincter Pylori. During the first ten or fifteen 

 minutes after the introduction of food the pyloric orifice is more or less 

 tightly closed. After this period it opens at irregular intervals to permit the 

 passage of liquefied food which is ejected by peristaltic waves for a distance 

 of two or three centimeters into the duodenum. The frequency with which 

 the pyloric orifice opens depends apparently on the degree to which the food 

 is softened. When the food is hard, the orifice is closed more tightly and 

 remains closed a longer period than when it is soft. 



The physiologic cause for the relaxation or inhibition of the sphincter 

 pylori appears to be the presence of free acid at the pylorus; its contraction, 

 the presence of free acid in the duodenum. With the neutralization of the 

 acid in the duodenum, its influence on the sphincter muscle is weakened, after 

 which the muscle again becomes susceptible to the inhibitor influence of the 

 acid within the stomach. It is probably for this reason that carbohydrates, 

 which do not absorb the acid, are discharged from the stomach early; that 

 the proteins, which postpone the appearance of free acid, are retained longer 

 and that fats, which check the secretion of gastric juice are discharged 

 slowly (Cannon). It should be emphasized, however, that the relaxation 

 and contraction of the pyloric sphincter, due to the action of free acid on 

 the gastric and duodenal sides, respectively, can take place independently 

 of the central nerve system, and through the intermediation of a local reflex 

 nerve mechanism, the myenteric plexus, in the walls of the pylorus. 



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. 10.32 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. 



