62 THE SIGNS OF LIFE [LECT. 



spot has crept to the left of the scale, therefore to the left signi- 

 fies ingoing, to the right outgoing ; but we shall verify this 

 presently with a bit of zinc. 



I tetanise the nerve for a second or two, and after a distinct 

 interval, long enough to allow the thought that there is no 

 effect, the spot sweeps across the scale to the right, signifying 

 that the skin, aroused by excitation of the sciatic nerve, has given 

 an outgoing current. And knowing what to expect, I took care 

 to shunt the galvanometer, for the resistance of the skin is so 

 low, and the voltage of response so great, that the spot would 

 assuredly have flown off scale if the galvanometer had not been 

 shunted. I wanted, however, to have a measurable effect, such 

 as should allow us to make further trials. Five minutes have 

 elapsed, and I repeat the excitation ; deflection to the right occurs 

 again, but is much smaller. It looks as if the nerve-skin pre- 

 paration were becoming fatigued. And while we wait for a 

 second five minutes to elapse, let us consider matters. How 

 has the nerve acted upon the skin ? What is the meaning of 

 the declining effect we are looking for? Is the response always 

 in this positive or outgoing direction ? 



All authorities who have worked at the subject are agreed 

 that it is upon the skin-glands that the nerve acts, and that the 

 skin effect is the sign of a glandulo-motor or secreto-motor 

 phenomenon. I share this view, and do not think it probable 

 that nerve has any connection with the general epithelial invest- 

 ment of the skin ; but, in remembrance of the fact that cutaneous 

 pigment cells are demonstrably influenced through nerves, I 

 make some mental reservation to the assumption that the effect 

 is exclusively glandular. The wearing-out of the response in 

 repetition is not due to wearing-out of the nerve, nor even 

 wholly, I think, to wearing-out of the discharging gland, but 

 more probably to a wearing-out of the junction between nerve- 

 fibre and gland-cell. 



The second five minutes is at an end, and now you see that 

 excitation gives a negative instead of a positive response, and 

 thus answers the last of the three questions we put a few 

 minutes ago. Clearly, the response is not always positive or 

 outgoing ; you have just seen that it may be negative or 



