CENTRAL NERVOUS SYSTEM. 



701 



tions. That a patient suffering from a lesion between the visual and motor 

 areas may be able to recognize an object and to indicate its use, it is sometimes 

 necessary that the object shall appeal to several senses. For example, the 

 name and use of a knife, when seen alone, may not be recalled, but when it is 



FIG. 198. Schema showing in a purely formal manner the different sort of afferent impulses whicfc may 

 influence the discharge of a cortical cell. 



taken into the hand that is, when the dermal and muscular sensations are 

 added to the visual one the response is made, though, acting alone, any one 

 set of sensations is inadequate to produce this result. 



Just where the block occurs in such a case it is not possible to say with 

 exactness, but the lesion lies, as a rule, between the sensory and motor areas 

 concerned, and by the damage to the pathway, it is assumed that one or more 

 groups of impulses are so reduced in intensity that they are alone insufficient 

 to produce a reaction ; and therefore it is only when the impulses from several 

 sides are combined that a response can be obtained. 



Variations in Association. It is a familiar fact that individuals differ 

 in no small degree in the acuteness of their senses i. e. in the power to dis- 

 criminate small differences, and this, too, when the sense-organs are normal. 

 Further, the powers of those best endowed are by no means to be attained by 

 others, however conscientious their training. Moreover, the central sensory 

 pathways differ widely. The inference is fair, therefore, that those who think 

 in terms of visual images, as compared with those who think in auditory 

 terms, do so by virtue of the fact that in the former case the central cells con- 



