306 Dr. A. D. Waller. Effect of Heat upon the [Feb. 20, 



nerve-photographs without any preliminary trials, as will be obvious on 

 consideration of the record given herewith. 



III. Of Skin. 



Frog's skin, which, according to my previous observations,* invariably 

 responds to local excitation by an outgoing current when led off by its 

 external, but not by its internal, surface, is a particularly satisfactory tissue 

 upon which to study the electrical effects of heat, for the electrical sign of 

 the local effect of excitation is the reverse of that of muscle or nerve, and it 

 possesses an effective (external) surface and an ineffective (internal) surface 

 that can be separately tested. 



Experiment. — Two unpolarisable electrodes A and B, in contact with the 

 external or effective surface of the skin to a galvanometer. A stirrup of 

 platinum wire in an accumulator and metronome circuit as for muscle. 



Heat under B gives a large effect in the negative direction, indicating 

 current in the skin from B to A (= B zincative), i.e. in the contrary 

 direction to that of the current aroused by electrical or mechanical excitation. 



Bepeating the trial with the skin turned round so that the electrodes A B 

 are in contact with its inner ineffective surface. Warmth applied as before 

 to B gives little or no deflection ; the deflection, if any, is in the opposite- 

 (negative) direction. 



A killed piece of skin gives little or no deflection from the warmed 

 spot B ; the deflection, if any, is small and in the positive direction. 



Thus in the living skin as in living muscle a current is aroused by warmth 

 which is antidrome to the current aroused by electrical excitation ; the facts- 

 in the two cases are as follows : — 





Local excitation of A. 



Local warmth to A. 





A negative 



A positive 



Skin 



A positive 



A negative 



Photographic records of the electrical effects of heat upon muscle, nerve,, 

 and skin. The connections in the three cases are with two points, A and B,. 

 as given above, heat being in each case applied at A or at B, as indicated 

 by the signal marks h, h, giving in the case of muscle and in that of nerve 

 heated at B response in the direction from A to B, and in that of the skin 

 from B to A. 



I. The muscle record consists of three successive positive responses to heat 

 * Waller, ' Eoy. Soc. Proc.,' vol. 68, p. 480, 1901, " Signs of Life/' 



