THE BRAIN 



349 



The Form of the Cerebral Hemispheres. — When the telencephalon expands 

 cranially, caudally, and at the same time ventrally, four lobes may be distin- 

 guished (Fig. 352) : (1) a cra.mal frontal lobe; (2) a dorsal parietal lobe; (3) a caudal 

 occipital lobe; and (4) a ventro-lateral temporal lobe. The ventricle extends 

 into each of these regions and forms respectively the anterior horn, the body, the 

 posterior horn, and the inferior horn of the lateral ventricle. The surface extent 

 of the cerebral wall, the thin, gray cortex, increases more rapidly than the un- 

 derlying, white medullary layer. As a result the cortex is folded, producing con- 

 volutions between which are depressions, the fissures and sulci. The chorioidal 

 fissure is formed, as we have seen (p. 342) , by the ingrowth of the chorioid plexus. 



Occipital lobe of 

 cerebrum 



Corpora 

 quadrigemina 



Hemisphere of 

 cerebellum 



Impression of 

 thalamus 



Temporal lobe 



Vermis rcrelu'lli 



Lalcral refers of 

 'ccnlyiiie fourth 



Fdxi-iriihis graciUs 

 Mcdiillii oblongata 



Fig. 353. — Dorsal view of the brain from a 100 mm. fetus (Kollmann). 



During the third month the hippocampal fissure develops as a curved infolding 

 along the median wall of the temporal lobe. Internally the infolded cortex forms 

 the hippocampus (Figs. 345 and 349). The lateral fissure (of Sylvius) makes its 

 appearance also in the third month (Fig. 352), but its development is not com- 

 pleted until after birth. The cortex overlying the corpus striatum laterally de- 

 velops more slowly than the surrounding areas and is thus gradually overgrown 

 by folds of the parietal and frontal lobes (fronto-parietal operculum) and of the 

 temporal lobe (temporal operculum). The area thus overgrown is the insula 

 (island of Reil) and the depression so formed is the lateral fissure (of Sylvius) (Fig. 

 355). Later, frontal and orbital opercula are developed ventro-laterally from the 



