718 PHYSIOLOGY OF DIGESTION AND SECRETION. 



waves of contraction appear at regular intervals. The pyloric 

 portion becomes lengthened and it may be noticed that in this 

 region the peristaltic waves become more and more forcible as 

 digestion progresses. These running waves or rings of contraction 

 serve to press the stomach contents against the pylorus. According 

 to Cannon, they occur in the cat at intervals of 10 seconds and each 

 wave requires about 20 seconds to reach the pylorus. While ni 

 human beings, to judge from the sounds which may be heard upon 

 ausculation when food mixed with air is given, they occur at intervals 

 of about 20 seconds. The obvious result of these movements is to 

 mix the food thoroughly, in the intermediate and pyloric portions 

 of the stomach, with the acid gastric juice and to reduce it to a thin^ 

 liquid mass, — the chyme. At certain intervals the pyloric sphincter 

 relaxes and the contraction wave squeezes some of the fluid con- 

 tents into the duodenum. The mechanism controlling the re- 

 laxation of this sphincter is obscure. It does not occur with 

 the apjjroach of each contraction wave, but at irregular inter- 

 vals. Canncm connects it in part with the consistency of the 

 food, but mainly with the effect of the hydrochloric acid in the 

 gastric secretion. Solid objects forced against the pylorus prevent 

 relaxation and retard the passage of the chyme into the intestine. 

 When liquid food alone is taken into the stomach numerous ob- 

 servations, made by means of intestinal fistulas, prove that the 

 material may be forced into the duodenum within a few minutes. 

 Hydrochloric acid in the stomach seems to favor or produce a 

 relaxation of the pyloric sphincter, while in the duodenum, on the 

 contrary, it causes a contraction of the sphincter. In this way it 

 may be imagined that after each ejection of acid chyme the sphinc- 

 ter is kept closed until the acid material in the duodenum is neutral- 

 ized, and so, automatically, a mechanism is provided by means of 

 which the duodenum is charged at intervals and at such times as it 

 is prepared to receive and neutralize a new quantity of the chyme. 

 According to this description, the portion of the food toward the 

 pyloric end of the stomach is the first to be thoroughly mixed with 

 the gastric juice, and to be broken down partly by digestion and 

 partly by the mechanical action of the contractions. This portion, 

 as it is liquefied, is expelled, and its place is taken by new material 

 forced forward from the fundic end. It would seem that this latter 

 portion of the stomach is in a condition of tone, and the pressure 

 thus put upon the contents is sufficient to force them slowly toward 

 the pyloric end as this becomes emptied. The older view was that 

 the contents of the stomach are kept in a general rotary movement, 

 so as to become more or less uniformly mixed; but Cannon's obser- 

 vations, and also those of Griitzner,* indicate that the material at 

 *Grutzner, "Archiv f. die gesammte Physiologie," 106, 463, 1905. 



