46 ANIMAL ELECTRICITY. LECTURE II. 



reoard it as matter for reoret that so much time 

 should have been necessary to investigate carbonic 

 acid at all thoroughly. And after all two years is 

 not so very long, but the investigation is not near 



its end. 



Let me at once, before entering upon any close 

 analysis, exhibit to you two observations that represent 

 the regular and typical action of carbonic acid upon 

 nerve. The first of these two figures shows that 

 carbonic acid in small quantity acts as a "stimulant." 

 The next figure shows that carbonic acid in large 

 quantity acts as a narcotic. It is in fact a typical 

 anaesthetic — excitant when its action is slight (fig. 16), 

 depressant when its action is complete (fig. 17). 



It will not be amiss if at this stage I invite you 

 to make a brief digression in order to recognise 

 the relations of carbon dioxide in the economy of 

 living matter, and perhaps broaden the view you 

 may have formed of the respiratory function. It 

 happened to me a few years ago to overhear a con- 

 versation in a conservatory between a young gentle- 

 man and a young lady. They were discussing the 

 plants, and the two disputants (they were of quite 

 tender years, nine and eight respectively) took up 

 what struck me as very typical standpoints. The 

 man-child said "of course plants breathe, how 

 could they live if they didn't breathe " ; the woman- 

 child said "of course plants don't breathe, how 

 can they breathe without lungs." Well, of course — 



