1882.] Excitability of Sensory Nerves of Man. 



367 



opportunity of noticing the occurrence of apparently similar pheno- 

 mena in the sensory nerves of the skin. 



This fact led us to undertake a preliminary series of experiments, 

 of which we propose to give here the main results. 



The methods we employed were precisely the same as those used 

 for the investigation of the excitability of the motor nerves.* One 

 electrode of small size — the exploring electrode — was fixed over the 

 nerve chosen for the experiment ; whilst the other electrode, of large 

 size, rested on a distant part of the body. In order to secure the 

 coincidence of the zones of polar alteration and of stimulation, the 

 polarising and testing currents were united in the same circuit. 



This is effected, when the induced current is used for testing the 

 excitability, by including the secondary coil in the circuit of the 

 polarising battery. When galvanic makes and breaks are so used, 

 the current of the testing battery is thrown into the circuit of the 

 polarising current without breaking the latter, by means of the double 

 key of Helmholtz arranged in the usual manner. 



The precautions we took for the elimination of errors arising from 

 changes in the resistance of the body and in the current strengths 

 during the experiments were either 1st, the galvanometric control 

 of the currents used ; 2nd, the intercalation of large additional 

 resistances in the circuit (viz., 10,000 ohms, which is about 8 times the 

 resistance of the human body under the conditions of our experiments) . 

 The influence of any changes in that resistance would thereby be 

 diminished in the same proportion. 



We usually employed the method of minimal stimuli, first noting 

 the current strength required to produce, by its action on the normal 

 nerve, a reaction in consciousness ; then finding the changes of the 

 current strength necessary to produce the same effect during and 

 after polarisation, anodic and kathodic. 



Two points of importance with reference to such experiments may 

 briefly be alluded to here : — First, the necessity of carefully distin- 

 guishing between the continuous sensation produced by the polarising 

 current when it has reached a certain intensity ; second, the necessity 

 of using a uniform rhythm of excitation, owing to the readiness with 

 which stimuli summate in the sensory nerves of man. The sensations 

 are referred either to the portion of the skin immediately under the 

 electrode or to the parts supplied by peripheral distribution of the 

 nerve. In experiments on mixed nerve-trunks care must be taken 

 to eliminate the possible admixture of sensations due to muscular con- 

 tractions. 



Our general conclusion is, that during and after the passage of a 

 galvanic current the alterations in the excitability of the sensory 



* "Proc. Roj. Soc," vol. 33, p. 353 j and "Phil. Trans.," 1882, p. 961. 



