ALIMENTATION AND DIGESTION 



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it occurs only when the animal is conscious; for if food be 

 introduced into the stomach of a sleeping dog, it evokes only 

 the most scanty secretion of gastric juice after the animal 

 has awakened. Moreover, both the amount and the efficiency 

 of the juice secreted vary directly with the enjoyment of the 

 meal. When meat is given to a dog which is not hungry, 

 no such abundant secretion of 

 gastric juice occurs as during 

 hunger. 



It is clear that we have here 

 to deal with a nervous process 

 more complicated than the sim- 

 ple reflex, and that the efferent 

 discharge to the stomach occurs 

 as the result of nervous proc- 

 esses taking place in the brain 

 in connection with the enjoy- 

 ment of food. In other words, 

 the more the food is desired 

 or enjoyed, the more efficient 

 will be this secretion of the 

 gastric juice. 



It is known that this " psy- 

 chic" secretion will continue 

 for several hours after an ordi- 

 nary meal, increasing in amount during the first hour or more 

 and gradually diminishing from that time onward (Fig. 57). 



2. Stimulation of the stomach by constituents of certain foods. 

 We have seen that the direct introduction of food into the 

 stomach (for example, into the stomach of a sleeping animal) 

 does not of itself evoke a secretion of gastric juice. Some 

 foods, however, contain substances which do evoke such a 

 secretion, the most important of these being certain con- 

 stituents of meat. Bouillon, for example, which is an extract 

 of meat, directly excites the wall of the stomach to secrete. 



FIG. 57. The curve of the "psychic" 

 secretion of gastric juice 



Vertical lines represent half-hour pe- 

 riods after taking the meal ; horizon- 

 tal lines, relative amounts of gastric 

 juice secreted 



