IV 



.] SKIN CURRENTS 61 



38. Indirect excitation. The effects on the skin of excita- 

 tion of its nerves are particularly interesting ; it is easy to 

 demonstrate them, but by no means easy to interpret them in 

 detail. I do not think that any interpretation yet offered has 

 properly embraced all the facts, and I shall not pretend to mend 

 matters much in this respect. But I will lay the facts before you, 

 more completely and more briefly than you will find them hitherto 

 described, by means of graphic records that exhibit better than 

 any narrative the character and dimensions of the phenomena. 



The experiment I am about to show you is one that was 

 first made by Roeber (on the suggestion of Rosenthal) more 

 than thirty years ago, and that has since been repeated with varia- 

 tions by Engelmann and by Hermann. The preparation is as 

 follows : A frog's sciatic nerve is dissected out in the usual way, 

 the foot is cut off, the skin of the leg is longitudinally divided on 

 the anterior aspect and peeled off the leg up to the knee-joint, 

 the leg is cut off just below the knee-joint, the femur and thigh 

 muscles are divided just above the joint. We now have a sciatic 

 nerve in connection with the skin of the leg, and the ends of the 

 femur and tibia serving as a handle. The skin is now laid over 

 one leading-off electrode, and the other electrode is brought into 

 contact with the other surface of the skin, the sciatic nerve is 

 laid across a pair of exciting electrodes that are connected to a 

 coil. We now have a nerve-skin preparation, the analogue of a 

 nerve-muscle preparation, quite as easy to make, and affording, 

 when made, an unfailing demonstration of a current of animal 

 electricity and of the action of nerve on skin. It ought long ago 

 to have become a regular lecture experiment and student's 

 exercise, yet somehow or other it has almost dropped out of 

 notice. Every student of physiology makes nerve-muscle pre- 

 parations by the score. I wonder how many students or teachers 

 have ever made a nerve-skin preparation, and what would be 

 said if, as a change from the regular round of about six practical 

 experiments (that cannot be properly done in the time allowed) 

 an examiner were to ask an honours candidate to show some- 

 thing by means of a nerve-skin preparation. 



Well, our experiment is waiting, and the spot has come to 

 rest. The normal skin current is ingoing and increasing, the 



