form in which it can be appreciated by those non-specialists most interested in the subject involved.
It is thus, to a certain extent, with the subject of the source of muscular power in the animal organism. It is needless to specify in this particular. Text-books and popular articles touching on the subject are continually asserting, as apparently unquestioned, theories which at the present time are either exploded or very much in doubt. It would seem, therefore, not without value to attempt, as far as practicable in a popular or semi-popular article, a general statement of the present condition of the theories on the source of muscular power, and of the main points of the evidence which tends to support these theories.
The general acceptance of the law of the conservation and correlation of physical forces had at once an important influence in directing attention to the source of muscular force. The idea was readily taken up that this form of force is at the expense of heat, which is produced by the oxidation of carbon and hydrogen in the body, the necessary oxygen being conveyed by the arterial blood to the muscular tissue. In other words, the somewhat trite comparison of the human body and the muscular system to an engine, which consumes just so much fuel to produce so much force, has pretty clearly formulated the idea as generally accepted. And so far as it goes the comparison is not bad.
When, however, we pass beyond this somewhat vague simile to an examination of the more intimate nature of these various processes, we find the questions raised are not so generally understood. Accepting that the muscular force is produced by the ultimate oxidation of carbon and hydrogen to carbonic-acid gas and water respectively, the next questions that suggest themselves are: "What is the immediate source of this carbon and hydrogen—the fuel material for muscular force?" and "What is the real nature of these processes which we call briefly oxidation?" The endeavors to answer these questions have given rise to many discussions and disputes, which are, even at the present day, by no means concluded.
Before taking up the discussion of the theories advanced to answer these questions, it will not be out of place to review very briefly the composition of the muscles and their general relations to the circulation—only in so far, however, as is necessary for a clear comprehension of the evidence and arguments involved in the discussion.
A muscle is essentially a collection of lengthened cells held together by a connective tissue. Each cell consists of a delicate cell-wall or membrane containing a fluid or semi-fluid mass of living (protoplasmic) matter. This gelatinous substance possesses the power of contraction under the stimulus of excitations of various kinds—nervous impulse, electricity, heat—and the cell becomes thereby shortened. This process, taking place simultaneously in all the cells of a given muscle