THREE TYPES OF RESPONSE 



63 



ingoing, and I may add, to complete the account, that it may 

 also be " mixed " negative then positive, or positive then nega- 

 tive, or positive negative positive.* 



And now that you have witnessed a representative experi- 

 ment, I will project on the screen the records of some previous 

 experiments. 



voit 

 0-01 



Second. 



OO3 J 



FlG. 27. Frog. Nerve-skin response, 

 (l) Positive or outgoing. (2) Mixed. (3) Negative or ingoing. 



| 39. Latent period. Here are a couple of photographic 

 records, taken on a more rapidly travelling plate, in order to 

 bring out more distinctly what was already obvious to simple 



* The accounts given by previous observers by Engelmann and by 

 Hermann in particular are not quite easy to reconcile with each other, and 

 with the description given in the text of these lectures. Thus Engelmann, in 

 1872, concluded that the usual effect of indirect excitation is a " negative varia- 

 tion " of the normal (ingoing) current. Hermann, in 1878, gives the usual 

 and principal effect as being a "positive variation" of the normal current. 



The two accounts are summarised in the following diagram, and their 



