firmed by Henneberg's experiments on oxen. More carbonic acid is excreted by day than by night, since more work is then done. But at the same time less oxygen is taken into the body in the daytime than during the night. For example, in one of Voit and Pettenkofer's experiments, for every 100 parts of oxygen which entered the system in the daytime 175 parts were contained in the carbonic acid excreted, while in the night the same relation was expressed by the number 58. When work was performed the difference was still greater. This and similar experiments show plainly that a large part of the carbonic acid excreted is formed at the cost of oxygen previously laid up in reserve, and that the increased rapidity of respiration during work is not for the purpose of supplying more oxygen, but of removing the carbonic acid.
It has been also shown that the amount of oxygen that can thus be stored up in health is proportioned to the amount of albuminoids in the food, and this is another indication of the importance of these bodies in the production of muscular power.
The necessity of this storing up of oxygen is strikingly shown by experiments on two diseases in which the patient is almost incapable of muscular exertion, viz., diabetes mellitus and leukæmia lienalis. In these diseases the total excretion and the total amount of food are not much different from those in health; but there is no such storing up of oxygen as in the healthy organism, and there is also an almost entire lack of strength.
This fourth condition is, for our present purpose, the most interesting and important of all. It shows that work is not produced by the direct oxidation of food materials by the oxygen of the blood, but that the muscles themselves contain a store of latent energy which the will can set free at pleasure, independently of oxygen, while the blood serves to wash out the waste products and gradually to renew the supply of force during those periods of rest of which this fact explains the necessity.
That the seat of this latent energy is in the muscles is shown by the fact that they are capable of contraction for a time after their blood-supply has been cut off, or even after their removal from the body. A frog's heart, when removed from the body and freed from all blood by injection of a weak solution of salt, will continue to beat for hours, and the whole animal under the same circumstances moves, leaps, and behaves in short like a living animal. Agassiz relates that on one occasion he captured a shark which fought as long and fiercely as is usual with these animals, but which, when finally secured, was found to have its gills eaten through by parasites, and almost all its blood replaced by sea-water. (Liebig.)
Like a bent spring the muscle contains a certain amount of potential energy, which the will can use at pleasure; but when the supply is once exhausted, when the spring has lost its tension, a further supply of force from without is necessary before more work can be performed.
