THE CENTRAL NERYOUS SYSTEM. 34* 



high above it, is an irregularly tabulated rounded portion, the 

 cerebellum (///) This is produced as a thickening of the wall 

 of the original tube; it forms the dorsal part of the meten- 

 cephalon. These two portions of the brain have a common 

 cavity, lying within the medulla oblongata and ventrad of the 

 cerebellum, and formed by a widening of the cavity of the 

 spinal cord ; it is known as the fourth ventricle. Craniad of 

 the cerebellum, separated from it by a deep transverse fissure, 

 are two very large portions, the hemispheres of the cerebrum 

 (//), separated from each other by a deep longitudinal fissure ; 

 they constitute together the telencephalon. The hemispheres 

 are produced by a sort of forking of the original central tube ; 

 by outgrowths on the sides of the tube, the central cavity 

 extending into the outgrowths. The two outgrowths extend 

 dorsad, as well as craniad and caudad from the original place 

 of origin, forming the larger part of the brain ; the extensions 

 of the central cavity which they contain are known as the 

 lateral ventricles. Finally, in front of the two hemispheres 

 are seen the two small olfactory bulbs (/) which are mere 

 extensions of the two hemispheres and contains cavities which 

 are extensions of the lateral ventricles. 



In a ventral view of the entire brain (Fig. 138) certain 

 parts are visible which in the dorsal view are hidden by the 

 large cerebral hemispheres. At the caudal end is seen, as 

 before, the medulla oblongata, or myelencephalon, and craniad 

 of this, on the ventral side of the cerebellum, a broad trans- 

 verse tract, the pons (z ) ; this, like the cerebellum, forms part 

 of the metencephalon. All this part of the brain formed by 

 the myelencephalon and metencephalon lies caudad of 

 (behind) the rest of the brain and is marked off from it by a 

 great fissure ; it therefore receives as a whole the name hind- 

 brain or rhombencephalon. 



Just craniad of the pons are seen two short diverging arm- 

 like bands of fibres (g), enclosing a small triangular space 

 between them. These two arms with the space between them 

 are all that is visible of a division of the brain which in dorsal 

 view is completely covered by the backward projection of the 

 cerebral hemispheres. This is the midbrain or mesencephalon. 



