Although they cannot tell the true significance of the electromotive change which marks the passage of an impulse, physiologists are in a much better position now than formerly to controvert certain popular misconceptions. There is no such thing as “nerve-force” in the vulgar sense. A nerve does not transmit energy to a muscle. The muscle obtains the energy which it dispenses when contracting from the foods with which the blood supplies it. The nerve transmits an excitation. Over-excitability is not a sign of strength, but of weakness. Nor is an impulse in a nerve an electric current. It may be generated by an electric shock, but a chemical stimulus is equally as effective. The slow rate at which it travels, as compared with electricity, puts it altogether out of comparison with an electric current. Its relatively rapid progress, on the other hand, equally excludes the hypothesis that it is a movement of ions, as that phenomenon is observed in solutions of salts. What is the nature of the process by which energy is conveyed along a nerve? When speaking of the passage of impulses from receptors to the central nervous system, and through this to effectors, we have used the vague expression “molecular change,” to avoid the necessity of being more precise. But the problem is of such profound interest that we look with eagerness for any hint of the direction from which light will eventually be thrown upon it. Recent discoveries regarding the nature of electricity, combined with investigations at present in progress as to the physical constitution of proteid substances, give more than a hint. Hitherto the choice has lain between a chemical and a physical explanation; now the border-line between chemistry and physics, always wavering, has disappeared. The hypothesis that an impulse is a progression of chemical change has meant in the past that the “wave” was due to the oxidation of substances contained in nerve, with liberation of CO₂ and H₂O. Various considerations render such metabolism of the substance of which nerve-fibres are composed improbable. In the first place, nerve-cell bodies contain a store of material, tigroids (p. 320), which is recognizably drawn upon during nervous activity. It would appear, therefore, to be the tigroids, and not the substance of the nerve-fibre, which supply the energy transmitted along a nerve. Then, again, the axon of a nerve-fibre, enclosed as it is in a tube of fat, is peculiarly ill-placed for the reception of the nourishment which would be needed to make up for waste, if its metabolism be fluctuating and at times excessive. Nor have nerves more than a very meagre blood-supply. Secondly, observation does not give any support to the hypothesis of fluctuating metabolism. A nerve does not give off more CO₂ when active than when passive. Nor does it become acid. Thirdly, nerves, or, to be quite accurate, medullated nerves, are indefatigable. Their capacity for conduction