Page:Popular Science Monthly Volume 43.djvu/530

514 constituents of foods, become necessary to liberate their needed supplies of energy.

As all the food constituents contribute to the blood-making processes, they all in like manner through digestive disintegration contribute to the supplies of energy required in the animal economy. The energy provided in foods in the potential form is quite as important in building animal tissues as the chemical elements entering into their composition, as when liberated in the form of heat it is utilized in constructive metabolism and stored again as potential energy in the animal tissues formed. The destructive metabolism taking place in these tissues, as an essential concomitant of their vital activities, again liberates energy in the form of heat, which, with that derived from the digestion of foods, is used, so far as needed, in the reconstructive process, and the balance appears as animal heat.

We have noticed the recently discovered continuity of the protoplasm of plants, but we can not fairly infer that there is a similar continuity of the protoplasm of the higher animals that have a highly specialized nervous system which brings the different organs and functions into harmonious action more completely and efficiently than they could be by simple threads of protoplasm like those which unite the cells of plants. The widely different products of destructive metabolism in the various tissues of plants and animals, aside from other considerations, furnish conclusive evidence that while the general rôle of protoplasm is everywhere the same, it must differ materially in composition and constitution in the different conditions in which it is found. As stated by Dr. Foster, "It is obvious that the varieties of protoplasm are numerous, indeed almost innumerable. The muscular protoplasm which brings forth a contractile katastate must differ in nature, in composition—that is, in construction—from glandular protoplasm whose katastate is a mother of ferment. Further, the protoplasm of the swiftly contracting striped muscular fiber must differ from that of the torpid, smooth, unstriated fiber; the protoplasm of human muscle must differ from that of a sheep or frog; the protoplasm of one muscle must differ from that of another muscle in the same kind of animal, and the protoplasm of Smith's biceps must differ from that of Jones's."

What determines these differences and gives direction to such diverse metabolic activities? Chemical and physical considerations fail to clear up the mystery of life and its varied manifestations. We may look upon protoplasm as the physical basis of life, and consider vital activities as resulting from its inherent properties; but this does not aid us in gaining a better knowledge of the mysterious endowments of living matter. What gives rise to these diverse properties in different species and in different