Page:Popular Science Monthly Volume 26.djvu/839

Rh animals, by which the whole body is made one interrelated unit, may be but a final outgrowth of the fibrillar-cell system. The fibril reticulum of the isolated cell becomes the nerve reticulum of the complex body, which is virtually converted into a single cell, with its intricate network of fibers.



N the course of these papers I have repeatedly spoken of the nitrogenous and non-nitrogenous constituents of food, assuming that the nitrogenous are the most nutritious, are the plastic or flesh-building materials; and that the non-nitrogenous materials can not build up flesh or bone or nervous matter, can only supply the material of fat, and by their combustion maintain the animal heat.

In doing so I have been treading on loose ground—I may say, on a scientific quicksand. When I first taught practical physiology to children in Edinburgh, many years ago, this part of the subject was much easier to teach than now. The simple and elegant theory of Liebig was then generally accepted, and appeared quite sound.

According to this, every muscular effort is performed at the expense of muscular tissue; every mental effort, at the expense of cerebral tissue; and so on with all the forces of life. This consumption or degradation of tissue demands continual supplies of food for its renewal, and, as all the working organs of the animal are composed of nitrogenous tissue, it is clearly necessary, according to this, that we should be supplied with nitrogenous food to renew them, seeing that the nitrogen of the air can not be assimilated by animals at all.

But, besides doing mechanical and mental work, the animal body is continually giving out heat, and its temperature must be maintained. 