1888.] The Spinal Cord, and the Cortex Cerebri. 21 



jected upon the plate. Its E.M.F. was from about O01 to O015 D. 

 The passage of the single break induction shock through the platinum 

 electrodes in either direction was followed by a small quick movement 

 of the mercury, which was invariably in the opposite direction to 

 that produced by the demarcation current. Its amount varied in 

 different animals from 1 to 2' 5 divisions of the micrometer eyepiece, 

 and it is shown as photographed in fig. 1 and fig. 2. After severing 

 the nerve from the bulbo- spinal system above the exciting electrodes, 

 the same effect was obtained ; its character, as shown by the movement 

 of the mercury was, however, different, being as we believe much 

 shorter in duration and less in amount. But our experiments not 

 being directed to the elucidation of this point, we will not speak 

 positively with regard to it. After a time, varying in different cases 

 from twenty minutes to three-quarters of an hour, the effect was no 

 longer visible. This movement of the mercury may be conceivably 

 due to the three following factors, working singly or in co-opera- 

 tion : 



(A.) Escape of the exciting induction current (uni-polar). 



(JB.) Electrotonic change. 



(C.) The true excitatory variation of the nerve. 



(A.) That it was not due to any escape of the induction current is 

 shown by the following facts : 



(1.) The variation was produced by the very weak induction 

 currents, such as those obtained when the Helmholtz wire is used, 

 and its character did not vary with increasing strength of the 

 current. , * . 



(2.) It was no longer perceptible when the nerve was ligatured 

 between the exciting and leading-off electrodes. 



(3.) As the nerve gradually died the effect became less, and was 

 no longer perceptible when the nerve was severed from the animal 

 and left for three-quarters of an hour. Moreover, when the nerve was 

 indifferently prepared the variation was absent, or else very small and 

 transient. 



(4.) The effect remained visible when the electrometer was short 

 circuited for T ^Vo second after the break of the exciting key. 



(B.) That it was not due to electrotonic change is shown by the 

 following additional facts : 



(1.) The direction of the effect was always the same, that is, 

 opposed to that of the demarcation current whatever the direction of 

 the exciting current. 



(2.) When the exciting electrodes were shifted to within a centi- 

 metre of the proximal leading-off electrode, an effect was produced, 

 the direction of which was dependent upon that of the exciting 

 current (fig. 3). This effect differed from that of the true variation 

 in other particulars, viz., its amount was dependent upon that of the 



