460 COMPARATIVE ELECTRO-PHYSIOLOGY 



of being rendered quantitative. As regards the latter, I 'have 

 already shown its practicability for experiments on excitatory 

 phenomena in vegetable tissues. Thus a single loop of 

 platinum wire may be made closely to surround the experi- 

 mental tissue, A definite current sent through the platinum 

 loop for a given length of time will now subject the encircled 

 area to a sudden thermal variation, which acts as a stimulus. 

 Successive closures of the circuit for a definite length of 

 time are ensured by means of a key actuated by a metro- 

 nome. The intensity of stimulus may be graduated in a pre- 

 determined manner by the adjustment of the heating-current. 

 Excitation may then be caused either by one or by a 

 summated series of thermal shocks. 



I was now desirous of determining whether this form of 

 stimulation would prove advantageous to experiments on 

 the nerve, and in the course of the investigation I found it to 

 be extremely convenient and appropriate. With good speci- 

 mens of nerve I have been able, using thermal stimulus, to 

 obtain long-sustained records of perfectly regular responses. 

 As regards its pliability and facility of application this form 

 of stimulus is quite unique. How many difficult problems 

 are made possible of attack by its means will be realised in 

 the course of the two following chapters, where the responsive 

 variations of different conducting tissues under changing 

 conditions are subjected to investigation. 



In order to obtain the electrical responses of animal 

 nerve that of frog, for example the distal contact is killed 

 and appropriate electrical connections made with the galvano- 

 meter. The heating current is then adjusted for the desired 

 amount of excitation. The thermal variation, it must be 

 remembered, should not be so great as to injure the tissue 

 in any way. The platinum loop is not in this case in contact 

 with the specimen, and this is the mode generally employed. 

 Should a more intense stimulation be desired, however, the 

 nerve may be allowed to rest on the platinum loop. In 

 such a case care must be taken to see that the rise of 

 temperature is not so great as in any way to injure the 



