DIGESTION AND ABSORPTION IN THE STOMACH. 771 



Normal Mechanism of the Secretion of the Gastric Juice.-— 



It has usually been assumed that the gastric glands are quiescent 

 when'the stomach is empty and are stimulated to activity during the 

 eating and digestion of food. According to the results published 

 by Carlson* this view is not wholly correct Even in the period 

 of fasting there is a small continuous secretion varying from 2 to 

 50 c.c. per hour. But during the act of eating and throughout the 

 period of gastric digestion the rate of secretion is increased greatly, 

 reaching a flow of as much as 3.5 c.c. per minute. The modern ex- 

 planation of the origin, maintenance, and regulation of this flow of 

 secretion is due chiefly to Pawlow. Contrary to a former general be- 

 lief, he showed that mechanical stimulation of the gastric mucous 

 membrane has no effect on the secretion of the tubules. This factor 

 may, therefore, be eliminated. In an ordinary meal the secretion 

 first started is due to the sensations of eating— that is, it is a psych- 

 ical secretion. The afferent stimuli originate in the mouth and 

 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 stomach itseh. It seems that some foods contain substances 

 designated as secretagogues, 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 secretagogues are lacking. Certain common articles of 

 food, such as bread and white of eggs, have no effect of this kind 

 at all. If introduced 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 diverted 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 appetite secretion; (2) the secretion 

 from secretagogues contained in the food; (3) the secretion from 

 secretagogues contained in the products of digestion. The manner 

 in which the secretagogues act cannot be stated positively. Since 

 the gastric glands possess secretory nerve fibers the first explanation 

 to suggest itself is that the secretagogues 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 

 * Carlson, Loc. cil. 



