308 - Multicellular Animals, Especially Man 



ach, is controlled only in small part by a 

 reflex mechanism, since cutting all the gastric 

 nerves merely reduces this fraction. Accord- 

 ingly, it may be concluded that a large part 

 of the production of the gastric juice is con- 

 trolled by some other mechanism, and this 

 proved to be the hormone gastrin. 



Gastrin is formed in the mucosa of the 

 pyloric part of the stomach, and is discharged 

 into the blood stream whenever the semi- 

 digested food mass comes into contact with 

 this mucosa. If an extract of the pyloric 

 mucosa is prepared and injected into the 

 blood, the gastric glands begin to secrete 

 shortly after the injection. Complete proof 

 of a hormonal agency in normal gastric secre- 

 tion depends, however, upon cross-circula- 

 tion experiments. The Pavlov pouch remains 

 in continuity with the other parts of the 

 stomach, and so it is not certain that all the 

 nerves to the pouch can be cut. But if the 

 blood streams of two dogs (A and B) are con- 

 nected artificially, by mutual junction estab- 

 lished between the major arteries (Fig. 16- 

 22), any substance present in the blood of 

 dog A must sooner or later be carried over 

 into the blood of dog B. In this case there 



is no possibility that nerve connections exist 

 between the organs of the two animals. 

 Nevertheless, when food is placed in the 

 pyloric stomach of dog A, the gastric glands 

 of dog B — as well as those of A — begin to 

 secrete, after a short lag. This secretion must 

 be effected by some substance carried by the 

 blood, and this is the hormone called gastrin. 



Some flow of gastric juice continues after 

 the stomach is emptied — that is, while the 

 digesting food is passing through the first part 

 of the small intestine — but this activity of 

 the gastric glands has not been studied ade- 

 quately. Possibly amino acids, sugars, and 

 other products of digestion, which begin to 

 be absorbed into the blood from the small 

 intestine, exert an influence upon the gastric 

 glands; or perhaps this final flow depends 

 upon a reflex or a hormone that has not yet 

 been recognized. 



The Churning Movements of the Stomach. 

 Each meal remains in the stomach until it 

 has been thoroughly liquefied and converted 

 into a smooth thick fluid called chyme. 

 Chyme has the consistency of a heavy cream 

 soup. The transformation of the ingested 

 food mass into chyme depends partly upon 



Fig. 16-22. By connecting the neck 

 arteries (carotids) of two dogs with 

 rubber tubing, as shown, the blood 

 flows through both animals alike in 

 the course of its travels. Such "cross- 

 circulation" experiments are espe- 

 cially useful in demonstrating control 

 of body activity by chemicals circu- 

 lating in the blood stream. 



