520 NERVE. 



that different points of its surface will show corresponding differences of 

 electrical potential. When a current is obtained in the experimental circuit 

 between any two points of the surface, and is counterbalanced by a suitable 

 external electromotive source, the latter is not a measure of the electro- 

 motive change in the interior of the investigated source, but only of the 

 surface differences. Supposing, therefore, that the surface itself is altered, 

 and its electrical resistance changed by increase of moisture, then the 

 current compensation is no longer adequate, since alterations in electrical 

 distribution occur in the new moist envelope. The estimation of the 

 amount of the electromotive force in resting nerve is in like manner that 

 of the amount of surface distribution, and the real value of the change present 

 at the electromotive source is not determined. Other things being equal, the 

 surface change varies directly with that of the interior; but the surface change 

 may be but a fraction of the amount of the electromotive difference present 

 within the nerve fibres. Since the nerve is a moist conductor, its electrical 

 resistance is increased by cooling and diminished by warming. The resist- 

 ance is very large ; in the case of the sciatic nerve of the frog it is about 

 100,000 ohms for each longitudinal centimetre of nerve; it is also greater to 

 currents flowing across the nerve fibres than to those flowing in the direction 

 of their axis, being five times as great in the former case. The resistance is 

 best determined by balancing the nerve resistance against a variable one, after 

 the method of Kohlrausch. In this a series of alternating induced currents 

 traverse a Wheatstone bridge and a portion of nerve ; a telephone suitably 

 connected indicates by its silence when the balance is complete. 



The influence of various conditions upon the amount and dura- 

 tion of the resting currents. — The amount of the electromotive differ- 

 ence between the surface and cross section of a nerve, as indicated by 

 the amount of the surface derivation, varies in accordance with the 

 structural character and physiological condition of the tissue. 



(a) The character of the nerve. — Any mixed nerve, such as the sciatic, t 

 will show an electromotive difference between surface and cross section. 

 The amount of this difference does not appear to be entirely dependent 

 upon cross sectional area, although the surface distribution must be 

 largely influenced by this. Thus both the slender nerve of the frog 

 and the larger one of the dog are capable of giving a high difference 

 (•02 D), whilst the large nerve of the horse and the smaller one of 

 the monkey give low differences ( - 008 D). 1 This is also the case in such 

 a nerve as the large second electrical nerve of Torpedo, which, with 

 24,000 fibres, gives only -007 D. 2 The amount is thus related to the 

 source of the nerve, i.e. the animal from which it is taken, and is probably 

 an expression of the varying susceptibility of the nerve to the particular 

 molecular change which produces the demarcation effect. This is itself 

 dependent upon the extent to which the uninjured surface of the nerve 

 retains its vital properties and still possesses the particular molecular 

 state which characterises the living tissue. All circumstances which 

 impair functional capacity diminish the magnitude of the effect. 



The differences just referred to may thus be related to variations 

 in the functional attributes of the nerves of the various animals 

 mentioned. But, in addition to this, the amount of the resting 

 electrical difference is associated with structural peculiarities of the 

 nerve fibres. Thus non-medullated nerves give far larger differences 

 of potential than medullated ones. The fine nerves in such inverte- 



1 See Biedermann, " Elektrophysiologie," 1895, S. 638. 



2 du Bois-Reymond, Sitzungsb. d. k. Preuss. Akad. d. Wissensch., Berlin, 1883. 



