Page:Life in Motion.djvu/164

144 animal starch, glycogen, and stored up in the liver for further use; lastly, the glycogen is again changed into sugar, either in the liver or in the muscles; and in the muscles the sugar is used up, being ultimately decomposed into carbonic acid and water. The splitting up of this sugar or carbo-hydrate in the muscle is, there is every reason to think, the main source of the liberation of the energy in the muscle. This view explains in particular the greatly increased consumption of oxygen and the greatly increased production of carbonic acid following active muscular exertion. At least a part of the carbonic acid is the waste product arising from the decomposition of the carbo-hydrate.

Now if the muscle receives no carbo-hydrate, or an inadequate supply of it, it does not follow that it will stop working. Experiment has shown the contrary. It will still work, using up the fat in the first instance; and if there is an inadequate supply of fat, it actually uses up proteid matter, or, in other words, it uses its own substance. If an animal is caused to work very hard, we do not find an increase in the excretion of the nitrogenous waste