iv GENERAL PHYSIOLOGY OF NERVOUS SYSTEM 215 



nearer the centre of the longitudinal surface of the nerve, when 

 the kathode is on the cross-section. Moreover, many conditions 

 that affect the demarcation current in one direction or the other 

 affect the action current in the same way. Non-medullated nerve- 

 fibres, which yield a more pronounced demarcation current, also 

 exhibit a stronger action current. In non-medullated fibres 

 the mere opening or closing of a constant current indicates on 

 the galvanometer a diminution in the difference of potential present 

 in the resting state, while a tetanising current (i.e. a succession 

 of stimuli of given frequency) is required to obtain the same effect 

 in medullated fibres. This is because in medullated nerve the 

 intensity of the electrical phenomenon is too low for the passage 

 of a single wave of the action current to act upon the galvano- 

 meter. But it can be demonstrated, as in muscle, that the 

 uniform negative variation shown by the galvanometer during 

 the tetanising stimulation of a nerve is the effect of a series of 

 discontinuous electrical changes which have the same rhythm as 

 the stimulus sent into the nerve. 



Bernstein (1867) demonstrated this with his differential 

 rheotome, by which the galvanometer circuit is rhythmically closed 

 for the briefest period at regular intervals which coincide with 

 each stimulation. He found that the negative variation starts at 

 once in the part excited, that it is propagated along the nerve at 

 the same rate as the excitation (27 m. per second at 15 C.) ; lastly, 

 that it remains a very short time at each point of the nerve 

 (0'0007 sec.), corresponding to a wave-length of about 18 mm. 



Wedensky adopted the telephone to render the rapid successions 

 of the currents of action perceptible to the ear on tetanising the 

 frog's sciatic. When connected with the nerve that is being 

 tetanised, the telephone gives the sound that corresponds with the 

 number of induction shocks from the exciting current. When 

 the strength of the shocks is increased, the sound in the telephone 

 is also strengthened till it reaches a maximum, after which no 

 further increase of current strength increases the effect in the 

 telephone. If the nerve is killed by ammonia every sound in the 

 telephone ceases. 



Gotch and Burch, by substituting a highly sensitive capillary 

 electrometer for the galvanometer, were able not only to demon- 

 strate the discontinuous character of the electrical changes 

 produced by faradisation of the nerve, but also to photograph the 

 action currents, as shown by the oscillations of the mercury 

 meniscus in the capillary. By this method they found that the 

 negative variation reached its maximum in O'OOl sec., and lasted 

 longer when the temperature was lower. Further, on comparing 

 the curves of the capillary electrometer with those obtained from 

 currents of known strength, they found that the negative variation 

 may amount to 0'03 volt at 5 C. 



