302 MESSRS. F. GOTCH AND V. HORSLEY 



have seen differentiation of a mixed nerve on applying the electrodes to one or other 

 side of the trunk of the hypoglossal above the descendens noni branch. 



We have, of course, found that great differences are produced according to variations 

 in the condition of the Mammalian nerve under varying circumstances. 



The nerve must not be dragged upon in any way and the electrodes must be in 

 good moist contact with the bared fibres, the intervention of fat or fascia or dried 

 epineurium being sufficient to diminish or prevent the excitatory effect. 



CHAPTER IV. THE RESTING ELECTRICAL DIFFERENCE IN THE MAMMALIAN 



NERVE AND SPINAL CORD. 



The experiments which form the subject of the succeeding sections furnished us 

 with a large number of observations as to the amount and character of the persistent 

 electromotive difference which exists in the Mammalian nerve and spinal cord 

 between the cross section and surface. 



The number alone of the experimental observations would be sufficient to warrant 

 their introduction at this stage in a chapter devoted to their consideration only ; but 

 as in addition they seem to furnish valuable side evidence as to the relations of the 

 spinal cord both to the cerebrum and the issuing nerves, such an exclusive study 

 becomes a matter of necessity. 



It is well known that when by means of appropriate contacts the cross section is 

 compared with the longitudinal surface of a living nerve, an electromotive difference 

 is found to exist, of such a character that if examined galvanometrically the surface 

 is positive to the cross section. The current associated with this difference was 

 termed by its discoverer, Dtr Bois-TiEYMOND, the nerve current (nervenstrom), and 

 by others, HERMANN, HERING, the demarcation current, since it is presumed by them 

 to have its seat in the zone of tissue which at once bounds and divides the region of 

 living from that of injured and dead or dying nerve. The use of either of these terms 

 is, we consider, objectionable in the present instance, since our object is merely to 

 state the actual fact without involving any cause thereof ; we will therefore designate 

 the persistent electrical difference just referred to the resting difference. 



The difference was in all cases estimated in the following manner. By means of 

 the long compensator, described in the preceding chapter on apparatus, a known 

 difference of electrical potential was introduced into one part of the electrode circuit 

 of a sign opposed to that present at the electrode contacts, and the former was then 

 adjusted until the galvanometer showed no current to be present in the circuit ; the 

 original difference between the two electrode contacts is thus given in terms of 

 decimal fractions of the constant external source of difference of potential. As 

 already indicated, this source was -the Callaud cell, the E.M.F. of which was itself 

 estimated by a similar balancing method in terms of a carefully prepared Daniell cell. 



