370 MKSSRS. P. GOTCH AND V. HORSLEY 



(1.) Relation of the Effect to tin- Stintxhis. 



A. Nature of Stimulus. 



(a.) Single Induction Shock. The effect can be evoked by a single stimulus, but in 

 that case gives only slight deflections in the galvanometer, but appreciable movements 

 in the electrometer. Thus, in one experiment (Cat 122) in which the dorsal end of 

 the experimental tract was connected with the non-polarisable electrodes and the 

 lumbar end with needle electrodes, on excitation by a single break induction shock 

 the electrometer showed a sudden transient movement of the mercurial meniscus of 

 considerable size, seven divisions of the eyepiece scale ; with the single make shock, 

 a movement of five divisions was observed, both movements being opposed in direc- 

 tion to that produced by the resting difference. 



(?>.) Repeated Induction Shocks. When the cord is excited by a rapid succession of 

 equal and alternately directed induction shocks, more pronounced electrical changes 

 are produced in the cord ; the amount of the galvanometric change becomes very 

 appreciable, amounting in some cases to two or three hundred scale, the amount of 

 the effect in both the galvanometer and the electrometer varying with the intensity 

 and direction of the successive stimuli. The best general idea of this effect is 

 obtained with the latter instrument. The mercury is seen to leap at the moment of 

 excitation in a direction opposed to that of the movement previously due to the 

 uncompensated resting difference, this sudden change of level amounting to from 10 

 to 15 divisions of the eyepiece; it then slowly continues to rise until the successive 

 stimuli cease, when a rapid subsidence to rather below its original level occurs. 



(c.) Mechanical Stimulus. If with one end of the experimental tract in connection 

 with the electrodes the other end receive a sudden mechanical stimulus, an electrical 

 effect is evidenced in both galvanometer and electrometer. The most effective 

 mechanical stimulus is that of sudden complete division or squeezing of the end of the 

 cord ; for this purpose ivory scissors were first used, but we afterwards found that if 

 insulating precautions were taken, sharp metal scissors could be employed with more 

 advantage, since the keenness of the blades ensured a clean cut and avoided the 

 dangers due to dragging on the cord. Such a division produces a small deflection of 

 20 to 30 scale in the galvanometer, and a pronounced movement of from 5 to 7 (eye- 

 piece divisions) of the mercurial meniscus. 



After injection of strychnia, the slightest mechanical irritation of the cord or 

 sensory impression evokes marked electrical effects in the galvanometer and electro- 

 meter. 



B. Intensity and Number of Stimuli. 



Other things being equal, the cord electrical effect varies directly with the intensity 

 and number of the successive stimuli, a limit being reached in this i-espect, the change 



