Staleness, again, which we may define as a failure of co-operation between muscles and will, and which is most noticeable in athletic pursuits (since in ordinary life slight sluggishness in co-operation is not practically noticeable) arises also from fatigue of the nerves, due to excessive work, want of nourishment, or monotony of employment. To take the smallest imaginable instance, supposing two men, well trained in matters of eye, attempt merely to hit a lawn-tennis ball backwards and forwards over the net, as easily and gently as possible, they cannot keep it up for a quarter of an hour. Each stroke is simplicity itself, less than the A B C of the game. Yet by the monotony and tiresome iteration of it, their nerves get bored; there is no failure of muscular power, but they are _unable_ to hit with the minimum of correctness after a very few minutes. Here is temporary staleness in its most elementary form, but it is some similar nerve-failure which is at the root of most staleness; incapability of correct work occurs without such fatigue of the muscle as would account for it; and to repeat the metaphor likening the muscle to the receiving instrument in a telegraph system, and the nerve to the wire through which the message passes, it may be a failure or a fault in the wire, though the instrument be intact, and the message (from the brain) be perfectly sent. That particular wire, or set of wires, must, therefore, be given rest, for it is mainly by rest that health and power come to nerves. In other words, change of exercise is required; or, if the whole system is a little out of gear, complete though temporary rest; or, again, a nerve-tonic may be needed; light, air, and cold water may set things right. Or, since the harmonious working together of all the parts of the body is conducive to correct work by any one nerve, it may be that the digestion is primarily at fault.