582 THE POPULAR SCIENCE MONTHLY. 



than this. Every elementary tract or process has found itself at dif- 

 ferent times excited in conjunction with many other tracts or processes, 

 and this by unavoidable outward causes. Which of these others it 

 shall awaken now becomes a problem. Shall 5 or c be aroused next 

 by the present a f We must make a further postulate, based, however, 

 on the undeniable fact of tension in nerve-tissue, and the summation of 

 excitements, each incomplete or latent in itself, into an open resultant ; 

 h rather than c will awake, if in addition to the vibrating tract a some 

 other tract d is in a state of sub-excitement, and formerly was excited 

 with h alone and not with a. In short, we may say : 



The amount of activity at any given point in the brain-cortex 

 is the sum of the tendencies of all other points to discharge into it, 

 such tendencies being proportionate (1) to the number of times the ex- 

 citement of each other point may have coexisted with that of the point 

 in question ; (2) to the intensity of such excitements ; and (3) to the 

 absence of any rival loccdity or process functionally disconnected with 

 the first point, into zvhich the discharges might be diverted. 



Expressing the fundamental law in this most complicated way 

 leads to the greatest ultimate simplification. This will now be seen ; 

 but the reader will bear in mind that our limits only allow us to treat 

 of spontaneous trains of thought and ideation, such as occur in reverie 

 or musing. The case of voluntary thinking toward a certain end must 

 be postponed to another opportunity. 



Take, to fix our ideas, the two verses from *' Locksley Hall " : 



" I, the heh* of all the ages in the foremost files of time," 

 and — 



" For I doubt not through the ages one increasing purpose runs." 



Why is it that when we recite from memory one of these lines and get 

 as far as the ages that portion of the other line which follows, and, so 

 to speak, sj^routs out of the ages, does not also sprout out of our mem- 

 ory, and confuse the sense of our words ? Simply because the word 

 that follows the ages has its brain-process awakened not simply by the 

 brain-process of the ages alone, but by it plus the brain-processes of all 

 the Avords preceding the ages. The word ages at its moment of strong- 

 est activity would, joer se, indifferently discharge into either " in " or 

 " one." So would the previous words (whose tension is momentarily 

 much less strong than that of ages) each of them indifferently dis- 

 charge into either of a large number of other words with which they 

 have been at different times combined. But when the processes of " So 

 I doubt not through the ages " simultaneously vibrate in the brain, the 

 strongest line of discharge will be that which they all alike tend to 

 take. " One " and not " in " or any other word will be the next to 

 awaken, for its brain-process has previously vibrated in unison not only 

 with that of ages, but with that of all those other words whose activ- 

 ity is just dying away. 



