Page:Popular Science Monthly Volume 43.djvu/527

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HOW PLANTS AND ANIMALS GROW.

these upward and downward transformations of matter and energy.

In the higher animals the various functions are more highly specialized, but we still find that protoplasm is the essential living substance of every tissue and the dominant factor in nutrition. It differs, however, from vegetable protoplasm in many of its properties, and it can not, as formerly assumed, be formed by plants, or built up from the simpler elements that plants feed on, and from which they construct vegetable protoplasm.

The proteids, fats, and carbohydrates which constitute the food of animals are, as we have seen, products of the destructive metabolism of the protoplasm of plants, and it was formerly supposed that they are transformed into animal proteids and fats, without any marked change in their chemical constitution. This assumption is, in fact, the basis of the fallacious theory of nutritive ratios, but it can not be reconciled with the known facts of animal nutrition. There appears to be conclusive evidence that the proteids, fats, and carbohydrates of the food can not be converted into the proteids and fats of the animal body without undergoing profound disruption and reconstruction through the agency of animal protoplasm; or, in other words, the products of vegetable protoplasm can not be made available in the construction of animal tissues without being resolved into simpler compounds and formed anew in the laboratory of animal life.

Without noticing the many details that would only tend to divert the attention of the general reader from the significance of the results produced, the essential or fundamental processes in the nutrition of the higher animals may be broadly stated as follows: In the first place, the proteids, fats, and carbohydrates of their food undergo a series of changes in the processes of digestion (using this word in its widest sense) that reduces them to simpler compounds and, in fact, almost to their elements, with a liberation of energy which is made available in the reconstruction of the disintegrated food constituents.

The activities of the various organs of nutrition are primarily directed to the elaboration of a nutritive fluid, the blood, which is distributed to all of the tissues through the circulatory apparatus provided for that purpose, funishing them the pabulum for their nutrition, and receiving the excretory and other products arising from their metabolism. "An average uniform composition of the blood" is maintained through the action of numerous glandular organs, and the drafts made upon it in the constant repair of the different tissues. The various ferments required in the disintegration or digestion of the food elements that are being transformed into blood are products of the destructive metabo-