760 OF THE FUNCTIONS OF THE NERVOUS SYSTEM. 



gives his attention to the sensorial impression, he may remember to have heard 

 the clock strike, and may even be able to retrace the number of strokes. 1 When 

 the attention is directed, however, to the sonorous impressions (as when we are 

 listening for the striking of the clock), or when it is not so closely fixed on any 

 other object as that it may be attracted by the sensations, the sounds are not 

 only recognized as proceeding from an external source, which is a simple act of 

 Perception, but, by a mental act which depends upon previous associations, the 

 sounds give rise to the complex idea of the striking of a clock, and are referred, 

 it may be, to some particular clock. Hence, when we say (as we commorly do) 

 that we have heard the clock strike, we affirm that which is not strictly correct ; 

 for that which we hear is simply the series of sounds, and it is by an intuitive 

 perception that we are led to consider those sounds as originating in an external 

 object; whilst the formation of a definite notion with regard to the nature of 

 that object is an act of judgment and comparison, guided by past experience. 

 When such an operation has been very frequently performed, however, the notion 

 comes to be so directly excited by the sensation, that it is uniformly and neces- 

 sarily called up when the attention is directed to the latter; the individual 

 being quite forgetful of the mental process by which this connection was origin- 

 ally established. 



792. Thus the formation of what have been designated as acquired perceptions, 

 in contradistinction to those of the intuitive kind, bears a striking analogy to the 

 process by which habitual movements come to be linked on to the sensations 

 that prompt them, so as at last to be automatically performed although originally 

 guided by the Will ( 749). And it can scarcely be regarded as improbable, 

 that, in the one case as in the other, the nervous mechanism grows to particular 

 modes of activity ( 726) ; so that successions of action are uniformly excited 

 by particular stimuli, which were not provided for in its original construc- 

 tion. Such a view harmonizes well with the fact that such associations, both 

 between sensations and respondent movements, and between sensations and 

 respondent ideas, are formed much more readily during the period of childhood 

 and adolescence, than they are after the full measure of development has been 

 attained; and that they are much more durable in the former case than in the 

 latter. For that which has been already pointed out with regard to the nutri- 

 tion of other tissues ( 591), may not unreasonably be applied to that of the 

 Nervous system; that, when once a certain mode of nutrition has been fully 

 established, it tends to perpetuate itself, provided that it be not altogether 

 unconformable to the original type. Throughout the whole constitution of Man, 

 physical and mental, we witness this capacity of adaptation to a great variety 

 of circumstances; and it seems to be purposely left to Man to educate himself 

 in accordance with those circumstances; so that he gradually acquires those 

 modes of action which in other animals are directly prompted by instinctive or 

 intuitive tendencies. The idea of the distance of an object, for example, is one 

 derived in Man from many sources, and is the result of a long experience; the 

 infant, or the adult seeing for the first time, has to bring the senses of sight and 

 of touch to bear upon one another, in order to obtain it; but, when once the 

 power of determining it is acquired, the steps of the process are lost sight of. 

 In the lower tribes of animals, however, in which the young receive no assist- 

 ance from their parents, there is an evident necessity for some immediate power 



1 It is curious that in so retracing a number, we are often assisted by mentally repro- 

 ducing the succession of strokes, imagining their recurrence, until we feel that we have 

 counted up to the impression that was left upon our sensorium. In the same way, if asked 

 how many stairs there are in a staircase which we are in the habit of using, we may not 

 be able to name the number ; yet, when actually ascending or descending, we are conscious 

 that we have arrived at the top or the bottom, by the completion of that series of sensorial 

 changes which has become habitual to us. 



