THE intimate nature of the nervous impulse has long been a subject for speculation by physiologists and psychologists. Simple models to explain the mode of transmission of an impulse along a nerve, like the electrical one of the marine telegraphic cable or like the chemical one of a train of gun-powder, have proved to be totally inadequate to explain the various phenomena observed in nerve. Until recently, the relative unfatiguability of nerve favoured a simple physical and non-chemical hypothesis. Modern refinements in technique, however, have shown that nerve during activity utilises chemical energy and produces heat. It is only in the last few years that the two phases of heat production in nerve have been analysed, and an account from the leading investigator in this field, Prof. A. V. Hill, appears in the special supplement to this week's issue of NATURE. Prof. Hill favours an electrochemical theory of a self-propagating wave of disturbance to explain the various forms of behaviour exhibited by an impulse in its passage along a nerve. Interesting experiments on the ionic distribution of potassium ions lend convincing support to the theory. The supplement will also be found to give a useful summing-up of the present position reached by investigators in this branch of nerve physiology.