MENTAL DEVELOPMENT 471 



which is instinctive in children and animals; and not only do 

 the feelings aroused in connection with it give pleasure, but they 

 are stimulating to growth of body and mind. Every movement 

 of the limbs leads to ingoing currents of nervous energy {vide 

 fig. 1). Bodily fatigue from exercise arises more from accumu- 

 lation of fatigue products (that is, chemical substances) in the 

 muscles than from exhaustion of the nervous structures ; indeed, 

 the nerves as conductors do not get fatigued. 



The Evolution of Association of the Eye and the Hand 



A study of the association of the eye and the hand is of great 

 interest in showing the reciprocal simultaneity in the develop- 

 ment of the visual directive and the tactile-motor executive 

 faculties. In the animal series it is not till we reach the 

 primates (apes, anthropoid apes, and man) that we find dissocia- 

 tion of the fore limbs from progression ; the nose is lifted from 

 the ground and the sense of smell and capture of food by the 

 mouth gives place to capture of food by the hand guided by 

 vision. The primates are microsmatic, that is to say the 

 olfactory nerves and the structures of the brain subserving the 

 sense of smell are relatively poorly developed, but the structures 

 of the brain which serve the function of vision, hearing, and touch 

 are largely developed. It is not till we reach the primates in the r 

 animal series that the eyes are set with their visual axes parallel 

 and that therefore these axes are capable of convergence ; con- 

 sequently by accommodation the image is always made to fall on 

 the yellow spot. Moreover, it is not till we reach the primates 

 that a yellow spot is found to exist. The panoramic vision of 

 the macrosmatic quadrupeds is replaced in the primates by 

 binocular stereoscopic vision. But with the development of 

 binocular stereoscopic vision, there has simultaneously developed 

 the sterognostic sense, or the sense arising by the association of 

 the experiences of the visual directive and tactile-motor executive 

 faculties, by which the mind can recall the visual image of an 

 object handled or touched. Every object seen is associated with 

 the experiences of touching and handling it, and makes us con- 

 scious of its realities of form, of smoothness, of roughness, of 

 hardness. A little reflection will show how great a part this 

 association of the eye and the hand has played in the pro- 

 gressive evolution of the brain as an organ of mind. Now some 

 people have the power of visualising, that is, summoning to the 

 3i 



