Til] THE DIGESTIVE ORGANS 37 



swallowed insieiul of entering the stomach forthwith passed (jul 

 of the l)ody. 



A dog so operated upon will eat with avidity and may continue 

 eating for hours, during which time gastric juice, free from ad- 

 mixture with other substances, passes out through the gastric 

 fistula. The juice thus obtained is found to be a clear fluid with an 

 acid reaction ; it contains about one per cent, of solids and ninety- 

 nine per cent, water. The solids consist of mucin, proteins, and 

 inorganic salts. It also contauis two ferments, pepsin which acts 

 upon proteins and converts them into simpler substances, called 

 proteoses and peptones, in the manner described in the second 

 volume, and rennin, which changes caseinogen (the phosphoprotein 

 of milk) into casein. There is however some reason to 

 doubt whether rennin is in reality an independent ferment, 

 since evidence has been brought forward suggesting that possibly 

 the conversion of casemogen into casein may after all be due 

 to pepsin. This ferment cati only act m an acid medium. In 

 the dog and in man the acid present is hydrochloric. The lactic 

 and other acids which are found in the gastric juice of herbivorous 

 animals are probably produced by the bacterial decomposition of 

 the food they swallow. As in the case of other ferments the activity 

 of pepsin and rennin is permanently destroyed by boiling. The 

 experiments by Pawlow described above indicate that the normal 

 stimulus for gastric secretion is the presence of food in the mouth 

 and that the process is of the nature of a nervous reflex. The 

 truth of this presumption is established by the fact that secretion 

 may not occur after the severance of the two vagus nerves which 

 supply the stomach, and the further fact that electrical stimulation 

 of these nerves is followed by secretion. It is clear therefore that 

 the efferent nerves concerned in the reflex are the two vagi, 

 while the afferent nerves are the sensory nerves of the mouth, 

 possibly assisted by the nerves of sight and smell. 



Edkins however has carried out experiments showing that 

 there is also a subsidiary mechanism for gastric secretion, for in 

 a dog in which the vagi have been severed and two gastric fistulae 

 have been established with two artificially separated portions of 

 the stomach, the introduction of food into one portion through the 

 fistula connecting that portion with the exterior, may be followed 

 by the outpouring of gastric juice through the other fistula. 

 Moreover injection of pyloric mucous membrane into the circulation 



