DIGESTION 191 



animals ; in the dog, for example, it may be removed experiment- 

 ally, for, as we shall see later on, protein digestion is provided 

 for elsewhere. But in the herbivora, especially ruminants, a 

 stomach is essential. The chief value of the stomach in those 

 animals which can be proved to live without it lies in the prepara- 

 tion of the food for subsequent digestion in the small intestines, 

 for it is quite undoubted that protein previously acted upon 

 by gastric juice is far more thoroughly handled by the pancreatic 

 fluid than protein not so previously acted upon. 



Influence of the Nervous System on the Secretion of Gastric 

 Juice. — By means of Pawlow's fistula, it has been proved that the 

 secretion of gastric juice is under the control of the nervous 

 system, the secretory fibres being contained in the vagus. 

 Stimulation of the peripheral end of the divided nerve causes 

 secretion after a slight delay ; the cause of the latent period is 

 unknown. No secretion results in consequence of mechanical 

 stimulation of the stomach wall, which is contrary to all pre- 

 viously accepted views. The sight of food, its smell, taste, 

 mastication, and swallowing, are direct excitants of secretion. 

 A secretion so obtained is known as psychical. Pawlow's experi- 

 ment of ' sham feeding ' a dog with a divided oesophagus, the 

 upper end of which is brought outside, has placed this beyond 

 doubt. The animal indulged in a meal which never entered the 

 stomach, but the effect of which could be ascertained through 

 the fistula. In addition to this reflex secretion, there is another 

 produced by the action of the food substances themselves ; such 

 substances, known as secretogogues, directly stimulate the 

 production of gastric juice, though all foods do not possess this 

 power. Bread, starch, fat, and white of eggs, introduced surrep- 

 titiously into a Pawlow's fistula so that the animal does not see 

 them, do not excite secretion ; while, on the contrary, meat 

 extract is most effective. 



Secretagogues may also influence the production of gastric 

 juice through the products of digestion. This chemical secretion 

 is difficult to explain ; it occurs after the vagus and all other 

 nervous connections are cut, which suggests that it does not 

 operate through a nervous reflex. It has been shown by Edkins 

 that the injection into the blood of an extract of pyloric mucous 

 membrane, specially prepared, causes a marked secretion of 

 gastric juice, and it is suggested that the chemical secretion 

 above referred to may be produced by the action of the secreta- 

 gogues upon the pyloric mucous membrane, resulting in the 

 formation of a gastric secretion, or gastrin, analogous to pan- 

 creatic secretion (yet to be studied), which, when absorbed into 

 the blood, acts as a chemical excitant, and stimulates the pro- 

 duction of gastric juice. 



