calls our attention to the double nature of the mechanism concerned in movement. Muscles are set in motion through the intervention of the nervous system. After they have worked to an unusual extent the nerve-centres connected with them grow tired. This, at least, is a legitimate inference from the fact that headache occurs when certain muscles of the eyeball have been subjected to an improper strain. But it must be remembered that the muscles of the eyeball never tire. They do not, like other voluntary muscles, give notice that they are in need of rest. It is not so clear that the central mechanism is in any way involved in the fatigue which is produced by excessive use of arms or legs. The muscles of the limbs (and the central nervous system) are protected by the sensations which originate in muscles when they are overworked. The fact that a weary man can, if a great emergency demands activity, use his muscles with as much vigour as if he were fresh from bed, has been cited as an argument in favour of the view that fatigue is of central origin; but it is an argument which works both ways. A strong emotion causes a fervent response from the nervous system. Tired muscles contract energetically when the impulses which reach them are sufficiently urgent. 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