230 ELECTRO-PHYSIOLOGY CHAP. 



with the data cited above for differences of excitability, and 

 susceptibility to injury, at different points of the uninjured 

 nerve. Mendelssohn convinced himself that the E.M.F. of the 

 axial current was greater in proportion with the rate at which 

 the excitatory impulses traversed the nerve in one or the other 

 direction, i.e. the more the nerve was excited within the organism. 



The great regularity of electromotive action in divided or 

 otherwise injured nerve obviously goes no further towards 

 establishing the pre-existence of an electrical potential within 

 such tissues than in the parallel case of muscle. Here, as there, 

 on the contrary, it must be affirmed that the perfectly uninjured 

 nerve is electrically inactive. It is obvious, in view of the mode 

 in which the nerve-fibres end in the peripheral organs, or central 

 system, that we cannot speak of leading off from a " natural 

 cross-section " (in the same sense as in muscle), especially as 

 not merely the motor end-organs (muscle), but others (e.g. gland- 

 cells) also, are proved to be, actually or potentially, the seat of 

 electromotive action. This applies, inter alia, to the organ which 

 seems at first sight best adapted to decide this question, viz. the 

 eye, as investigated by du Bois-Eeymond and others. Some 

 account of its electromotive activity will be given below. 



Electromotive action in nerve, as in muscle, is a vital property 

 of the living tissues. The nerves of a corpse gradually (albeit in 

 most vertebrata very slowly) cease to exhibit any difference of 

 potential between a fresh demarcation surface and points on the 

 uninjured superficies. It is intelligible that this should occur 

 sooner in warm- than in cold-blooded animals, as also that nerves 

 left in the body should preserve their normal properties longer 

 than excised nerves ; and that excitability should decline most 

 rapidly in the central tracts that are, generally speaking, the 

 least capable of resistance. Steiner (7) finds that the E.M.F. of 

 the nerve current increases within a certain range with rising 

 temperature, reaching its maximum between 14 and 25 C. At 

 boiling-point the current is reversed, according to du Bois-Eey- 

 mond, as Harless also finds at a certain stage of drying. Electro- 

 motive activity may persist for a long time during the process 

 of degeneration suffered by nerves that have been separated from 

 their centres, which is also natural, seeing that the medullary 

 sheath is first to be disintegrated in medullated fibres. Schiff 

 in id Valentin (8) found that nerves of birds and mammals, 



