THE NERVOUS SYSTEM. 



553 



growing more rapidly but later the fronto-parietal, thereby changing the 

 direction of the Sylvian fissure from an oblique to the more horizontal angle 

 characteristic of man as compared with the ape. In the meanwhile the 

 development of the frontal lobe leads to its also overlapping the insula. If the 



Parietal lobe 



Occipital lobe 



Mesencephalon 

 Cerebellum 



Frontal lobe 

 Insula 



Bulbus olfactorius 



Gyms olfactor. lat. 



Gyrus semilunaris 

 Gyms ambiens 



FIG. 484. Lateral view of the brain of a human foetus at the beginning of the 

 4th month. Kollmann. 



frontal lobe fully develops, it forms a U-shaped operculum between the fronto- 

 parietal and the orbital, if it does not so fully develop it forms a V-shaped 

 operculum, and a still less developed condition is shown by a Y-shaped arrange- 

 ment in which the frontal lobe does not completely separate the fronto-parietal 



Corpus callosum 

 Gyrus cinguli | 



I 



Sulcus corp. callosi 

 I Splenium 



|, Fissura parieto-occip. 



Cavum septi pellucidi 

 Lamina rostralis 

 Area parolfactoria ^ 

 (praeterminalis) 



Cuneus 



Fissura calcarina 



N. olfact. | | Fiss. rhinica 

 N. optic. Lob. temp. 



FIG. 485. Median view of the left half of the brain of a human foetus at the end 

 of the 7th month. Kollmann. 



and orbital opercula. The opercula cover the fore-part of the Sylvian fossa 

 during the first year. Conditions of arrested development are thus indicated by 

 the Y-shaped anterior ascending branch of the Sylvian fissure coupled with an 

 absence of the pars triangularis and also by a partial exposure of the island 



