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: (1) the cen- 

 tral sulcus, or fissure of Rolando, which forms the dorso-lateral boundary hne 

 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 

 between it and the parieto-occipital fissure the cuneus 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 



Corpus callosum 

 Gyrus cinguli 



Sulc. Corp. callosi 

 Splenium 



I 



Pariclo-occipiial fissure 



Space of 

 septum 

 pellu- 

 cidum 

 Rostral 

 lamina 

 Parol- 

 factory 

 area 



Cuneus 



Cat- 

 - carine 

 fissure 



Olfactory lobe Fissioui rhinica 



Optic nerve Temporal lobe 



Fig. 356. — Median surface of the right cerebral hemisphere from a seven months' fetus (KoHmann). 



posterior horn 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 arrangement and 

 with the fissures mark off from each other the various functional 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 differ- 

 entiated the three primitive zones t)^ical of the neural tube: the ependymal, 



