Nothing so definitely removes muscle from the category of machines as its liability to fatigue. To speak of a muscle as tired is, of course, to transfer to an object a term which is applicable only to a phenomenon of consciousness; but it is necessary, unless a cumbrous expression is to be used, to designate thus the effect upon the muscle of prolonged activity. The petrol may be low in the tank, but the quantity burnt in the cylinder at each stroke is not reduced. If an isolated muscle is repeatedly stimulated by an electric current of a certain strength, the response which it makes improves for the first two or three induction shocks; then it begins to weaken. At each succeeding spasm the muscle shortens a trifle less than before. More remarkable than the diminution in the amount of work done by a muscle which is growing tired is the prolongation of the time taken both in contracting and in relaxing. Further, it has been shown that the fatigue which accompanies the contraction of an isolated muscle is not a condition dependent upon the shrinking of the store of energy which it possessed when it was first thrown into activity. Muscles undisturbed as to blood-supply, and contracting under the direction of the Will, also exhibit it. Speaking generally, it may be said that the tiring of muscle is not so much due to the exhaustion of its store of food as to accumulation of products of action. Vigour is restored to a tired muscle by passing through its bloodvessels a stream of salt-solution, which brings it no food, but washes away some of its waste. But the problem is far more complex than this. The machinery is not simply clogged with the products of its own activity. If the blood of a tired animal is injected into the vessels of one that is rested, the muscles of the latter exhibit the phenomena of fatigue. Evidently muscle is self-protective. During activity it prepares a “fatigue-substance” which poisons its own nerve-endings, making them worse conductors from nerve to muscle of the commands which descend from the brain. Not only does the fatigue-substance dull the nerve-endings in the particular muscle which has contracted, but, being distributed by the blood to the whole body, it produces a general effect. If the legs have been severely worked, they exhibit fatigue in the highest degree; but after a long walk the arms also are less ready and less capable than the state of their nutrition warrants.