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immediately enter into the system of tlie animal ; whilstgum, starch, sugar, &c., must undergo a change beforethey can constitute a part of the body of an animal.
284. It has already been stated that vegetable andanimal oils consist of the same three elements ; manyof these oils contain precisely the same proportions ofcarbon, oxygen, and hydrogen: hence there is goodreason to believe that the oil existing in many vege-tables used as food may contribute to the formation offat, without undergoing any change.
285. Bearing in mind, then, that the strength of manand animals depends mainly on muscle, and that theformation of muscle is greatly dependent on the amountof organic matter containing nitrogen in their food, itbecomes a matter of the first importance to study themode of increasing the quantity of these matters infood.
286. Some animals feed wholly on vegetable food ;others feed wholly, or in part, on flesh : in either casethey derive their nitrogen, or the substances containingit, from plants. Animals do not appear to have anypower of absorbing nitrogen from the air ; all the albu-men, fibrin, &c., which they contain, is therefore eitherdirectly or indirectly obtained from plants.