while the same process of tear and wear goes on in a muscle, the muscle, being a living thing, has the power of self- repair. It is always engaged in mending itself, building up so as to make good the waste, and in this way for a long time it is able to work efficiently. The substances needed for building it up are brought to its own door by the blood, many of them ready made, and it takes them into itself and repairs the machinery. These substances for repair are no doubt the proteids, the carbo-hydrates, the fats, saline matters, and water. They all seem to be necessary for the upkeep of the muscle-substance.
But our little machine not only keeps itself in repair, but it can excite chemical changes in certain matters brought to it, and by these changes energy is liberated. There are strong grounds for holding that the carbo-hydrate matters are changed or altered in this way by the action on them of the living muscle-substance. The history of these carbo-hydrates is very wonderful. Entering the body mainly as starch, they are changed into sugar; then they pass into the blood and are carried to the liver; then they are reconverted into the
