WHAT IS A LIVING ANIMAL? 295 



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 lim- 

 ited 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, tem- 

 perature, 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 gene- 

 rated in living bodies by the successive compositions and decomposi- 

 tions, the integrations and disintegrations, the electrolytic associations 

 and disassociations of ions and electrons resulting from animal metab- 

 olism, if no arrangement had been provided by which the energy de- 

 veloped could be secured to the use of the individual producing it, in- 

 stead 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." 10 

 Among other explanations he suggests that this may be because the 

 envelope of the corpuscles refuses passage to the electric charge pro- 

 duced 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 sub- 

 stance? The answer will depend upon our definition of the word 



10 " Human Physiology," p. 35, 3d ed., 1899. 



