320 ZOOLOGICAL PHILOSOPHY 



2. That, in acts of intelligence, the organ of understanding is only 

 passive and is prevented from reacting by its extreme softness ; it 

 acquires no activity from the nervous fluid but merely impressions, 

 of which it preserves the tracings ; the part of the fluid, which works 

 in the various portions of this organ, is modified in its movements 

 by the tracings already present, and at the same time traces more ; 

 so that the organ of understanding, which has only a narrow channel 

 of communication with the rest of the nervous system, uses only a 

 part of the whole fluid of the system. Moreover this narrow channel 

 of communication preserves that part of the nervous fluid, contained 

 in the organ of intelhgence, from the general agitation which occurs 

 in emotions of the inner feeling, and in the formation of sensations, 

 except when that agitation is of great intensity, in which case nearly all 

 the functions and faculties of the system are disturbed. 



On these grounds, it appears probable that the entire mass of nervous 

 fluid secreted and contained in the system is not at the disposal of the 

 inner feehng of the individual, but that some of it is held in reserve 

 to provide for the continuance of the vital functions. Hence, just as 

 there are muscles independent of the will, and others which only enter 

 into activity when excited by the inner feeling driven by the will or 

 some other cause ; in the same way, no doubt, one part of the nervous 

 fluid is less at the service of the individual than the other, lest the 

 whole fluid should be drained away from the vital functions. 



Indeed, since the nervous fluid is never used without a proportional 

 amount of loss, it follows necessarily that the individual is only free to 

 use up a certain part of it : untoward effects ensue even when this part 

 is run too low, for then some of that held in reserve becomes available 

 and the vital functions suffer accordingly. 



I shall have further opportunities later on for extending these 

 various remarks on the nervous fluid ; but let us first enquire what 

 is the mechanism of sensations, and how the marvellous faculty of 

 feeling is produced. 



