MENTAL DEVELOPMENT 



301 



The new-born male brain weighs 321 grams; the female 

 361 grams. In the course of the first nine months the weight of 

 the brain is doubled, and microscopic examination shows why 

 this is. The myelin insulating material has been deposited 

 around a large bulk of the axon processes of the neurones and 

 the white matter has in consequence greatly increased. The 

 neurones have not increased in numbers, they have increased in 



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Fig. 2. — Diagram after Ramon y Cajal to show the phylogenetic and ontogenetic develop- 

 ment of a psycho-motor neurone. 



A, frog ; B, newt ; C, mouse ; D, man. It will be noticed that in ascending the zoological scale there is 

 an increase in complexity of the neurone and in the multitude of points of contact produced especially 

 by an increase in the dendrons and dendrites, also but to a less degree by the collaterals of the axon. 

 a, b, c, d, e, show the development of a psycho-motor cell in the human embyro as it grows. 

 Neurones may be arrested in their growth, and in the brains of idiots an arrest takes place. 



complexity and preparedness for function. The weight of the 

 brain still continues to increase for the same reason, and in the 

 course of the first three years the weight is treble that at birth. 

 After this the addition to the brain weight gradually diminishes 

 in amount and only slowly continues to increase in the male sex 

 up to nineteen or twenty ; in the female up to sixteen to 



