VIII 



CONDUCTIVITY AND EXCITABILITY OF NERVE 



99 



a neutral period, in the strict sense of the word, is not always 

 demonstrable, and does not usually exist, as meaning an 

 interval of current intensity at which neither muscle gives 



FIG. 165. Contraction curves of adductor muscle (upper) and abductor muscle (lower) in the cray- 

 fish claw, excited from the common nerve with increasing strength of tetanising induction 

 currents. The figures mark the distance of coil in centimetres. 



any sign of response. There is always a given strength of 

 excitation at which the contraction of both abductors and 

 adductors is very feeble, but the stimulation is never wholly 

 ineffective. In such a case a slight alteration of the secondary 

 coil in one or the other direction suffices to elicit maximal 

 contractions from either adductors or abductors. 



In order to ascertain the presence or absence of a " neutral 



FIG. 166. Contraction curves with indirect, tetanising excitation of adductor (upper) and abductor 

 (lower) muscles of crayfish claw, with gradual approximation of secondary coil. 



zone," it is convenient to excite the nerve continuously, while 

 steadily pushing up the coil. The abductor muscle is first to 



