ix ELECTRICAL EXCITATION OF NERVE 185 



2. Excitation of Nerves in Crayfish 



On the crayfish claw, under certain conditions, the excita- 

 tory effects of the constant current may be remarkably developed 

 (Biedermann, 43). It has already been stated that with tetanising 

 excitation of the claw -nerve, the tonically contracted adductor 

 muscle relaxes at about the same comparatively low strength of 

 current at which the abductor contracts vigorously ; while strong 

 currents, again, throw the former into tetanic contraction, the 

 abductor either suffering no visible change of form, or, if there is 

 any tonus, becoming relaxed. Hence there would appear to be 

 a complete antagonism of excitatory conditions in the nerves 

 corresponding with the two muscles. 



The results of excitation with the constant current, on the 

 other hand, are much more complicated. In the first place, there 

 is never a " neutral zone " of current strength as defined above, 

 although striking and perfectly regular differences of action 

 between currents of different strength are by no means want- 

 ing. In agreement with the excitatory reaction on tetanising 

 the nerve by means of alternating currents, e.g., we find with 

 closure of a battery current that the excitatory effects pre- 

 dominate or appear alone in the abductor, the inhibitory effects 

 at the adductor, with low intensity of current, while with stronger 

 currents the contrary effect appears. In detail, however, the 

 effects are far harder to analyse, because at each adequate ex- 

 citation both impulses (excitation and inhibition) usually appear, 

 so that in tracing the changes of form in one of the two tonically 

 contracted muscles, highly complicated curves may arise, which 

 are only intelligible on the ground of the previous data. 



The effects of excitation are best seen in the atonic adductor 

 muscle, where the results agree throughout with experiments on 

 other nerve-muscle preparations, and correspond perfectly with 

 Pfliiger's law of contraction. Medium currents here work in- 

 dependent of the direction in which they are passed through 

 the nerve, exciting both at make and at break, while a strong 

 descending current excites at make, a strong ascending current 

 on the contrary at break only. It is to be remarked for these 

 experiments that every stronger excitation gives rise to a more 

 or less prolonged tetanic contraction of the muscle, so that per- 

 sistent excitation by the constant current is here the rule (supra). 



