Page:Popular Science Monthly Volume 75.djvu/299

Rh With these preliminaries, I submit the following definition of a living being. It is this: A living body, whether a simple cell or a fully developed mammal, consists of a temporary aggregation of a limited number of material particles, call them what we may—molecules, atoms, ions, electrons—whose actions and reactions between each other, and between themselves and their environing conditions (light, temperature, air, water, food, terrestrial magnetism, gravitation, etc.) are of such a kind as to generate electro-magnetic energy, which energy is and necessarily must be secured to the use of the individual producing it, by a semi-porous limiting external envelope which provides the individual with electric insulation from its surroundings.

It is upon this external electric insulation that I desire to insist as a necessary part of everything that can truly be said to "live, move and have its being." Vain and useless indeed would be the energy generated in living bodies by the successive compositions and decompositions, the integrations and disintegrations, the electrolytic associations and disassociations of ions and electrons resulting from animal metabolism, if no arrangement had been provided by which the energy developed could be secured to the use of the individual producing it, instead of instantly flashing back to the earth whence it came, which it inevitably would do, in the absence of such insulation.

That this insulatory covering really exists, in the case of animals, eggs, seeds, etc., has been shown by the experiments before mentioned. That the individual cells of the body—the histological units—are also provided with the same electric insulation, may be more difficult to demonstrate. But such demonstration is not altogether wanting. The red corpuscles of the blood are, in a measure, insulated from the serum in which they float. "The intact red corpuscles," writes Stewart, "have an electric conductivity so many times less than that of serum that they may, in comparison, be looked upon as non-conductors." Among other explanations he suggests that this may be because the envelope of the corpuscles refuses passage to the electric charge produced by the dissociation of ions within them.

In the developing ovum, according to this view, the ectoderm ought to be an insulator. I can give no proof of this, but it is significantly suggestive that the cerebro-spinal axis of the embryo (which we should think ought to receive a specially good insulation) is clothed on its outside by an investment from the ectodermic layer, produced by an invagination of that structure to form the medullary groove and canal in which the central nervous system pursues its development.

Finally, is the protoplasm of animal organisms a really living substance? The answer will depend upon our definition of the word