CHAPTER II. 
METABOLISM. 
General Conception.—By the various processes of digestion 
and resorption the epithelium of the alimentary canal extracts from 
the crude materials eaten those ingredients which are fitted to 
nourish the animal and transmits them more or less directly to the 
general circulation which carries them to all the tissues of the body. 
While these ingredients are many in number and diverse in charac- 
ter, yet the vast mass of them, aside from the water in which most 
of them are dissolved, may be grouped under six heads, viz., ash 
ingredients, albuminoids or bodies related to the alburminoids, 
amides and other crystalline nitrogenous substances, fats, carbo- 
hydrates, and organic acids, and these, together with relatively 
small amounts of other materials, may be regarded as constituting 
the real food of the organism. 
As was pointed out in the Introduction, the cells of which the 
living tissues of the animal body are composed are the seat of con- 
tinual chemical change. On the one hand, the digested ingredients 
of the food which are brought to them by the circulation are being 
built up into the structure of the body. On the other hand, the 
material of the cells is undergoing a continual process of breaking 
down and oxidation, uniting with the oxygen supplied by the blood 
to form the waste products which are removed from the body 
through the organs of excretion. These excretory products are 
substantially carbon dioxide, water, and urea and similar nitroge- 
nous substances. 
The general term Metabolism is commonly used to designate the 
totality of the chemical and physical changes which the materials 
of the resorbed food, or of the tissues formed from them, undergo in 
being converted into the excretory products. Similarly, we may 
speak in a more restricted sense of the metabolism of a single ingre- 
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