STUDIES IN DIGESTION 27 



a very remarkable illustration of adaptation of the 

 saliva to the food supplied. Meat calls forth a viscid 

 mucous saliva, to assist in swallowing. Biscuit 

 excites a watery flow containing more ptyalin. 

 Sand, which will be spat out, evokes a copious watery 

 secretion. 



We have known for half a century that the 

 secretion of saliva is a nervous reflex. The up path 

 may be the nerves of smell, of sight, or of taste ; 

 the down paths are the chordae tympani, lesser 

 superficial petrosals, and the sympathetic nerves. It 

 has only lately been shown that the secretion of 

 gastric juice is, at first, a nervous reflex of which 

 the vagus is the down path, but that as the stimulus 

 of appetite wears off, this mechanism is replaced by 

 a chemical excitation of the glands. The secretion 

 of the pancreatic juice is controlled from the first by 

 a chemical messenger. 



THE SECRETION OF GASTRIC JUICE. 

 The secretion of gastric juice was investigated by 

 Pawlow and the St. Petersburg school in a fascinat- 

 ing research on dogs. A sort of gastrostomy was 

 performed in such a manner that an isolated portion 

 of stomach was brought into communication with 

 the surface, and shut off to some extent from the 

 rest of the cavity of the viscus, yet without sacrificing 

 its vascular or nervous supply. Then, to avoid ad- 

 mixture of gastric juice with the food, the oesophagus 

 was divided and brought out in the neck, so that 

 food taken by mouth merely fell out again in the 

 neck. Genuine feeding had to be carried out through 



