and after muscular action. 2. The examination and comparison of the blood coming to the muscle, and that leaving it, during rest and exertion. 3. The examination of the gases given off or absorbed by the active muscle after excision from the animal, and under the influence of artificial irritation. 4. The influence of continuous muscular exertion on the respired gases and on the waste products of excretion.
1. With regard to the changes in the muscular tissue, it has been noticed that the proportion of water in the muscles is increased or the proportion of solids diminished by work, the amount of substances soluble in water is diminished and the amount soluble in alcohol increased; and particularly that glycogen disappears and sugar is increased (the latter probably as a product of fermentation at the expense of the glycogen).
2. Changes produced in the blood are for the most part difficult to trace with certainty; but, it has been observed that the blood coming from the active muscle contains more carbonic acid and less oxygen than that coming from the resting muscle; and, further, that the carbonic acid is increased in greater proportion than the oxygen is diminished. We shall recur to this later.
3. Investigations into the changes which occur in gaseous atmospheres surrounding an excised muscle made to contract under the influence of electricity are interesting and instructive. G. Liebig found that the excised muscle gave off carbonic acid and took up oxygen, but that muscular contraction took place also when the surrounding atmosphere contained no oxygen, carbonic acid being given off, however, in this latter case also. Later observers confirmed these observations, and Matteucci considered, from his experiments in the same direction, that the carbonic acid was not produced at the expense of the oxygen of the surrounding atmosphere, but from oxygen held in some form of combination in the muscular tissue itself. Herrmann found that a portion even of the oxygen absorbed from the air was absorbed in consequence of incipient putrefactions.
4. Investigations under the fourth head, as to the effect of muscular exertion on the general relations of respiration and excretion, have been very elaborate and very numerous. Pettenkofer and Voit, Ludwig and Sczelkow, and others, have investigated the relations of carbonic acid and water given off to food and oxygen consumed as influenced by muscular exertion. Their investigations have shown that the oxygen consumed and carbonic acid and water given off are largely increased by muscular exertion. This had been noticed as a general fact by Lavoisier a half-century or so earlier, but the experiments of the above-named investigators were carried on with a care and thoroughness which left little to be wished for in that direction.
Whether the subject of the experiment be kept on a constant diet during both work and repose, or whether it be allowed to eat and drink according to desire, or even if no food be permitted during the experi-
