128 DIGESTION 



this process was put on a scientific basis by the experi- 

 ments of Pavlov. Pavlov's procedure was as follows. 

 He first cut the oesophagus in the neck, and brought the 

 two ends to the surface, to which he sutured them. When 

 he gave the dog food it merely fell out of the upper opening. 

 When he wished to feed the dog he inserted food into the 

 lower opening. He then made a permanent opening or 

 fistula into the stomach, and so sutured the mucous mem- 

 brane that the part of the stomach cavity which opened 

 by the fistula did not communicate with the main stomach, 

 but at the same time it preserved its normal nerve-supply. 

 In this way there was formed a small sac opening to the 

 exterior, the secretion of which was a measure of the 

 secretion in the whole organ. This sac did not become 

 contaminated with food. By this means Pavlov was able 

 to investigate the changes which occurred in the stomach 

 when food was shown to the animal, but not masti- 

 cated ; when it was masticated, but did not reach the 

 stomach ; when it was inserted into the stomach without 

 the animal's knowledge; and when it was masticated and 

 inserted into the stomach. 



The results may be briefly stated. The same stimuli 

 which provoke secretion of the salivary glands — sight, 

 smell and taste of food — stimulate production of gastric 

 juice. When the vagi are cut this effect is abolished, 

 showing that these nerves form the efferent path of the 

 reflex. Gastric secretion, therefore, begins before food has 

 entered the stomach. It is important to notice that 

 psychical secretion only occurs when the stimulus is 

 associated with a pleasurable sensation. Mastication of 

 inedible substances, hke small stones, is ineffective. 



The secretion of gastric juice continues long after the 

 food has been taken. This is no mere prolongation of the 

 reflex effect, for it occurs independently of the vagi when 

 these nerves are cut after the preliminary secretion has 

 begun. Nor is it due to the mechaniQal irritation of the 

 food against the stomach- wall. It is due to the action of 



