DIGESTION l8l 



enlargement of the cardiac end of the stomach the term receptive relaxa- 

 tion has been given. 



The investigations of Cannon previously referred to, show that not only 

 the sphincter cadiae, but other portions of the stomach walls exhibit differ- 

 ent forms of activity which for convenience of description are separately 

 described by him as follows : 



1. The Movements of the Pyloric Portwn.Thz movements of this region 

 of the stomach were first studied and sketched by means of the Rontgen 

 rays by Cannon. Some of the changes in shape due to these movements 

 are shown in Fig. 76. Within five minutes after a cat has finished a meal 

 of bread there is visible near the duodenal end of the antrum or vestibule 

 a slight annular contraction which moves peristaltically to the pyloric orifice; 

 this is followed by several waves recurring at regular intervals. Two or 

 three minutes after the first movement is seen, very slight constrictions ap- 

 pear 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 or vestibule 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 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. 77. 



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. 



