ELECTROMOTIVE CHANGES IN NERVE. 527 



Before proceeding to discuss in detail the characters of this electrical 

 response, one other aspect of the negative variation must be alluded to, 

 namely, its relation to the cross sectional difference. The negative 

 variation is dependent not merely upon effective stimulation, but 

 upon the condition of the nerve contacts, and is thus related to the 

 resting nerve current. The larger the demarcation difference, the larger, 

 cceteris paribus, will be the negative variation. Most of the conditions 

 alluded to in the previous section as affecting the amount of the resting 

 difference, influence in similar manner that of the negative variation. 

 Thus the spinal cord and those non-medullated nerves which give large 

 demarcation currents give with appropriate stimulation large excitatory 

 effects. 



At this stage it will be convenient to anticipate a portion of the 

 subject matter of the succeeding pages, and state in general terms what 

 is known as to those electromotive changes in an excited nerve which 

 produce the familiar negative variation. When the excitation producing 

 this galvanometric deflection is a rapid series of stimuli, each of these 

 evokes an excitatory state of brief duration, with its accompanying 

 electromotive change. A corresponding series of such states and changes 

 will thus be evoked at the seat of stimulation, and be propagated in 

 strict succession along the nerve. The arrival of each member of the 

 series in the neighbourhood of the contact on the longitudinal surface 

 is contemporaneous with a brief electromotive change at this point, 

 such that the tissue there becomes for a short period galvanometrically 

 negative to the unexcited portions. It is the sum of such a series which 

 affects the galvanometer, and if, on the subsequent arrival of the 

 several transmitted states at the cross section, similar changes were 

 produced, it will be obvious that the tissue here would also exhibit the 

 same condition, when compared with any unexcited portions. Under 

 these circumstances the recording instrument would be acted upon by a 

 series of brief currents of alternate direction, one from cross section to 

 surface being in an extremely brief period (less than -001 sec), followed 

 by one from surface to cross section. It is scarcely necessary to 

 point out that under such conditions there would be no deflection of the 

 galvanometer needle. The considerable deflection which occurs when 

 one contact is upon the surface and the other on the cross section, thus 

 shows that the electromotive change on the arrival of the excitatory 

 state under the former contact is far more intense than that occurring 

 subsequently in the neighbourhood of the cross section, and thus the 

 change resulting in galvanometric negativity of the surface contact 

 becomes prominent. With repeated stimuli the deflection is, in reality, 

 the sum of a series of such electromotive changes under the surface 

 contact, corresponding to the series of stimuli by which they are evoked. 

 Hence the relation of the negative variation to the cross sectional 

 difference must be to a large extent dependent upon the circumstance 

 that, under these conditions, one contact alone is upon tissue which 

 gives the full excitatory response. When both contacts are so arranged 

 on the surface that there is little or no difference, it is obvious that 

 there must be little or no galvanometric indication of negative variation. 



The discontinuous character of the electromotive change evoked 

 in nerve by a series of excitations. — Non-medullated nerves are 

 remarkable, inasmuch as the closure or opening of a single galvanic 

 current may cause an obvious galvanometric indication of a diminution 



