a compound made up of carbon and oxygen, it is evident that the muscle must have got oxygen from somewhere. The only thing we can say about this is that the muscle takes it from matters in its own substance that contain oxygen.
We have learned, then, that an active muscle becomes acid, that it uses up oxygen, and that it produces carbonic acid. Other chemical changes happen in a muscle that I will not attempt to demonstrate, as the methods by which this can be done require time and many refined appliances not suitable for the lecture-room. I shall merely mention them. Thus a peculiar kind of starch (glycogen), formed in the liver, is carried to the muscles by the blood, and is there consumed. We do not know, however, how the muscle uses the glycogen, whether it uses it directly or whether it first splits it up and then uses some product of its decomposition. So long ago as 1845, von Helmholtz pointed out that by exercise the substances that can be dissolved out of muscle by water are diminished, while those soluble in alcohol are increased, indicating that the one set of substances was
