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Page:Popular Science Monthly Volume 18.djvu/416

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energy, integration is also a universal concomitant of vitality, so that for practical purposes life may be provisionally defined as the continuous individual integration and differentiation of material energy.

While these two correlated processes pertain to every variety of life, the physiological expedients by which their respective activities are sustained must vary in conformity with the specific requirements of different structures. A simple unit of protoplasm effects all its vital purposes through direct interchange with its environment, without the necessity of any intermediate provision. But, in higher organisms, life is indissolubly associated with certain accessory processes, and, in these cases, though the molecular interactions on which its essential attributes immediately depend are directly imperceptible, yet it is possible to prove its existence or non-existence by sensibly demonstrating the presence or absence of these its inseparable concomitants.

Man with his powers unimpaired manifests his vitality in unmistakable terms, but conditions not incompatible with resuscitation may occur wherein all his functions are so reduced as to be directly imperceptible. In such cases, to prevent premature burial, it is important to discover some sign absolutely diagnostic of real or apparent death.

An essential characteristic of living bodies is their power of actively maintaining a degree of varying integrity of constitution in opposition to destructive influences. This requires the incorporation of extraneous materials and their conversion into definite specific structures, and always involves the immediate apposition of ingredients, as well as a reciprocal state of the parts to be nourished. Although such intimate reciprocation of living structures and nutrient materials must always exist, the means whereby it is effected varies exceedingly in different instances. In the lower order of beings it is accomplished very simply, the medium which they inhabit offering directly the requisite pabulum, which their own condition enables them to assimilate without any preparatory elaboration. In more complex organisms a definite correlation of parts is necessary to elaborate the crude materials of food, as well as to bring them into immediate relation with the various tissues.

In some simple forms vital action may be suspended indefinitely by desiccation, being restorable by moisture, and even in some higher cold-blooded animals a state of temporary negation may be induced by congelation, the vital powers returning concurrently with the absorption of heat. In man it is quite different: the animal functions may be suspended, and even some of the organic processes interrupted, without extinguishing life, but there are certain of his functions the cessation of which for a limited period must inevitably cause death.

As to their vital significance, man's functions may be classified into essential and supplemental—the former including such as can not be