DIGESTIVE SECRETIONS AND THEIR CONTROL 439 



a substance which has been named prosecretin. This substance 

 reacts with hydrochloric acid to form pancreatic secretin, the 

 hormone for exciting the pancreas to secrete. But this region of 

 the small intestine comes in contact with hydrochloric acid only 

 at the moment when a mass of food is entering it from the stomach ; 

 we have previously seen that this passage of food occurs only 

 when the food is mixed with excess of hydrochloric acid. Thus 

 the production of the hormone is confined to the time when its 

 stimulating function is required. 



The Control of the Bile Flow. It has been shown recently 

 that the bile, which, although secreted continuously, is poured 

 out only when food enters the small intestine, is controlled by the 

 same hormone, secretin, which excites the flow of pancreatic juice. 

 Under the stimulation of this hormone the gall bladder contracts, 

 forcing its contents through the bile-duct into the intestine. 



The Control of the Succus Entericus is at present wholly un- 

 known. Whether it is constantly present in the intestine or 

 whether its secretion is controlled by a hormone remains to be 

 determined. It is worth noting, however, that there are probably 

 not many periods, except during prolonged fasting, when intestinal 

 digestion is not going on, so that a continuous secretion of intes- 

 tinal juice would be less wasteful than of the other digestive juices. 



Digestive History of a Meal. We can summarize the whole 

 process of digestion as well, perhaps, by following the course of 

 an ordinary meal through the digestive tract as in any other way. 

 We shall disregard the accessories, and consider only the nutrients 

 proper, since, as we have seen, the digestive process concerns it- 

 self with these alone. The meal, then, is a mixture of carbo- 

 hydrates, proteins, albuminoids, and fats. 



In the mouth the food is reduced to a semi-liquid alkaline mass, 

 containing no large particles, by the combined action of chewing 

 and mixing with the saliva. The salivary glands are reflexly 

 excited to secrete their juice by the presence of the food in the 

 mouth. The enzym of saliva, ptyalin, begins its digestive action 

 on the starch, converting it to maltose. By the act of deglutition 

 the food, when sufficiently mixed with saliva, is passed on to the 

 stomach. If the chewing and swallowing of the food is attended 

 with agreeable emotions, there is aroused a reflex secretion of 

 gastric juice; the so-called "psychical" secretion. 



