VI 



.] SECRETO-MOTOR EFFECTS 99 



terminal connected with the right pad, sends the spot off to 

 your right ; I know, therefore, that on excitation of the left sciatic, 

 the spot will go to your right. I know that the skin under its 

 control is rendered zincative (galvanometrically negative accord- 

 ing to physiologists, electro-positive according to physicists but 

 in any case in the direction of the arrow on the black board 

 from outer to inner surface of that skin, i.e., in an ingoing 

 direction). 



The spot has " flown off scale " as Biedermann so often 

 expresses it to your right (I am giving a demonstration, and 

 not making a measurement). In order to save time to make 

 the converse experiment without undue delay, I bring the spot 

 back by the counter current of a compensator. I have used 

 .015 volt; that therefore has been the approximate electro- 

 motive value of the response by no means an inconsider- 

 able value. 



I now apply similar excitation to the sciatic nerve of the 

 right side. You think nothing happens, but before you have 

 made up your mind that nothing happens, the spot flies off to 

 the left. Opposite side opposite direction of course. Ingoing 

 current in the right side gives deflection to the left, just as 

 ingoing current in the left side gave deflection to the right ; and 

 the pause perceptible to most of you at the second observation, 

 but not at the first, was the latent period between cause and 

 effect, between excitation of the sciatic nerve and response of 

 the cutaneous gland-cell. I guess it to be about three seconds, 

 but we will measure it presently. 



This obvious delay between excitation and response should 

 make its mark upon your memory. It is a sure and reassuring 

 sign that we are dealing with a true physiological response, 

 outside all possibility of coarse physical fallacy. The response 

 I mean the indirect response of the skin to excitation of the 

 nerve can be elicited by a single induction shock. This in 

 itself is rather curious ; we should have expected a visceral 

 (sudo-motor) nerve to need prolonged or summating stimuli 

 for its effective excitation ; you see for yourselves, however, 

 that a single shock causes an unmistakably delayed, well- 

 marked and somewhat prolonged response. 



