470 DIGESTION 



Boldyreff, or it may in part be due to a psychic stimulation dependent 

 upon the thought of food. That the latter is probably not the cause, is 

 indicated by the fact that, at least in Carlson's patient, the psychic juice 

 could not be made to flow short of giving food. 



The sham feeding causes stimulation of the gastric secretion through 

 impulses transmitted to the stomach along the vagus nerves; for it has 

 been found, in animals in which the vagus nerve has been cut, that the 

 sham feeding no longer induces a secretion of gastric juice. The ques- 

 tion therefore arises as to how the nerve center is stimulated. Three 

 possible causes may be considered: (1) mechanical^ stimulation of the 

 sensory nerves of the mouth; (2) chemical stimulation of these nerves; 

 (3) the agreeable stimulation of the taste buds and olfactory endings 

 concerned in the tasting of food. In investigating these possibilities, 

 mechanical stimulation was readily ruled out by showing that mere 

 taking of solid matter in the mouth did not excite any secretion, although 

 it might cause a flow of saliva. Mere chemical stimulation could not be 

 the cause, for no secretion was induced by placing substances such as 

 acetic acid or mustard oil in the mouth. By exclusion, then, it would 

 appear that the adequate stimulus must consist in the agreeable stimula- 

 tion of the taste buds, etc. that is to say, in the gratification of appetite. 



Further justification for this conclusion was readily secured by noting 

 that foodstuffs for which the animal had no particular desire or appe- 

 tite failed to excite the secretion. Most dogs, for example, although 

 they may take it, are not particularly fond of bread, and when fed with 

 it, these animals did not produce any appetite juice. In one animal that 

 showed considerable liking for bread, active secretion occurred when he 

 was fed with this foodstuff. 



Pavlov further noted that usually it was not necessary actually to 

 allow the animal to take the food into his mouth, but that mere teasing 

 with savory food was sufficient to cause the secretion, and that in 

 highly sensitive animals even the noises and other events usually asso- 

 ciated with feeding time were sufficient to excite the secretion. In the 

 case of a hungry animal, the mere approach of the attendant with food, 

 or some other noise or action definitely associated with feeding time, 

 was a sufficient excitant. The appetite juice when started was found 

 to persist for some time after the stimulus causing it had been removed. 



Carlson has succeeded in confirming in man most of these observa- 

 tions. He noted, however, that the secretion produced by seeing or 

 smelling or thinking of food is much less than would be expected from 

 Pavlov's observations on dogs. Even when his subject was hungry, 

 Carlson did not observe that the bringing of a tray of savory food into 

 the room caused any secretion of gastric juice. It is, of course, to be 



