790 NERVE 



positive break-contraction is, for a similar reason, stronger than the 

 negative. 



With a ' weak ' current, the only contraction is a closing one at the 

 kathode; with a ' medium ' current there are both opening and closing 

 contractions at the positive pole, and a closing but no opening con- 

 traction at the negative (Practical Exercises, p. 846). 



Conductivity of Nerve. The disturbance which is called the 

 nerve-impulse, once set up, is propagated along the fibres. Are 

 the changes in the nervous substance involved in the initiation of 

 the disturbance at a given point identical with those involved in 

 its transmission from one point to the next, or are they different ? 

 This is a question which has been much discussed, and many 

 attempts have been made to prove that the two processes can be 

 dissociated by acting on nerves with substances like carbon dioxide, 

 ether, and alcohol, which gradually suspend their functions, by 

 cutting nerves off from the circulation and allowing them to die 

 gradually, by depriving them of oxygen and in other ways. Many of 

 the results obtained from such experiments seem at first sight to 

 be favourable to the view that the local change is different from the 

 propagated disturbance. Nevertheless, careful examination of the 

 results on which such statements are based indicates that none of 

 them supplies a crucial test of the question at issue. For example, 

 when a stretch of frog's sciatic nerve is treated with ether or another 

 of the narcotics which act on nerve, and the strength of stimulus 

 determined which is necessary to elicit a contraction when applied 

 to an untreated portion more remote from the muscle than the 

 narcotized area, this strength is found, for some time after the 

 application of the narcotic, to be just the same as it was previous 

 to the application. ' The conductivity ' of the narcotized stretch 

 appears to be unaltered. On the othxar hand, the stimulus, when 

 applied within the narcotized region, must be strengthened, and 

 the narcotic appears to have diminished the ' excitability ' of the 

 nerve. When the narcotic has acted for a longer time, the reverse 

 effect appears. No stimulus, however strong, applied to the central 

 non-narcotized stretch will cause a contraction, the ' conductivity ' 

 having been apparently totally abolished by the narcotic, whereas 

 a strong stimulus applied in the narcotized region will still cause 

 a contraction, showing that ' excitability ' still remains. As to 

 the facts there is general agreement; it is their interpretation 

 which is in doubt. Now, it has been shown that in passing along 

 a narcotized nerve the propagated disturbance diminishes in pro- 

 portion to the length of nerve which it has to traverse. Accordingly 

 in the second stage of narcosis the failure of the stimulus applied 

 to the upper part of the nerve to elicit a contraction is explained 

 most naturally as due to the extinction of the disturbance, which 

 must pass through the whole narcotized region, whereas the dis- 

 turbance set up by stimulation in that region succeeds in reaching 



