292 MESSRS. F. GOTCH AND V. HORSLEY 



perceptible with the secondary coil at 2000, the tingling effect becoming first dis 

 tinctly felt at 4000, and painful at 12,000. 



The arrangement of the exciting circuit is given in the preceding figure 1, p. 289, 

 the only feature calling for remark being the use of a revolving mercurial key in the 

 secondary circuit. 



This key consisted of a hard paraffin bed, supporting two semi-circular narrow 

 vulcanite troughs containing mercury. Into these dipped the two ends of a loop of 

 stout platinum wire, which was carried by a horizontal revolving vulcanite arm. 

 Each mercurial pool was connected with one terminal of the secondary induction coil, 

 so that when the platinum loop connected the two pools a short circuit was made for 

 the secondary coil. The arm was driven by a clock at such a rate that a whole 

 revolution occupied a period of 10 seconds. 



By means of the key (D.B.) shown in the plan, the secondary coil was additionally 

 short-circuited, and the revolution of the arm thus made ineffectual ; when, however, 

 it was desired to excite the preparation, the key was opened for one revolution only, 

 and the secondary coil was thus disconnected from any short circuitous arrangement 

 during 5 seconds. The interrupted induction currents were thus allowed to traverse 

 the tissue between the platinum points of the exciting electrodes for this period only. 



(b.) Mechanical stimulation. The importance of obtaining evidence of physiological 

 effects by different methods, as a control to the definite results obtained with 

 electrical excitation, led us to employ mechanical stimulation wherever practi 

 cable. The simplest form of mechanical stimulus, and the most effectual, is un 

 doubtedly that obtained by the complete and sudden severance of a tract of fibres. 

 This we accomplished, when the position of the parts allowed of it, by the use of sharp 

 scissors, thus severing, for instance, the sciatic nerve, and observing the electrical 

 change which the division evoked in the spinal cord, and vice versd. As, however, in 

 some cases the metal blades of the instrument coming into contact with the moist 

 tissue caused localised electrical changes, derivations from which affected the 

 galvanometer and electrometer, we substituted the stimulus occasioned by the sudden 

 tightening of a ligature, and also that caused by crimping the tissue, nerve, or cord, 

 by squeezing it suddenly (sometimes so as to divide it) between the jaws of a pair of 

 specially constructed ivory scissors. 



The method of stimulus being used only as a control, the disadvantage of few 

 repetitions being possible was not a serious one. 



(c.) Chemical Excitation. In addition to the previous methods of excitation, we 

 have made use. of strychnia, in intraperitoneal injections, in order to obtain pronounced 

 reflex discharges from the spinal cord. 



Further, we have frequently employed, with notable advantage, the method of 

 exciting the brain first investigated by MAGNAN, viz., the production of nerve 

 impulses from central apparatuses by the toxic influence of certain substances, 

 especially the essence of absinthe. 



