ANIMAL ELECTRICITY. LECTURE III. 69 



resistance in this or that direction of the central 

 nervous system — the " Bahnung " of German psycho- 

 logists — is conceivably due to a facilitation of 

 transmission along some line previously traversed by 

 nerve-impulses, and therefore with its excitability 

 sharpened by CO^. We may even be tempted to 

 cast our imagination much further back — to the 

 very origin of specially conductile tissue within a 

 mass of homogeneous protoplasm. Picture to your- 

 self a first linear discharge of action within such 

 mass ; carbonic acid is evolved along this line, and 

 the subsequent discharge of action will be in this 

 rather than in other lines. That will have been a 

 primitive form of " Bahnung " in the sense with 

 which Spencer's writings have made us so familiar, 

 and to my mind a knowledge of the relations be- 

 tween carbonic acid and nerve makes this idea more 

 concrete and tangible by suggesting a possible^ I had 

 almost said probable — physico-chemical mechanism of 

 the result. And if there has been a disintegration, 

 there follows along the same line a reintegration of 

 matter, by which the nerve-path becomes organically 

 as well as functionally constituted. We know that 

 wear is followed, and more than made good, by repair ; 

 we also know that one of the products of wear is 

 carbonic acid. I wonder does this carbonic acid 

 become altogether dissipated ; may it not perhaps be 

 relnvolved in some storage combination, as the nerve- 

 fat perhaps, that is so prominent a constituent of fully 



