ANIMAL ELECTRICITY. LECTURE III. 63 



Here is another and less pronounced effect of 

 carbonic acid upon nerve of the third stage (fig. 30). 

 The positive effect is diminished. 



Compare now with these three typical and charac- 

 teristic effects of carbon dioxide, the effects produced 

 upon nerve — either upon the same nerve, or at least 

 upon nerve in a similar state — by prolonged artificial 

 activity. You will recognise that in each separate 

 case the effect of activity is similar to the effect of 

 carbon dioxide. Considering the conditions of the 

 case — that the a /r/6>r/ probable product of activity is 

 carbon dioxide, we are, I think, entitled to conclude 

 that the similar series of effects has been due to a 

 common cause — to known CO^ introduced from with- 

 out in one series ; to the hitherto unknown CO^ 

 evolved from within in the second series. The nerve 

 itself has served as the reagent indicative of the 

 presence of the latter. 



Here is the nerve giving a small negative deflec- 

 tion followed by a large after-deflection, altered as 

 you have seen by CO^ from without, and similarly 

 altered in consequence of tetanisation. (Compare 

 figs. 26 & 27). 



Here is the nerve giving a positive deflection, 

 reversed to a negative deflection by CO^ from without, 

 and here is the same nerve exhibitino- a similar 

 reversal in consequence of tetanisation. (Compare 

 figs. 28 & 29). 



Here, finally, is the nerve of which the positive 



