MENTAL DEVELOPMENT 473 



based the desires which excite the brain, through the senses, to 

 explore the external world in order to gratify them. This is 

 well exemplified by observing that an infant at first conveys 

 all objects, that it sees and grasps, to its mouth. A simple 

 elemental sensation soon after birth becomes impossible ; for 

 every simple sensation tends to reflex activation, and each phase 

 in that motor reaction which occurs is immediately followed by 

 incoming sensory stimuli registering in the mind the successive 

 movements brought about {vide fig. i). At the same time each 

 experience perfects the association of the sense of movement 

 with the mental image of the sensation ; thus the memory of the 

 visual image is associated with the memory of the movements of 

 the eyes necessary for it to be clearly seen ; likewise the memory 

 of the visual image of an object is associated with the memory 

 of the movements of the arm and hand by which it was grasped. 

 Thus it may be truly said that the muscular sense contributes 

 to every other sense, and all the sensory areas of vision, hearing, 

 smell, taste, and touch become linked up by the bonds of asso- 

 ciative memory with the muscular sense. Now the muscular 

 sense is combined with the active sense of touch; but it is 

 better to speak of it as the kinaesthetic sense, for this includes 

 the sensation arising from the stretching of tendons, the move- 

 ments of joints, as well as of movements of the muscles. All 

 the sensory receptor spheres of the brain are associated with 

 the voluntary efferent motor sphere (vide fig. 3), and every 

 sensation in the infant tends to activation, that is motor 

 expression; for it is by handling and touching parts of its 

 own body that it becomes aware of its own personality, and 

 by motor reaction to sensory stimulus it learns the reality 

 of things in the world external to it ; consequently with the 

 progressive evolution of the child's mind there is constant 

 sensori-motor association. Not only is there association of 

 each sensory sphere with the motor and kinaesthetic spheres, 

 but there is also an association of all the sensory spheres 

 with one another; so that a simple sensory stimulus from 

 within or without the body revives in the memory a com- 

 plexus of previous sensory experiences which are termed 

 " percepts." The perceptive faculty of associative memory of 

 concrete images of previous experiences with elemental time 

 and space relations and the acquisition of appropriate motor 

 reactions under the influence of the will, is also possessed by 



