LECT. vii.] THE HUMAN SKIN 115 



finger. This is the third fence ; for take the shock as weak as 

 you like, and you may be sure that the patient will jump, shift 

 contact, disturb compensation, and have to begin over again 

 many times, before he is able to keep his finger quiet while a 

 sufficiently strong shock is passed through it. But let us say 

 that he has reached this point, and that his finger is in circuit, 

 steady and currentless, and unmoved when a shock is passed. 

 You would find, even now, that the response is uncertain, 

 irregular, and capricious, always open to the objection that 

 contact between skin and electrodes has been altered during 

 experiment. And if you reflect upon the conditions of 

 experiment, I think you may fairly abandon it at this point. 

 For you have at best the resultant of two opposite effects at 

 the two electrodes, which resultant will depend upon at least two 

 or three unknown variables. You do not know what is the normal 

 direction of skin response, nor whether one or other portion of 

 skin is physiologically more or less effective, nor whether there are 

 polar differences when one or other portion is anodic or kathodic. 

 The original question cannot be answered by this apparently simple, 

 but in reality most objectionable and complicated experiment. 



I am not sure yet, in spite of several trials of the point, 

 whether or no a conclusive answer is to be obtained as to the 

 nature and direction of excitatory currents in the intact human 

 skin. I am, however, quite sure that no such answer is to be 

 got with a single pair of electrodes, and that we must first study 

 the separate polar effects by the ABC method, B C A 

 exciting through A B, leading off through A C 

 or B C after previous compensation. 



I did this two years ago, obtaining what 



then appeared to me to be sufficiently constant 



and regular effects ; the formula of skin response 



was thus > ~4 



The responses Nos. i and 4, being antidrome to the exciting 

 current, I regarded as equivocal ; Nos. 2 and 3, being homodrome 

 to the exciting current, as unequivocal. All the responses 

 were " ingoing " at the excited spot, a direction that is gener- 

 ally assumed to be the normal direction of skin-current in the 

 cat and in man. But there was the principal and unavoid- 



