432 DIGESTION 



associated with the taking of food. It .is from the psychic rather than 

 from the physiologic standpoint, therefore, that these observations are 

 of importance, for they permit us, by objective methods, to study on 

 dumb animals problems that would otherwise be beyond our powers of 

 investigation. Many of the results, with their bearing on the functions 

 of the higher nerve centers, have been discussed elsewhere (Chapt. XCVII). 

 Meanwhile, however, even at the risk of repetition it may not be out of 

 place to cite a few of the most interesting experiments. 



If we tease a hungry animal with food for which he has a great appe- 

 tite, a copious secretion of saliva immediately occurs. If we go on teas- 

 ing him without giving him food, and repeat this procedure on several 

 succeeding days, it will be found that gradually he no longer responds 

 to the teasing by increased salivation. Evidently, therefore, the reflex 

 is conditioned upon the animal's afterward receiving the food. 



The experiment may be performed in another way. If, for example, 

 we offer the animal some food for which he has no appetite, no secre- 

 tion of saliva will occur; but, if at the end of the process we give him 

 appetizing food, it will be found after repeating this procedure on 

 several successive days that the presentation of the unappetizing food 

 calls forth a secretion. He has learned to associate the presentation of 

 unappetizing food with the subsequent gratification of his appetite. The 

 experiment can even be performed so that a definite interval of time 

 elapses between the application of the stimulus and the salivation: if 

 the animal is teased on successive days with food for which he has an 

 appetite but is not given the food until after ten or twenty minutes, 

 presentation of this food will come to be followed by salivation not 

 immediately, but after the exact interval of time that had been allowed 

 to intervene in the training process. During this interval there must be 

 an inhibition of psychic stimulation of the salivary centers by other nerve 

 centers. It is of great interest that this inhibition may itself be inhib- 

 ited by various forms of stimulation of the nervous system (see page 858). 



THE SECRETION OF GASTRIC JUICE 



Methods of Investigation 



There being no common duct, the secretion of the gastric glands is a 

 much more difficult problem to investigate than is that of glands which, 

 like the salivary, are supplied with ducts. One of the most interesting 

 chapters in the history of physiology concerns the methods which from 

 time to time have been evolved for the collection of this juice and for 

 studying the digestive processes in the stomach. Prominent among the 

 problems confronting the earlier investigators was the question, whether 



