72 THE SIGNS OF LIFE [LECT. 



we shall do so by observing the separate partial reactions A C 

 and B C. Both these reactions will be found to be outgoing at 

 A and at B, being directed (in the skin) from C to A and from 

 C to B. 



45. There has been one very striking feature in the fore- 

 going experiments that must at once have attracted your 

 attention, namely, the surprisingly strict and limited localisation 

 of the reaction. With a layer of skin only a fraction of a 

 millimetre thick between our exciting electrode A and B, we 

 saw a great reaction when the external electrode A and an 

 indifferent electrode C were connected with the galvanometer, 

 but no reaction at all when the connection was with B and C ; 

 yet in this case also we had quite close above B an active 

 area A that might have been expected to alter the potential 

 of B. 



J. S. MacDonald, to whom I had the pleasure of showing 

 this experiment, said, " Oh, there must be a membrane." I 

 think he is quite right, but I am unable to analyse the mode of 

 action of this membrane. 



This strictly local character of the response which in the 

 present case of electrodes A and B in close juxtaposition is as 

 difficult to understand as it is easy to demonstrate is a singularly 

 favourable condition as regards the experimental application of 

 the blaze test to skin and to other animal and vegetable tissues, 

 in which there is little or no diffusion of the local polar effect 

 of excitation. In the cases of muscle and of nerve these local 

 effects are to some extent masked by the propagated distal 

 effects of excitation, and require for their manifestation current 

 strengths greatly in excess of those sufficient to give maximal 

 propagated effects. In a piece of skin, or in a leaf or petal- 

 provided they are alive, the excited and therefore blazing spot is 

 limited as to depth and breadth, and the exhaustion of that spot 

 by excessive stimulation does not sensibly modify the vitality of 

 parts in its close proximity. Of course the result is a question 

 of degree ; the entire interpolar length of a tender shoot traversed 

 longitudinally by violent currents is stunned (or killed) and may 

 shrivel up and be obviously dead in a day or two / a flat leaf 



