ALL our sensations, all our movements, most of the activities of our nervous system, depend upon a certain transmitted disturbance which we call the nervous impulse: this, in the study of nerve activity, is what the atom, the electron and the quantum are to chemistry and physics. A rapid reaction to events occurring at a distance is necessary for efficient working. Special nerve cells, therefore, have been developed in all the larger animals: from these the axon or nerve fibre runs out, which is only 3 μ to 25 μ in diameter but may be many metres in length. Along these fibres wave-like messages are sent.