THE BRAIN 



351 



In fetuses of six to seven months, four other depressions appear which 

 later form important landmarks in the cerebral topography. These are: 

 (i) the central sulcus, or fissure of Rolando, which forms the dorso-lateral 

 boundary line between the frontal and parietal lobes (Fig. 355); (2) the 

 parieto-occipital fissure, which, on the median wall of the cerebrum, is 

 the line of separation between the occipital and parietal lobes (Fig. 356) ; 

 (3) the calcarine fissure, which includes the cuneus between itself and the 

 parieto-occipital fissure and marks the position of the visual area of the 

 cerebrum ; (4) the collateral fissure on the ventral surface of the temporal 

 lobe, which produces the inward bulging on the floor of the posterior horn 



Cyrus ci 



Corpus callosum 

 ' 



Sale. corp. callosi 

 Splenium 



Parieto-occipital fissure 



Space of 



septum 



pellit- 



cidum 



Rostral 



lamina 



Parol- 



factory 



area 



Cutifia 



Cal- 

 carine 

 fissure 



Olfactory lobe \ Fissura rhinica 



Optic nerve Temporal lobe 



FIG. 356. Median surface of the right cerebral hemisphere from a seven months' fetus 



(Kollmann). 



of the ventricle known as the collateral eminence. The calcarine fissure 

 also affects the internal wall of the ventricle, causing the convexity termed 

 the calcar avis (hippocampus minor). 



Simultaneously with the development of the collateral fissure, appear 

 other shallower depressions known as sulci. These have a definite arrange- 

 ment, and, with the fissures, mark off from each other the various func- 

 tional areas of the cerebrum. The surface convolutions between the 

 depressions constitute the gyri and lobules of the adult cerebrum. 



Histogenesis of the Cerebral Cortex. In the wall of the pallium are 

 differentiated the three primitive zones typical of the neural tube: the 

 ependymal, mantle, and marginal layers. During the first two months 



