230 



THE NERVOUS SYSTEM 



overlies the corpus striatum and develops more slowly than the surrounding areas 

 (labelled lateral fissure. Fig. 164). Folds from the surrounding cortex close in 

 over the insula, burying it from sight in the adult brain. These folds are known 

 as the opercula, and the deep cleft which separates them as the lateral fissure. 



Development of the Cerebral Cortex. At first the pallium, like other parts 

 of the neural tube, consists of three primitive zones: the ependymal, mantle, 

 and marginal layers. But during the third month neuroblasts migrate outward 

 from the ependymal and mantle layers into the marginal zone and there give 

 rise to a superficial layer of gray matter the cerebral cortex. Nerve-fibers 

 from these neuroblasts and others growing into the hemisphere from the thala- 



Suicus postcenlralls 



Sulcus centralis 



Lobus 

 parietalis 

 superior 

 Supra- 



Occipital 

 pole 



Inferior 



frontal 



sulcus 



Ascend- 

 ing 

 ramus 

 Lateral 

 fissure 

 (Syhii) 



Temporal 

 lobe 



Superior temporal gyms Middle temporal gyrus 

 Fig. 164. Lateral view of the right cerebral hemisphere from a seven months' fetus. (Kollmann.) 



mus accumulate on the deep surface of the developing cortex and form the 

 white medullary substance of the hemisphere. As the brain increases in size 

 the area of the cortex expands out of proportion to the increase in volume of 

 the white medullary layer upon which it rests, and is thrown into folds or gyri 

 separated by fissures or sulci. All the larger mammalian brains present well- 

 developed gyri, while the smaller brains are smooth; and it would thus appear 

 that the size of the brain is an important factor in determining the* amount of 

 folding that occurs in the cortex. 



As we shall learn, the cortex does not differentiate in exactly the same man- 

 ner throughout, but may be subdivided into structurally and functionally dis- 



