62 ANIMAL ELECTRICITY. LECTURE III. 



Let US now take nerves of these three stages, 

 giving respectively the three types of electrical 

 response just enumerated, and examine upon them the 

 alterations of response brought about by carbonic acid. 



The alterations produced in fresh nerve you have 

 already witnessed. The negative variation was aug- 

 mented. Those produced in transitional and stale 

 nerve, especially the latter, are under all circumstances 

 more uncertain ; we cannot unfailingly obtain a varia- 

 tion of the desired type, and even when we have 

 obtained it — we cannot confidently predict the altera- 

 tion that may result from our interference. And this 

 is the worst month (February) in which to attempt to 

 make such delicate demonstrations. 



But the records of past experiments are of equal 

 value with the experiments themselves, and the 

 examples already alluded to in my second lecture, 

 and to which I now return, are representative of 

 the regular results of several hundred experiments, 

 all the records of which are indexed and open to 

 your inspection. 



Here, then, is the effect of carbonic acid upon 

 nerve of the second stage (fig. 26). The negative 

 effect is increased, the positive after-effect is dimin- 

 ished. 



Here is the effect *of carbonic acid upon nerve of the 

 third stage (fig. 28). The positive effect is abolished 

 and replaced by a negative effect. 



