The nerves which go to the muscles send their branches to every fibre, like the fine cords of a match which serve the purpose of firing the powder in the mines at a distance, or, to make use of another simile nearer the truth, although still very far from reality, like the thin wire which conducts the electric spark to a cartouch of dynamite. When the nerve discharges its influence into the muscles, a very rapid molecular change takes place in the substance contained in the cartouches or muscular boxes, which contract, pressing their ends close together. Scarcely has the action of the nerves ceased than they relax and resume their previous form. It is well known, that when a muscle contracts, it becomes thicker and shorter; it suffices to take hold of the arm a little above the elbow and then to bend it in order to feel how the biceps muscle swells and hardens. Wherever a muscular contraction is produced, we may always imagine it as the transverse thickening of an innumerable series of little prismatic boxes, which at the same time shorten in the direction of the fibres. The blood, which circulates in all the most remote corners of the organism, brings, so to speak, fresh explosive materials to charge the muscles again, cleansing it also from the soot and scoriæ. The movement of the blood in the cells and fibres of the body is like a brook flowing through a village, from which every house may draw water for its needs, and into which any useless articles may be thrown and so carried away. If we stop our ears with our fingers, we hear a dull noise, like the distant roar of cannon or a prolonged peal of thunder. This thunder is produced by the contraction of the muscles, for the nerves do not exercise a constant action on the fibres, but develop their influence in very rapid, irregular shocks, like the rattle of musketry in a battle.