THE CENTRAL NERVOUS SYSTEM. 341 
high above it, is an irregularly lobulated rounded portion, the 
cerebellum (///7). 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 
(/7), 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; 
—hby 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 (7); 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. 
