652 COMPARATIVE ELECTRO-PHYSIOLOGY 



the complete obliteration of the negative element an already 

 painful sensation may be converted into pleasurable. 



Of the scheme thus laid down, the first part will be 

 carried out in the present, and the second in the succeeding, 

 chapters. 



Turning now to our sensations themselves, it may be well 

 to consider some of the characteristics of the central perceiving 

 apparatus. As a detector of -nervous changes the brain is 

 undoubtedly the most delicate of instruments, surpassing in 

 this respect not only the galvanometer, but also the 

 Kunchangraph. It fails, however, strictly speaking, as an 

 accurate metrical apparatus. It is not able to discrimi- 

 nate quantitatively, for instance, by means of sensation, 

 through any wide range, between the finer differences of 

 intensity in the nervous impulses it receives. In the pain- 

 and- pleasure series, again, the distinctions which it is able to 

 make are, to a certain extent, of a merely qualitative 

 character, unmistakable only as between the two extremes 

 of the series, the intervening region tending to be somewhat 

 indefinite. The sensitiveness of the physical instruments, 

 Kunchangraph and galvanometer, is always constant and 

 reliable. For example, in the galvanometer, by adjusting 

 the controlling magnet, we can obtain varying degrees of 

 sensibility, which at any particular adjustment will remain 

 constant. But in the perceiving apparatus, not only is the 

 sensitiveness of different individuals widely different, but even 

 in a single individual it undergoes great variation under 

 different conditions. 



By deliberate attention or inhibition, as by raising or 

 lowering of the controlling magnet in the galvanometer, the 

 sensitiveness of the perceiving field can be almost indefinitely 

 varied. Pursuing this analogy of the galvanometer further, 

 we find that in the brain, instead of a single coil, with its 

 one pair of terminals, there are many coils with many pairs 

 of terminals, receiving impulses from every part of the 

 organism. Confining our attention, moreover, to any single 

 circuit among these, we find again that the impulses it 



