i 92 TEXT-BOOK OF PHYSIOLOGY. 



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 sec- 

 onds to move from the middle of the stomach to the pylorus. At all 

 periods of digestion the waves recur at intervals of almost exactlyjen 

 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. 



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 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 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 usually described. On the 

 contrary, the movement through the pre-antral tube and antrum is 

 in general a progressive though an oscillating one. As the con- 

 striction 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 

 gastnc 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. 



