The action of the heart itself, for instance, which is, in ordinary circumstances, entirely automatic and absolutely essential to life, consists in the alternate contraction and expansion of muscle, which sends the blood to all parts of the body; and the heart, like all other muscles, can be made stronger with suitable exercises, can be overtaxed by undue strain, and can lapse into feebleness and degeneration if it is not rightly and fully used. For the health of all muscles, apart from their proper nourishment by food and air, lies in their proper use. All muscles which are within our reach, so to speak, which by an effort of will on our part are made to move some portion of the body, are controlled by nerves, which transmit the message, as through a telegraph wire, from the brain to the required place. By constant use it is possible, as in the case of walking (in itself a rather complicated movement), so to accustom the nerves to send their messages that, after a time, the action of the muscles become automatic, and we are conscious of no effort of will to make them work. The instinct of self-protection is another instance of this automatic use of muscles; a man will put up his arm or duck his head to avoid a blow without consciously bidding the muscles of the arm or neck to act. Not only does continued use of a proper kind give this speed to the communication between brain and muscle, but--granted proper nourishment--it gives strength and health to the muscles used; and, broadly speaking, the man who has _all_ the muscles of his body in perfect working order, is physically a thoroughly healthy man.