THE CEREBELLUM. 237 



the same as elsewhere except that the fourth or fiber layer is 

 largely occupied by a number of gray masses or nuclei. These 

 gray masses form the roof of the fourth ventricle at its anterior 

 end. In this region are recognized near the median line the 

 paired nuclei tecti or jastigii; lateral to them on either side the 

 smaller nucleus globosus and nucleus emboliformis; and farther 

 laterally the large nucleus dentatus which in man is convoluted 

 like the lower olive but in lower mammals has a simple compact 

 form. In the region of these nuclei is the commissure of the 

 deep or white layer of the cerebellum. 



The general features of the vertebrate cerebellum may now be 

 summarized. It consists in all classes of the dorso-lateral walls 

 of the metencephalic segment which are arched over and connected 

 above the ventricle, where there is a commissure of molecular 

 layer fibers. In most vertebrates the lateral walls bulge outward 

 and forward as the auricular lobes, the floccular lobes in man. 

 In selachians the cerebellum contains a part of the fourth ventricle 

 which extends up into all the folds (Fig. 120). In mammals 

 the great increase of the white layer has obliterated the cavity 

 in the several cerebellar folds and its position is indicated only 

 by the branches of the arbor vitae. In median sagittal section 

 the roof of the fourth ventricle extends up into the cerebellum 

 in the form of an inverted letter V. The apex of the A enters the 

 base of the arbor vitae and separates the gray masses beneath 

 the anterior lobe from the nodulus. The position of these deep 

 gray masses and the commissure which passes through them 

 corresponds fully to that of the velum medullare anterius and 

 neighboring parts in selachians. The nodulus, floccular lobe and 

 the acustic nuclei bear the same relations in mammals as do the 

 caudal border of the cerebellum, auricular lobe and tuberculum 

 acusticum in selachians. The mammalian vermis is formed by a 

 great thickening of what corresponds to the mid-dorsal region 

 of the cerebellum of any lower vertebrate. The hemispheres 

 correspond to the dorso- cephalic wall of the lateral lobes in lower 

 vertebrates. At the base of the lateral walls in fishes lie the 

 secondary gustatory nuclei whose commissure crosses through 

 the velum. In mammals in the region corresponding to the velum 



