650 PHYSIOLOGY 



a direction away from the body, and therefore in the third plane of space. 

 Thus gradually are acquired not only ideas of extension, distance, and space, 

 but certain movements are correlated with stimulation of definite regions 

 of the skin or of the retina. Tactile and retinal impressions therefore acquire 

 local sign, and power is acquired of moving the limbs to a degree and in a 

 direction adapted to stimuli arising from any part of the tactile or retinal 

 surfaces. The child gradually acquires the power of following a bright 

 object with its eyes, i.e. of contracting the ocular muscles so as to keep 

 the retinal image of the object on the fovea centralis, and up to adult age 

 we are still engaged in this balancing of muscular movement against sense 

 impressions a balancing in which the muscular sensations are the constant 

 guide and criterion of success. Only by the muscular sensations are we 

 informed whether our willed movement has been carried out or not. It 

 is in virtue of the muscular and allied sensations that we are able to clothe 

 our visual and tactile sensations with properties of extension, solidity, and 

 resistance, which create them in consciousness as parts of a material world. 



