The almost instantaneous transmission of sensation and the motor impulse, by the different parts of the nervous system, is one of the mysteries of the organism. This class of phenomena has been compared to those produced in nature by electricity or magnetism, and the question has been raised whether the nervous system is not under the influence of an imponderable fluid produced in its substance, or drawn from the same source as all the elements of animate matter. Various names, such as nervous fluid, nervous force, the active principle of the nerves, have been given to the agent whose hypothetical existence permits us to explain the nervous functions, as we explain the action of the galvanic pile or the movements of the magnetic needle. The admirable discovery of Galvani seemed to prove the analogy, if not the identity, of the electric and nervous fluids. Naturalists and physicians have striven to establish by the aid of experiment, that electricity is developed in the nervous centres and circulates in the nerves. But up to the present time, the most delicate instruments in the hands of the most skilful observers have failed to detect in the nerves the slightest electric current, and nothing authorizes us to consider the nervous force as identical with electricity. May we not consider them as at least analogous? They may both be developed by friction, by chemical combinations, by heat, etc.; both are rapidly transmitted, and both cause an elevation of the temperature, and the composition or decomposition of certain products. But while it is true that motor impulses are transmitted with great speed, comparable to that of the electric fluid, the nervous system contributes only indirectly to produce animal heat, and nothing here suggests a current heating a metallic wire. In fact it is only by hypothesis that we assume the influence of nervous force on the chemical operations of life, otherwise than in giving activity to the organs intrusted with these operations. But we must, notwithstanding, admit a certain analogy between nervous phenomena and electrical phenomena. Further research will doubtless throw light upon this question, which is so eagerly studied, and which would perhaps have already been solved if we could compare the reactions of inert matter to the transformations of organized matter, and the phenomena which are purely physical with those in which life takes part.