MENTAL DEVELOPMENT 465 



feeding, is that poor and destitute women unable to purchase 

 milk are very numerous. They themselves have large families 

 which, owing to their own nutritional failure, they are not able 

 to rear. 



Stimulus in Relation to Development of the Brain 



There are two other factors to consider beside innate 

 potentiality of the neurones and their supply of the necessary 

 materials for growth by a pure and adequate blood supply. 

 They are the stimulus to growth by the physical and chemical 

 excitation of the nerve endings in the sense organs and bodily 

 structures. Let us consider this a little more fully. The 

 infant learns to know its own existence and the desires 

 necessary for its life by its organic sensibility ; the nerve 

 endings in the skin, muscle, tendons, and joints carry messages 

 continually to its brain, inciting the desire to breathe, to take 

 nourishment, and to perform the calls of nature. The special 

 sense organs associated with the muscle sense — which con- 

 tributes to every other sense — are especially represented in the 

 grey matter covering the brain ; they are the avenues of 

 intelligence and by motor reaction and adaptation the source 

 of information concerning the external world. As I pointed 

 out in my first lecture, preparedness for function by myelination 

 is first shown in the structures of the cortex which serve as 

 the arrival platform of sensations of organic and bodily sensi- 

 bility, of smell and of taste ; then of vision, and lastly of hearing ; 

 these, combined with the kinaesthetic sense, constitute the 

 primary perceptive centres. A simple experiment shows that 

 the chains of neurones which constitute the peripheral receptor 

 (sense organ), the transmitter, and the central perceptor have 

 the power of transforming cosmic energy into neural energy. 

 The experiment is this : if you take a pair of fine electrodes con- 

 nected with an electrical apparatus discharging an interrupted 

 electrical current, and place them on the tongue, a sensation of 

 taste is produced ; if on the skin a vibratile sensation is felt ; 

 if the eyeball is excited a bright light is seen ; and if the nerve 

 of hearing is stimulated a noise is heard. Since the stimulus 

 does not vary in any one of these experiments it necessarily 

 follows that each sensory nervous mechanism has the power of 

 transforming the stimulus and producing a specific effect on 

 consciousness. The neurones then not only act as receptors, 



