VIII 



CONDUCTIVITY AND EXCITABILITY OF NERVE 



109- 



of the reflex centre, so that in the one case the direct excitability 

 of the motor fibres of the spinal cord appears to be heightened, 

 in consequence of a previous excitation of the central end of the 

 sciatic, while in the other the reflex functions of the lumbar 

 enlargement are favoured by previous tetanisation of the spinal 

 cord. And it has, in fact, been found that single descending 

 induction currents, which, when applied directly to a fresh section 

 on the ventral surface of the frog's spinal cord, are per se 

 ineffective, do produce marked excitatory effects after a pro- 

 longed reflex tetanus has been induced by excitation of the central 

 end of the sciatic. Conversely, reflex stimuli that were formerly 



FIG. 170. a, b, Incomplete tetanus of frog's gastrocnemius on excitation of the cord (divided above) 

 with a rapid succession of induction shocks. Single and previously ineffective opening shocks 

 then excite strongly, if led iu at the part of the cord that has just been tetanised, by the 

 same electrodes. 



ineffective can be made to act by protracted and immediately 

 antecedent tetanisation (Biedermann, 37). 



A 7 ". CHANGES PEODUCED IN NERVE BY ACTIVITY 



No less difficult than the establishment of the specific 

 excitability of the nerve is the Question whether (as seems a priori 

 the more probable) the course of the excitatory process in the nerve 

 is associated with metabolism, and in what degree. Two methods 

 are here conceivable. It might be possible to demonstrate altera- 

 tions in the chemical composition of the nervous substance either 

 directly by means of prolonged excitation, or indirectly by 

 investigating the laws of fatigue, and recovery, in the nerve. As 

 regards the first question, it is even harder to decide this point in 

 nerve than in muscle, owing partly to its smaller bulk, and partly 



