for days after continued muscular exertion, would be in harmony with such an origin, as they might indicate a gradual replacement of glycogen consumed, at the expense of albuminoid material with elimination of urea as a a waste product. Sugars (grape-sugar and maltose) absorbed from digestion or formed from liver-glycogen, are doubtless consumed in the tissues and organs and assist in producing animal heat. Whether muscular tissue consumes these sugars in greater quantity than other tissues it is difficult to say with certainty.
We come now to the second question as to the nature of this decomposition to which we have alluded as oxidation. This question is still contested. The older theory is that the oxygen, taken up by the blood, is given up in the form of active oxygen, or ozone, and by its energetic oxidizing power burns up or oxidizes the carbon and hydrogen of the fuel-material, with formation of carbonic acid and water.
The newer theory is that the decomposition processes are essentially fermentative in their character; that under the influence of appropriate ferments the substances combine with water, splitting up into simpler and simpler products with evolution of heat or force, as is the case with all fermentative changes. The oxygen present in the arterial blood gives these processes the character of fermentative changes in the presence of oxygen; secondary oxidation takes place, as in putrefaction in presence of air, the final products being mainly carbonic acid and water, as also is the case in putrefactive processes.
Some of the objections raised to the older theory are that we know of no similar changes produced by ozone in watery solutions, such as exist in the animal organism; that the oxygen obtained from the arterial blood under the air-pump contains no ozone. Also certain compounds are found in the blood and tissues which are essentially deoxidized products, which could not be supposed to exist in the presence of ozone, but the presence of which accords with the supposed fermentative character of the processes (Hoppe-Seyler). The fact that the evolution of carbonic acid from the contracting muscle is in great part independent of the presence of oxygen at the time would harmonize also with such a fermentative character of the changes, as carbonic acid is the product of many fermentative changes out of the presence of oxygen, as, for example, of the alcoholic fermentation of sugar. Matteucci's supposed storing up of oxygen in some form of combination in the tissues would then be interpreted rather as the storing up of fermentable substances (like glycogen) rich in oxygen. The combustion theory, on the other hand, would seem to demand that the evolution of carbonic acid and consumption of oxygen should be simultaneous, which is apparently contradicted by the experiments of G. Liebig, Matteucci, and others above mentioned. It would exceed our limits to enter more fully into a discussion of these two opposing theories. The conflict between them is still in progress, and new evidence is constantly accumulating. Both theories agree in this, that
