36 ESSENTIALS OF PHYSIOLOGY. 



being used. A single induction shock is applied to the nerve at a 

 distance from the non-polarisable electrodes, and the movement of the 

 mercury in the capillary electrometer or of the string of the string 

 galvanometer is photographed on a rapidly moving plate. In this w#y 

 it is shown that the part of the nerve which is excited at any instant is 

 negative to all other parts of the nerve. 



By recording the electric response, it can be shown that when any 

 part of a nerve is stimulated the disturbance is propagated in both 

 directions. If the middle of a length of nerve is stimulated by a single 

 induction shock, each end of the nerve being in circuit with a galvano- 

 meter by means of non-polarisable electrodes, both galvanometers record 

 the passage of a current of action. Apart from the electric response, 

 the changes produced in a nerve by the passage of an impulse are very 

 slight. No appreciable production of heat can be detected by the most 

 delicate thermo-electric methods. Chemical changes must take place, 

 because the nerve fibres lose their irritability when completely deprived 

 of oxygen, and the loss of irritability occurs more rapidly if the nerve 

 is stimulated ; but the true nature of the chemical changes is unknown. 

 Possibly the changes which occur during the propagation of the dis- 

 turbance are largely physical, and depend upon changes of surface 

 tension. 



The Velocity of a Nervous Impulse. The rate at which an impulse 

 is transmitted in the nerve of a frog may be measured by using a 

 muscle-nerve preparation and recording the contraction of the muscle 

 on a rapidly moving drum or pendulum myograph, (1) when the stimulus 

 is applied as far as possible from the muscle, and (2) when it is applied 

 close to the muscle. The two tracings are recorded on the same 

 abscissa, the point of stimulation being at the same point of the tracing 

 for both. The difference in latent period is measured by means of a 

 time tracing taken from a tuning-fork giving 250 double vibrations 

 per second. This difference gives the time taken by the impulse to 

 travel along the length of nerve between the two points stimulated. 

 In the case of frog's nerve the rate of transmission is found to be about 

 28 metres per second. In human nerves it is estimated to be about 

 four times this rate. 



Conditions which affect Excitability and the Propagation of the 

 Disturbance in Nerve. It has already been pointed out (p. 30) that 

 nerve fibres cannot be fatigued. Their excitability and conductivity 

 may, however, be affected by temperature, drugs, and the passage of a 

 constant current, as well as by the passage of a previous impulse along 

 the nerve, and by injury. 



Generally speaking, the irritability of nerve for induction shocks is 



