AXIS-CYLINDER (ELECTRICAL SUBSTANCE). 271 



lary matter (Markstoff), or myeline, that in extremely 

 large quantity fills up the interval between the axis- 

 cylinder and the sheath in primitive nerve-fibres. 

 If the nutrition of a nerve suffer disturbance, this sub- 

 stance diminishes in quantity and indeed may under cer- 

 tain circumstances totally disappear, so that a white 

 nerve may be again reduced to the condition of a grey 

 or gelatinous one. This constitutes grey atrophy, or 

 gelatinous degeneration, in which the nerve-fibre in itself 

 continues to exist, and only the peculiar accumulation 

 of medullary matter has been affected. Herein you may 

 seek for an explanation of the circumstance, that in many 

 cases where, in accordance with the results of anatomical 

 investigation, it was formerly thought one might expect 

 to find a part completely incapable of fulfilling its func- 

 tions, proof has been afforded by means of clinical obser- 

 vation, aided by electricity, that the nerve is still capa- 

 ble of performing its functions, although in a less degree 

 than normal. Hence too it is manifest that the medulla 

 cannot be the constituent in which lie vested the func- 

 tions of the nerve as such. To the same conclusion phy- 

 sical investigations also have generally led, and at the 

 present time therefore the axis-cylinder is pretty gene- 

 rally looked upon as the really essential constituent of 

 the nerve, which is also present in pale nerves, whilst in 

 white ones it can only be distinctly isolated by the sepa- 

 ration of the investing medullary sheath. The axis-cylin- 

 der would therefore seem to be the real electrical sub- 

 stance of natural philosophers, and we may certainly 

 admit the hypothesis which has been advanced, that the 

 medullary sheath rather serves as an isolating mass, which 

 confines the electricity within the nerve itself, and allows 

 its discharge to take place only at the non-medullated 

 extremities of the fibres. 



The peculiar nature of the medullary matter most fre 



