FUNCTIONS OF THE BRAIN 



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tipn in the cortex. In the development of a neuron four stages can be 

 distinguished: (i) Cells without processes; (2) the appearance of pro- 

 cesses, first the axon and then the dendrites; (3) the formation of col- 



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Fig. 392. Flechsig's Developmental Zones (after Flechsig). Outer surface of human 

 cerebral hemisphere. Primary zones (i-io), darkly shaded; intermediate zones 

 (11-31), less deeply shaded; terminal zones (32-36), unshaded. 



laterals; (4) myelination or the formation of the medullary sheath 



(Fig. 33.3. ? 8 54). 



Myelination occurs in the cerebral convolutions in a regular order. 

 In some areas the fibres may be medullated three months before birth, 



Fig. 393. Flechsig's Developmental Zones (after Flechsig). 

 cerebral hemisphere. 



Inner surface of human 



in others not till six months later. For instance, the Rolandic and 

 olfactory regions, the calcarine portion of the occipital lobe associated 

 with vision, and the portion of the temporal lobe associated with 

 hearing, are plentifully provided with medullated fibres a short time 

 after birth, at any rate before the first month, whereas the remaining 

 regions of the cortex are completely, or almost completely, free from 



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