64 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION 



The articles we eat, besides being largely insoluble, differ 

 very materially in their composition from any substances 

 found as parts of the body tissues. Even those undigested 

 substances most closely resembling living tissue will no.1 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. How- 

 ever, 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 suffi- 

 cient quantity into the circulation, as by rectal enemata, hun- 

 ger is relieved. The true seat of this sensation is undoubt- 

 edly in the cells themselves, it being simply a call from them 

 for more material to take the place of their worn-out con- 

 stituents. 



Cold weather demands an increase in the amount of food, 

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



Seat of Thirst. The demands of the cells for water is re- 

 ferred 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 absorption may take place from its mucous mem- 

 brane. But, if water in sufficient amount be placed into the 

 circulation in any way satisfaction ensues. Next to the de- 

 mand 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 conditions, such as 

 external moisture and temperature, exercise, etc. 



Classification of Foods. A very large number of sub- 



