340 THE GROWTH OF THE BRAIN. 



right-handed person it is the injury to the left cerebral 

 hemisphere between the sensory and motor regions that 

 most often gives rise to symptoms of aphasia. The 

 symptoms vary with the location and extent of the lesion, 

 but depend on the stoppage or diminution of the nerve 

 impulse at some point between its arrival at the cortex 

 and its transfer to the efferent central cells, used for the 

 response. The study of aphasic patients has brought out 

 the fact that any one or more of the several events in- 

 volved in recognising the bell by means of the principal 

 senses, and expressing this recognition in the usual 

 way, by the written or the spoken word, may drop out. 

 So, for example, the patient may be unable to speak or 

 write the word " bell " when a bell is simply seen, but at 

 the same time be able to do both if it is also rung. Or 

 the sound may elicit only the written but not the 

 spoken word, and so on through the possible combina- 

 tion of sensations and forms of expression. 



Thus the impulses reaching an expressive centre, and 

 proving too weak to cause the discharge of it, may still 

 prove efficient if combined with the corresponding im- 

 pulses from another sensory centre to which the object 

 also appeals. It is therefore the passing of the impulses 

 from all the sensory centres stimulated by the object that 

 gives the basis for the most perfect response. Clinical 

 studies of the kind furnish grounds for the idea that the 

 presentation of an object to any one of the senses revives 

 the mental image of that object in terms of the other 

 senses which may be, and formerly have been, excited 

 by it, and that the more vivid these associated images, 

 the more complete and clear is the conception. As the 

 possibility of forming the extra images is curtailed, the 

 conception becomes weaker, more special, and less 

 reliable. 



When the sight of the bell causes it to be named, the 



