44 ANATOMY OF THE CENTEAL NEETOtJS SYSTEM. 



The most manifold tracts and centers serve sensibility, and in verte- 

 brates, especially in man, who is able to give information in regard to the 

 perception of certain impressions, we have found a multiplicity of sensations. 



The sensory control required by apparently similar movements is not 

 always the same. Particularly in the higher animals there seem to be 

 more factors entering into this control than in the lower ones. But even 

 mammals may, at times, through habit, etc., learn to dispense with one or 

 another such factor; i.e., may be successful in performing acts through the 

 agency of elements which in lower stages of evolution were simply regulative 

 in their action. 



The importance of the single forms of this sensory regulation of en- 

 tirely elementary motor mechanisms is well illustrated by the shrewd ex- 

 periment of J. Eichard Ewald. If one remove the labjointh on both sides 

 of a dog, the general muscular tone, and with it the power to maintain the 

 vertical equilibrium, suffers so as to render walking and standing impossible. 

 But this is gradually recovered from, the tracts for tactile and other im- 

 pressions making up for the loss. Eemove now both the cortical motor 

 areas for the legs, and the severe motor disturbance reappears, the animal 

 cannot produce co-ordinated, or, indeed, any regular movements. Still, 

 here follows slowly a recovery. But the dog is in a serious state, being re- 

 duced to visual control for all his movements. When the room is darkened, 

 or his eyes are bandaged, he falls helplessly to the ground. 



Lower vertebrates — frogs, for example — cannot conceal the loss follow- 

 ing removal of the labyrinths, because with them the possibility of substi- 

 tuting other forms of sensation from the cerebral cortex for those lost is 

 very slight. They remain unable to jump after removal of the labyrinth. 



The foregoing shows how complicated, even in the most ordinary act, 

 the mechanism required for its performance is. By study and experimenta- 

 tion, one may, perhaps, recognize in the central organ at least enough of 

 this mechanism to serve in explaining physiological and psychological 

 processes. 



