90 MANUAL OF PHYSIOLOGY. 



Animals use complex food-stuffs to renew their tissues, which 

 they are constantly oxidizing, and by this means the energy for 

 the performance of their various active functions is set free. 



Although the various kinds of food-stuffs used by animals are 

 so highly organized in comparison with those used by plants, yet 

 they cannot be admitted at once into the economy without having 

 undergone a special preparation, which takes place in the diges- 

 tive tract, where the various food-stuffs are so changed as to allow 

 them to pass into the fluids of the body. 



We shall first consider the chief varieties of food-stuffs, next 

 their preparation for absorption, and then the means by which 

 they are distributed to the tissues. The last step in tracing the 

 assimilation of the food is to follow the intimate processes which 

 go on between the blood carrying the nutriment and the different 

 tissues. This most interesting but difficult question shall receive 

 our attention in a subsequent section. 



Food. There are two portals, namely, the lungs and the ali- 

 mentary canal, by which new materials normally enter the ani- 

 mal body. 



Within the lungs the blood comes into close relation with the 

 air, and takes up oxygen from it. The oxygen is then carried to 

 the various tissues, where it aids the combustion accompanying 

 the life and functions of these tissues. Although oxygen is the 

 most abundant element in the body, taking part in almost every 

 chemical change, and its continuous supply is more immediately 

 necessary for life than that of any other substance, yet it is not 

 counted as food, because tissue oxidation is artificially distin- 

 guished from tissue nutrition. 



The details of the union of oxygen with the blood will be found 

 in the Chapter (XIX.) on Respiration. 



It is then only to the liquid and solid portions of the material 

 income of an animal that, in short, which it must busy itself to 

 obtain that the term "food" is applied. These are introduced 

 into the alimentary canal, where the truly nutrient materials are 

 separated and prepared for absorption into the blood, while the 

 portions which are not useful for nutrition are carried away as 

 excrement. One is, therefore, quite prepared to hear that the 



