64 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION 



parts of the body tissues. Even those undigested substances 

 most closely resembling living tissue will not be utilized by the 

 cells when presented to them by being injected into the blood. All 

 the articles which we use for food must undergo a special process, 

 called digestion, before they can be absorbed by the tissues. 



Seat of Hunger. Food is taken into the body in obedience 

 to an expressed want on the part of the system. The desire for 

 food the sensation of hunger is referred, in a rather indefinite 

 way, to the stomach. That sensation is ordinarily satisfied by 

 the introduction of food into the stomach. However, this does 

 not necessarily mean that its seat is in that organ, since removal 

 of the stomach by no means prevents hunger. But, if nutritious 

 material be introduced in sufficient quantity into the circulation, 

 as by rectal enemata, hunger is relieved. The true seat of this 

 sensation is undoubtedly in the cells themselves, it being simply 

 a call from them for more material to take the place of their 

 worn-out constituents. 



Cold weather demands an increase in the amount of food, as 

 also do physical and psychical activity, certain drugs, etc. 



Seat of Thirst. The demand of the cells for water is referred 

 to the fauces and throat, but this is no more the seat of thirst 

 than is the stomach of hunger. The taking of water into the 

 mouth alone will not quench thirst, except in so far as absorp- 

 tion may take place from its mucous membrane. But, if water 

 in sufficient amount be placed into the circulation in any way 

 satisfaction ensues. Next to the demand for oxygen, that for 

 water is the most imperative which comes from the tissues ; that 

 is, they can live much longer without solid food than without 

 water. The amount necessary is manifestly subject to many con- 

 ditions, such as external moisture and temperature, exercise, etc. 



Classification of Foods. A very large number of substances 

 are taken into the alimentary canal as food; but examination 

 reveals that all such materials contain one or more of a very 

 few classes of food stuffs. These may be divided as follows: 



