CENTRAL NERVOUS SYSTEM. 261 



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 arc by no means to be attained br- 

 others, however conscientious their training. Moreover, the sensory path- 

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

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

 so by virtue of the fact that in the former case the central cells concerned in 

 vision are distinctly the better organized, while in the latter case it is those 

 concerned in hearing. 



In the same way, the power of expression varies in an equally marked 

 degree, and the capacity for the expression of ideas by means of the hand, in 

 writing, is by no means necessarily equal to the power of expression by means 

 of spoken words. In the former case we have the results of the play of 

 impulses from the several sensory centres on the motor area for the hand, and 

 this is reinforced by the sight of that which has been written, whereas in the 

 latter case impulses from these same sensory centres play upon the area which 

 controls the muscles of phonation, and this reaction is reinforced by the sound 

 of the words uttered. Of course, in the case of a defective, like a blind-deaf- 

 mute, the expression of thought is by movements of the fingers, and this is 

 reinforced by the tactile and muscular sensations which follow these move- 

 ments. 



It is not by any means to be expected that the anatomical connections 

 which render such reactions possible will be equally perfect for the different 

 sensori-motor combinations, or the same combinations in different persons, 

 and hence the powers of the individual will be modified by the varying per- 

 fection of these paths. From this it also follows that the same lesion as 

 grossly determined, will not produce identical results in the two persons, for 

 it will not effect the damage of structural elements which are strictly com- 

 parable. 



Latent Areas. — It has been plain from an examination of the foregoing 

 figures, as well as from the descriptions, that there must be a large portion of 

 the cortex which, so far as has been observed, may be called latent. These 

 areas, which include nearly the entire ventral surface of the hemispheres, a 

 large part of the mesial surface, and on the dorsal and lateral aspects a large 

 portion of the frontal, parietal, and temporal lobes together with the island, 

 certainly require a word. 



These last correspond with the "association centres" as described by 

 Flechsig. To direct stimulation they give no response. From any one 

 portion of the latent area the connections are not massive enough to 

 permit of impulses which will cause a muscular contraction, and hence 

 these impulses coming from one locality to a discharging cell form only a 

 fraction of the impulses which control it. For this reason the significance 

 of these parts fails to be clearly evident upon direct experiment. 



The cortex of the frontal lobes has some connections with the nuclei of 

 the pons, and so with the cerebellum. The more recent experiments on th« 



