The Digestive System - 307 



the digestion of an average meal ranges usu- 

 ally between 400 and 800 cc. This copious 

 flow is induced partly by a series of well- 

 defined reflexes and partly by a hormone 

 called gastrin. The flow of gastric juice com- 

 mences at the very beginning of a meal, and 

 this early secretion of juice is mainly of reflex 

 nature. But the gastric juice continues to 

 flow until shortly after the stomach is emp- 

 tied, and this late sustained secretion is 

 effected mainly by the gastric hormone. 



Pavlov was one of the first investigators to 

 study the secretion of the gastric juice, and 

 he devised the experimental techniques that 

 are shown in Figures 16-20 and 16-21. If a 

 dog with an esophageal fistula (Fig. 16-20) is 

 fed, the food of such a "sham feeding" never 

 reaches the stomach. Nevertheless about one 

 quarter of the normal flow of gastric juice 

 takes place. This fraction of the total gastric 

 juice is entirely reflex in origin, since it is 

 abolished when the gastric nerves are cut. 

 Like salivation, this gastric reflex is partly 

 unconditioned, involving the taste buds as 

 primary receptors, and partly conditioned, 



Fig. 16-20. Showing an esophageal fistula, used in 

 "sham feeding" experiments. (From The Living Body, 

 by Best and Taylor. Holt, Rinehart and Winston, Inc.) 



involving other receptors of smell and sight, 

 etc., as substitutes for the taste buds. 



Isolating a small part of the stomach, called 

 the Pavlov pouch (Fig. 16-21), and leading 

 this pouch to the external surface of the 

 abdominal wall permitted Pavlov to observe 

 the flow of gastric juice under a variety of 

 conditions. Introducing food directly into 

 the stomach — without allowing the animal 

 to taste, smell, or see the food — calls forth 

 almost three-quarters of the total normal pro- 

 duction of the juice. This flow, which results 

 from the mere presence of food in the stom- 



Fig. 16-21. Upper drawings show Pavlov's 

 method of fashioning a gastric pouch, or 

 miniature stomach. A horizontal incision 

 (A-B) is made, which causes minimal in- 

 jury to the gastric nerve and blood supply. 

 (D) vagus nerves. The flap (C) is turned 

 down and the pouch (S) isolated (by 

 sutures) from the main cavity of the 

 stomach (V) as shown in the right-hand 

 sketch. (A) abdominal wall. The lower 

 drawing illustrates the manner in which 

 pure gastric juice can be collected from 

 the miniature stomach while digestion is 

 proceeding in the main part of the 

 stomach. (From The Living Body, by Best 

 and Taylor. Holt, Rinehart and Winston, 

 Inc.) 



