DIGESTION OF FOOD. 361 



It is remarkable in the highest degree that man's appetite, 

 or the instinctive choice of food, has proved wiser than our 

 science. It would be impossible even yet to match, by calcula- 

 tions based on any data we can obtain, a diet for each man equal 

 upon the whole to what his instincts prompt. With the lower 

 mammals we can prescribe with greater success. At the same 

 time chemical and physiological science can lay down general 

 principles based on actual experience, which may serve to cor- 

 rect some artificialities acquired by perseverance in habits that 

 were not based on the true instincts of a souiid body and a 

 healthy mental and moral nature; for the influence of the 

 latter can not be safely ignored even in such discussions as the 

 present. These remarks, however, are meant to be suggestive 

 rather than exhaustive. 



We may with advantage inquire into the nature of. hunger 

 and thirst. These, as we know, are safe guides usually in eat- 

 ing and drinking. 



After a long walk on a warm day one feels thirsty, the 

 mouth is usually dry; at all events, moistening the mouth, 

 especially the back of it (pharynx), will of itself partially re- 

 lieve thirst. But if we remain quiet for a little time the thirst 

 grows less, even if no fluid be taken. The dryness has been 

 relieved by the natural secretions. If, however, fluid be intro- 

 duced into the blood either directly or through the alimentary 

 canal, the thirst is also relieved speedily. The fact that we 

 know when to stop drinking water shows of itself that there 

 must be local sensations that guide us, for it is not possible to 

 believe that the whole of the fluid taken can at once have en- 

 tered the blood, 



Hunger, like thirst, may be mitigated by injections into the 

 intestines or the blood. It is, therefore, clear that, while in the 

 case of hunger and thirst there is a local expression of a need, 

 a peculiar sensation, more pronounced in certain parts (the 

 fauces in the case of thirst, the stomach in that of hunger), 

 yet these may be appeased from within through the medium 

 of the blood, as well as from without by the contact of food or 

 water, as the case may be. 



Up to the present we have assumed that the changes 

 wrought in the food in the alimentary tract were identical with 

 those produced by the digestive ferments as obtained by extracts 

 of the organs naturally producing them. But for many reasons 

 it seems probable that artificial digestion can not be regarded as 



