854 



A MANUAL OF PHYSIOLOGY 



of localization in the cortex. In the development of a neuron four 

 stages can be distinguished : (i) Cells without processes ; (2) the 

 appearance of processes, first the axon and then the dendrites ; 



FIG. 361. 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. 



(3) the formation of collaterals ; (4) myelination or the formation of 

 the medullary sheath (Fig. 305, p. 752). 



Myelination occurs in the cerebral convolutions in a regular order. 

 In some areas the fibres may be medullated three months before 



FIG. 362. FLECHSIG'S DEVELOPMENTAL ZONES (AFTER FLECHSIG). 

 Inner surface of human cerebral hemisphere. 



birth, 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, 



