The nerve-current sets free the energy already stored in the muscle. The muscle substance is a magazine or store of energy, and this energy is set free by the molecular action of the nerve.
Having got hold of these general notions as
Fig. 16.—Battery of galvanic elements. Z, zinc plate; C, carbon plate. Arrows show direction of current.
to how a nerve acts on a muscle, let us consider for a little the nature of the electric shocks we employ for stimulating the nerve.
I have here a number of galvanic elements joined together so as to constitute what we term a battery. Electricity is generated in these elements in consequence of chemical changes occurring in them, and we may suppose this electricity to flow like a current out by this wire, along any circuit formed by a con-
