12 RESPONSE IN THE LIVING AND NON-LIVING 
The following is a tabular statement of the two 
types of response : 
I. Negative variation.—Action current from more 
excited to less excited—cuproid change in the excited— 
e.g. fresh muscle and nerve, stale retina. 
Il. Positive variation.—Action current from less 
excited to more excited—zincoid change in the excited 
—e.o. stale nerve, fresh retina.' 
From this it will be seen that it is the fact of the 
electrical response of living substances to stimulus that 
is of essential importance, the sign plus or minus being 
a minor consideration. 
Universal applicability of the electrical mode of 
response.—This mode of obtaining electrical response is 
applicable to all living tissues, and in cases like that of 
muscle, where mechanical response is also available, it 
is found that the electrical and mechanical records are 
practically identical. 
The two response-curves seen in the accompanying 
diagram (fie. 5), and taken from the same muscle by the 
two methods simultaneously, clearly exhibit this. Thus 
we see that electrical response can not only take the 
place of the mechanical record, but has the further 
1 T shall here mention briefly one complication that might arise from 
regarding the current of injury as the current of reference, and designating 
the response current either positive or negative in relation to it. If this 
current of injury remained always invariable in direction—that is to say, 
from the injured to the uninjured—there would be no source of uncertainty. 
But it is often found, for example in the retina, that the current of injury 
undergoes a reversal, or is reversed from the beginning. That is to say, 
the direction is now from the uninjured to the injured, instead of the 
opposite. Confusion is thus very apt to arise. No such misunderstanding 
can however occur if we call the current of response towards the more 
excited positive, and towards the less excited negative. 
