The anatomy of the nervous system, from the standpoint of development and function . tex which 229 230 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 inover the insula, burying it from sight in the adult brain. These folds are knownas the opcrciila. and the deep cleft which separates them as the lateral fissure. Development of the Cerebral Cortex.—At first the pallium, like other partsof the neural tube, consists of three primitive zones: the ependymal, mantle,and m
The anatomy of the nervous system, from the standpoint of development and function . tex which 229 230 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 inover the insula, burying it from sight in the adult brain. These folds are knownas the opcrciila. and the deep cleft which separates them as the lateral fissure. Development of the Cerebral Cortex.—At first the pallium, like other partsof the neural tube, consists of three primitive zones: the ependymal, mantle,and marginal layers. But during the third month neuroblasts migrate outwardfrom the ependymal and mantle layers into the marginal zone and there giverise to a superficial layer of gray matter—the cerebral cortex. Nerve-fibersfrom these neuroblasts and others growing into the hemisphere from the thala- Sidcus postccntralis Sulcus centralis Lohusparietalis superior Supra-marginaland an-gular lateralfissure Middle temporal sulcus Occipitalpole Inferior frontal sulcus. Temporallobe Superior temporal gyrus Middle temporal gyrusFig. 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 thewhite medullary substance of the hemisphere. As the brain increases in sizethe area of the cortex expands out of proportion to the increase in volume ofthe white medullary layer upon which it rests, and is thrown into folds or gyriseparated by fissures or sulci. All the larger mammalian brains present well-developed gyri, while the smaller brains are smooth; and it would thus appearthat the size of the brain is an important factor in determining the amount offolding 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- THE EXTERNAL CONFIGURATION OF THE
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