318 MESSRS. K GOTCH AND V. HORSLEY 



ia the cord by stimulating either the cortex of the brain, the different regions of the 

 cord itself, or the issuing posterior roots. These effects resemble in character those 

 evoked in the nerve trunks themselves ; the electrical change commences with the 

 stimulation, is opposed in direction to the resting difference, and subsides on the 

 cessation of the stimulus. Following this true excitatory effect is an after-effect, 

 which is characterised by an increase in the previous difference. 



In any given experiment the after-effect shows itself thus. The cord having been 

 exposed, divided, prepared, and connected in the usual way with the electrodes by its 

 cross section and its longitudinal surface, the amount of the resting difference was 

 noted. This difference rises as just stated for the first few minutes, and then remains 

 very nearly steady. The cord is now excited in any of the three ways mentioned 

 above ; an electrical change occurs, i.e., a current opposed to the difference is present 

 in the galvanometer circuit which, on the cessation of the stimulation, subsides. The 

 galvanometer needle, which evidences the existence of this current, having moved 

 from its resting zero position through a certain deflection, returns to its previous 

 position. If the nerve were the tissue under investigation, this return would be not 

 quite complete, but with the spinal cord it is characterised by continuing beyond the 

 zero position to a point considerably the other side, after which the needle slowly 

 moves back again towards zero, which, however, it does not reach. As the result of 

 excitation, therefore, the previously balanced difference is no longer compensated ; a 

 current persists in the circuit in the direction of that pi-oduced by the difference, and 

 to compensate this, and thus bring the needle back to zero, the balancing circuit has 

 to be readjusted. 



In short, the excitation has caused, after the usual negative variation, a 

 permanent rise in the resting difference between the two electrode contacts. This, 

 positive after effect is not unknown in other structures, it occurs occasionally in both 

 muscle and nerve preparations of the Frog (HERING, HEAD), but the conditions 

 necessary for its production are but imperfectly ascertained. It is, however, as far as 

 we know, invariably present in the case of the spinal cord of the Cat and Monkey, 

 though its amount is very variable in different preparations. 



In almost all cases the amount of the rise is dependent rather upon the condition 

 of the preparation than either the intensity or duration of the cord stimulation. It is 

 greatest as a consequence of the earlier stimulations, and becomes less and less with a 

 repetition of the stimulus. 



If, therefore, a series of experiments are made involving the stimulation of 

 the cord at successive intervals, the resting difference rises at first rapidly, and then 

 more slowly, until after 20 to 30 stimulations in from 30 to 45 minutes, the rise 

 finally practically ceases. 



The stimulus used, whether applied to the cord or to structures connected with the 

 cord, was in all cases that of the alternating induced current previously alluded to in 

 the chapter on the experimental method. The amount of the total rise observed in 



