782 PHYSIOLOGY OF DIGESTION AND SECRETION. 



nostrils; the efferent path, the secretory fibers, is through the vagus 

 nerve. This reflex insures the beginning at least of gastric digestion, 

 but its effect is supplemented by a further action arising in the stom- 

 ach itself. It seems that some foods contain substances designated 

 as secretogogues, that are able to cause a secretion of gastric juice 

 when taken into the stomach. Thus, meat extracts, meat juices, 

 soups, etc., are particularly effective in this respect; milk and water 

 cause less secretion. In other foods these ready-formed secreto- 

 gogues are lacking. Certain common articles of food, such as 

 bread and white of eggs, have no effect of this kind at all. If intro- 

 duced into the stomach of a dog through a fistula so as not to arouse 

 a psychical secretion, for instance, while the dog's attention is di- 

 verted or while he is sleeping, they cause no flow of gastric juice 

 and are not digested. If such articles of food are eaten, however, 

 they cause a psychical secretion, and when this has acted upon the 

 foods some products of their digestion in turn become capable of 

 arousing a further flow of gastric juice. The steps in the mechanism 

 of secretion are, therefore, three: (1) The psychical secretion or ap- 

 petite secretion; (2) the secretion from secretogogues contained in 

 the food; (3) the secretion from secretogogues contained in the prod- 

 ucts of digestion. The manner in which the secretogogues act 

 cannot be stated positively. Since the gastric glands possess 

 secretory nerve fibers the first explanation to suggest itself is 

 that the secretogogues by acting on sensory fibers in the gastric 

 mucous membrane reflexly stimulate the secretory fibers. This 

 explanation, however, is rendered untenable by the fact that the 

 effect of these substances is obtained after complete severance 

 of the nervous connections of the stomach. If, therefore, this 

 so-called chemical secretion is produced by a nervous reflex, the 

 nerve centers concerned must lie in the stomach itself, the reflex 

 must take place through the intrinsic ganglion cells. Another 

 more probable explanation has been offered. Edkins * has shown 

 that decoctions of the pyloric mucous membrane, made by boiling 

 in water, acid or peptone solutions, when injected into the blood 

 cause a marked secretion of gastric juice. These substances when 

 injected alone into the blood cause no such effect, and decoctions 

 of the mucous membrane of the fundic end of the stomach are 

 without action on the gastric secretion. This author suggests, 

 therefore, that the secretogogues, whether preformed in the food or 

 formed during digestion, act upon the pyloric mucous membrane 

 and form a substance which he designates as gastrin or gastric se- 

 cretin, and this substance after absorption into the blood is carried 

 to the gastric glands and stimulates them to secretion. The effect 

 is, therefore, not a usual nervous reflex, but an instance of the 

 stimulation of one organ by chemical products formed in another. 



* Edkins, "Journal of Physiology," 1906, xxxiv., p. 133. 



