702 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



cerned 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 im- 

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

 this is reiuforced 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 the 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 rein- 

 forced by the tactile and muscular sensations which follow these movements. 



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 perfection of 

 these paths in the several cases. From this it also follows that the same 

 lesion as grossly determined will not produce identical results in the two per- 

 sons, for it will not effect the damage of structural elements which are strictly 

 comparable. 



Pathways through Gray Matter. Moreover, what is true of the spinal 

 cord is also probably true of the cortex viz. that while the long tracts are 

 the usual and preferred pathways between centres, shorter tracts formed by a 

 large series of cells often serve as the pathway, and impulses may under some 

 conditions find their way from one part of the cortex to another by way of 

 these more complex tracts. 



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 

 regions, 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 and temporal lobes, certainly require a word. 



The various forms of investigation yield negative results. The speech- 

 centre is, strictly speaking, neither a motor nor a sensory portion of the cortex, 

 and yet when it is damaged the function of speech is disturbed. We have 

 come to look upon the speech-centre as containing cells by way of which im- 

 pulses pass to the centres controlling the muscles of phonation. This relation 

 suggests that the rest of the cortex called latent may act in a similar manner, 

 and that by way of it pass impulses which modify the discharge of the motor 

 areas proper. From any one portion of the latent area, however, the connec- 

 tions are not massive enough to permit of impulses which will cause a contrac- 

 tion, and hence these impulses coming from one locality to a discharging cell 

 form only a fraction of the impulses which control it ; and for this reason the 

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



