398 DIGESTION 



time. Milk and gelatin solution are also direct excitants of gastric 

 secretion apart from the water in them. Starch, fat, and egg-white 

 are totally inert. After section of both vagi in dogs, no marked 

 qualitative or quantitative changes have been observed in the 

 gastric juice. The secretion caused by the presence of food in the 

 stomach is still obtained when, in addition to the vagi, all other 

 nerves which can possibly connect the central nervous system with 

 the organ have been severed and the sympathetic abdominal 

 plexuses have been destroyed (Popielski). We must therefore sug- 

 pose that the gastric glands, while normally under the control of a 

 nervous mechanism in the upper portion of the cerebro- spinal axis 

 whose efferent fibres run in the vagi, are also capable of being locally 

 stimulated through the peripheral ganglia in the stomach walls or 

 the chemical action of the products of digestion absorbed into the 

 blood. Edkins showed that the injection of food substances or the 

 products of their digestion (broth, dextrin, peptone) or of acid into 

 the blood caused no secretion of gastric juice, while the injection 

 of an extract of the pyloric mucous membrane, made by boiling it 

 with water, acid, or peptone, excited a certain amount of secretion. 

 He therefore concluded that the secondary secretion of gastric juice 

 is determined, not by local stimulation of a reflex mechanism in the 

 gastric wall, but by the production in the mucous membrane of the 

 pyloric end of a chemical substance, the gastric secretin or gastric 

 hormone,* which is absorbed by the blood, and acts as an excitant 

 to all the gastric glands. The cardiac mucosa was found incapable 

 of forming this substance. 



It is not to be imagined that the ' psychical ' secretion and the 

 secretion called forth by the direct action of the food or food- 

 products in the stomach perform independent offices. They can, 

 in various instances, be shown to supplement each other. For 

 example, not more than one-half or one-third of the gastric juice 

 secreted during the digestion of bread or boiled egg-albumin can 

 be ascribed to the psychic effect. Yet these substances, when 

 introduced directly into the stomach, cause practically no secretion. 

 We must suppose that during the digestion of the bread and albu- 

 min by the psychically secreted juice certain products analogous to 

 those in the meat extract are formed, which act as chemical excitants 

 of the local secretory apparatus. The psychic juice is indispensable 

 in this case to start the process, ' to set the stove ablaze,' as Pawlow 

 puts it. In the case of meat it is not indispensable, since the meat 

 can chemically excite the gastric glands; but it greatly hastens the 

 process of digestion. These facts emphasize the importance of 



' Hormone ' (from 6pfj.au, I arouse or excite) is the name given to a sub- 

 stance which, carried by the blood from the place where it is formed, acts as 

 a chemical messenger in exciting the activity of some more or less distant 

 organ. The classical example is the pancreatic secretin which, manufactured 

 in the intestinal mucosa, excites the secretion of the pancreatic juice. 



