THE BRAIN 



349 



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 underlying, white medullary layer. 

 As a result, the cortex is folded, producing convolutions 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. During 

 the third month the hippocampal fissure develops as a curved infolding 

 along the median wall of the temporal lobe. Internally, the infolded 



Occipital lobe of 

 cerebrum 



Corpora, 

 quadrigemina 



Hemisphere of 

 cerebellum 



Impression of 

 thalamus 



Temporal lobe 



Vermis cerebelli 



Lateral recess of 

 fourth ventricle 



Fasciculus gracilis 

 Medulla oblongata 



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



cortex forms the hippocampus (Figs. 345 and 349). The lateral fissure 

 (of Sylvius) makes its appearance also in the third month (Fig. 3^2), 

 but its development is not completed until after birth. The cortex over- 

 lying the corpus striatum laterally develops more slowly than the sur- 

 rounding 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 frontal lobe. These are not approximated over the insula until 

 after birth. The frontal operculum is included between the anterior limbs 

 of the Sylvian fissure, and the extent of its development, which is variable, 

 determines the form of these limbs. 



