178 TEXT-BOOK OF PHYSIOLOGY. 



These investigations show that different portions of the stomach walls 

 exhibit different forms of activity, which for convenience of description are 

 separately described by Cannon as follows: 



i. The Movements of the Pyloric Part. Within five minutes after 

 a cat has finished a meal of bread there is visible near the duodenal end 

 of the antrum a slight annular contraction which moves peristaltically to 

 the pylorus; this is followed by several waves recurring at regular intervals. 

 Two or three minutes after the first movement is seen, very slight constric- 

 tions appear near the middle of the stomach, and, pressing deeper into the 

 greater curvature, course slowly toward the pyloric end. As new regions 

 enter into constriction, the fibers just previously contracted become relaxed, 

 so that there is a true moving wave, with a trough between two crests. When 

 a wave swings round the bend in the pyloric part, the indentation made by 

 it deepens; and as digestion goes on the antrum elongates and the constrictions 

 running over it grow stronger, but, until the stomach is nearly empty, they 

 do not entirely divide the cavity. After the antrum has lengthened, a wave 

 takes about thirty-six seconds to move from the middle of the stomach to 

 the pylorus. 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. Be- 

 tween the rings of constriction the stomach is bulged out, as shown in the 

 various outlines in Fig. 77. 



Movements of the Pyloric Sphincter. During the first ten or fifteen 

 minutes after the first constriction of the antrum the pylorus is 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 

 pylorus opens depends apparently on the degree to which the food is softened. 

 When the food is hard, the pylorus closes 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 nerve system. 



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

 antral part 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 antrum to succeed that which 

 has been prepared and ejected into the duodenum. As the pre-antral tube 

 is emptied of its contents the longitudinal and circular fibers of the fundus 



