vin CONDUCTIVITY AND EXCITABILITY OF NERVE 95 



currents at lower points. V. Eleischl, indeed, makes the matter 

 still more complicated by dividing the entire nerve into a number 

 of segments, each of which is more susceptible to the descending 

 current in its upper half, to the ascending current in its lower, and 

 to both currents equally at the centre ("equator"). 



In view of these several experiments it seems a priori most 

 probable that excitability in the 

 normal nerve of the living animal 

 is equal at all points of its course. 



Electrical excitation is reallv leSS FlG - IS*. Curve of excitability along the 

 . . sciatic nerve. (Heidenhain.) 



fitted for determining this than 



chemical or mechanical stimuli. With the last, Tigerstedt (46) 

 has actually ascertained the equal excitability of all points of the 

 uninjured nerve. There is therefore no " avalanche " increment 

 of excitation, or, at least, no fact is known which compels us to 

 assume its existence. 



Certain differences (which seem, however, to be caused less by 

 local disparity of the true excitable substance of the axis-cylinder 

 than by local irregularity of development in the investing 

 medullary sheath) do nevertheless exist between the central and 

 peripheral tracts of nerve. These are indicated not merely by 

 histological evidence (Clara Halperson, 47), but also by the fact 

 that the upper end of the frog's sciatic is more susceptible to the 

 action of poisons (alcohol, etc.), than the deeper segments. 



The observations of Clara Halperson show for electrical, and 

 those of Efron (48) for other (chemical, thermal, mechanical) 

 stimuli, that the responsive capacity in the upper segments of 

 the nerve is greater in itself, and is raised more rapidly, and to a 

 higher degree, by reagents which augment excitability, as well as 

 by heat, than is the case in the lower segments. 



The application of toxic substances, or of cold, again takes 

 effect sooner in the upper segments. Efron found, e.g., with 

 two frog's nerves (of equal length and equal excitability), one of 

 which was treated above, and the other below, with dilute amyl 

 alcohol, that the excitability of the upper tract of the nerve dis- 

 appeared completely, while that of the lower was still uninjured. 

 In view of the intimate connection between excitability and 

 conductivity, it is not remarkable that conductivity should be 

 differently affected in the upper and lower parts, like excitability. 

 According to Efron, who depressed the excitability of the middle 



