adopt a kind of materialism, the fundamental idea of which is that the only matter concerned by its transformations in the production of muscular force is food. If a theory, with this idea as its basis, could be substantiated, it would indeed be an advance in positive knowledge, so great that its importance could hardly be over-estimated; and it is not surprising that the simplicity of the explanations of various physiological processes afforded by such an hypothesis should bring to its support many earnest and able advocates.

Since the enunciation of the laws of the correlation and conservation of forces, which are now almost universally accepted, it has seemed impossible to successfully controvert the notion that every manifestation of force in animal bodies, not excluding man, is dependent upon some kind of transformation of matter. Physiologists cannot comprehend the idea of the existence of any force unconnected with material changes, any more than it is possible to conceive of the absolute destruction of an atom of matter or of the generation or creation of something out of nothing.

Taking Nature as she now appears to us, there seems to be little or no basis for what may be termed an immaterial physiology. The researches which I have made into the question of the source of muscular power are not in any way opposed to the known relations between matter and force; they have been directed simply toward the solution of the problem whether the food be concerned directly, by its transformations, in the production of muscular power, or whether muscular effort involve changes in the muscular substance itself, this substance being destroyed as muscular tissue, discharged from the body in the form of excrementitious matter, and the waste being repaired by food. The gravity of this problem can be appreciated when it is remembered that complete and able treatises on physiology have lately been written upon the basis of the idea that food is directly concerned in the production of force, and that the muscular system, like the parts of a steamengine, has no relation to the force developed, except that it consumes food and transforms it into energy, as a mechanical apparatus consumes fuel.

A logical method of inquiry to apply to this question is to disturb the natural balance between ordinary muscular work and the quantity of food, by increasing the work; then to calculate the income and outgo of matter and to ascertain, if possible, what is consumed in the production of force over and above that which can be accounted for by the food taken, assuming that this food is used either in repairing the muscular tissue consumed in the work or in the direct production of the work itself. If it can be shown by such a method of inquiry that excessive and prolonged muscular work consumes a certain amount of muscular tissue, it then becomes a question whether such work involve processes of destruction and nutrition of muscular substance, differing in kind as well as in degree from those which take place in ordinary muscular effort. But I shall not attempt here to prejudge any of the questions that will be involved in the discussion of the facts that I have at my command.

This essay appeared in the Journal of Anatomy and Physiology, Cambridge and London, for October, 1877, and, by some inadvertence, I did not

receive the proofs before it was printed. The typographical errors, both in the figures and the text, were quite important. As it is, I have attempted, in this publication, to present an accurate statement of my own observations and what seem to me to be the logical conclusions to be drawn from these as well as from experiments made by others upon the human subject under conditions of rest and of muscular exercise.

NEW YORK, December, 1877.