QUANTITATIVE STIMULATION OF NERVE 459 



may obviously be very disturbing, and are indeed pro- 

 ductive of fallacies in vivisection and also in experi- 

 ments with the galvanometer, if not avoided by due 

 precautions. Hering has pointed out that in experiments 

 such as the investigation of the negative variation of 

 nerve-currents, in which galvanometers and exciting 

 circuits are separated by a long tract of nerve, the most 

 complete insulation of the two circuits is no guarantee 

 against the overflow of induced electricity through the 

 interpolar part of the nerve into the galvanometer 

 circuit. . . . This kind of unipolar stimulation is an 

 obvious danger in all experiments on action-currents and 

 negative variation in nerve, while it shows what narrow 

 bounds restrict the intensities of current that may be 

 safely used in these experiments.' l 



From this it will be seen how important it is to have at 

 our command some non-electrical form of stimulation, when 

 the response to be recorded is electrical. Heidenhain em- 

 ployed a mechanical form of stimulation, by which the nerve 

 was subjected to blows from an ivory hammer, which was 

 kept vibrating by means of an electro-magnetic arrangement. 

 The employment of this mode of stimulation would there- 

 fore eliminate all that uncertainty arising from the possible 

 escape of current which is inseparable from the use of 

 electrical stimulus. Though this method must be regarded 

 as one of great value, yet it is impossible to say how far the 

 excitability of a given point in a structure so delicate as 

 nerve will remain unmodified under the repeated action of 

 such blows. In any case, it appeared desirable to inquire 

 whether there was no other non-electrical form of stimulus 

 that could be rendered practicable. 



Besides the mechanical, the only remaining non-electrical 

 forms of stimulus are the chemical and the thermal. Of 

 these, the former is obviously incapable either of repetition or 



1 Biedermann, JEfectro-Pfyszofogy ( English translation), 1898, vol. ii. pp. 222- 

 223. 



