Several experimenters have corrected or perfected some points, but the general doctrine has not been shaken by the recognition of its secondary and very subtle difficulties, several of which still puzzle physiologists. It is, indeed, undeniable that the greater part of the reactions which occur in the system, with the production of heat, do bring out, as a result, the exhalation of watery vapor and carbonic acid from the lungs; but these two gases cannot arise from a direct combustion of hydrogen and carbon, because the system does not contain such substances in a free state. They represent really only the close of a succession of transformations, often distinct from combustions, properly so called. On the other hand, these are not the only residue of the chemical operations performed in the vital furnace. Besides the water and carbonic acid thrown off by animals in breathing, which are like the smoke of this elaboration of nutrition, they excrete by other channels certain principles which are, as it were, the scoriæ. Now, these principles of disassimilation, among which should be noted urea, uric acid, creatine, cholesterine, etc., could not be results of pure combustion, and they denote that the circulating current is the seat of extremely manifold reactions, the laws of which we are only beginning to gain a glimpse of.
The latest advances of chemistry allow us, indeed, to follow the linked sequence of the gradual transformations of nutritive substances into the cycle of vital operations. It is well, at the outset, to fix exactly the seat of these phenomena. They take place in all the points of the system traversed by the capillary vessels. The glands, the muscles, the viscera, in brief, all the organs, are in a state of constant burning—they are every instant receiving oxygen, which brings about alterations of various kinds in the depth of their substance. In a word, every organ breathes at all its points at once, and breathes in its special way. Certain physiologists of the present day are wrong in localizing the phenomena of breathing in the capillary vessels. They are merely the channels of transfer for oxygen, which, by exosmosis, penetrates their thin walls, and then effects, by direct contact with the smallest particles of the organized mass, the chemical action which keeps up the fire of life. It is easy to prove this by placing any tissue, lately detached from the body, in an oxygenated medium. We remark in this case an escape of carbonic acid, together with a development of heat, and this possibility of breathing outside the system proves clearly that such act can be accurately compared, as Lavoisier thought, to the combustion of any substance. The only difference is with regard to intensity. While a candle or a bit of wood burns rapidly, with a flame, the combustible materials of organic pulp unite with oxygen in a more slow and quiet manner, less violently and manifestly.
The blood, which flows and reflows incessantly in the most slender vessels of our bodies, and charges itself full with oxygen every time
