268 Dr. N". H. Alcock. On the Negative Variation [Jan. 17, 



resistance of the mammalian nerve between the exciting electrodes 

 must be borne in mind in these comparisons. 



Gotch* has recently stated that in determining the sub-maximal 

 response of frog's nerve, the excitation of a smaller number of fibres is 

 a far more potent cause than the varying response of each fibre, and it 

 seems very probable that the higher threshold and wide range of 

 excitation in mammalian nerves is due to the failure of the exciting 

 •current to reach the more distant fibres, protected as they are by inter- 

 vening fibres and connective tissue, and not to any essential difference 

 in the nerves. 



The negative variation commonly persists without great alteration 

 under ordinary conditions for at least 4 — 8 hours post-mortem. The 

 longest time I have seen was in Experiment C df (internal popliteal of 

 the hedgehog, 28 hours post-mortem) ; the right and left median nerves 

 the kitten in Experiments C c " and C cf gave a small and rapidly 

 diminishing response 19 hours post-mortem. 



The earlier observers (Valentin, Fredericq, Hermann) have stated 

 that they have found the negative variation to persist for days, and to 

 last longer than in frog's nerve. I am unable to confirm this ; even 

 in the hedgehog the nerves are much more short-lived than in the 

 frog under similar conditions, and the phenomena referred to were 

 probably of a different nature to those examined here, viz., electro- 

 tonic spread or ordinary diffusion. 



f -00/ 



> v. 



Fig. 1. — Rabbit. R. ext. popliteal. Normal series of negative variations. 

 Exp. J3 a . Reads from left to right. 



* Gotch, ' J. PBysiol. ' vol. 28, p. -10. 



