In a general view of the cat’s brain from the dorsal side (Fig. 137) four subdivisions are discernible. At the caudal end is a small stalk-like portion (_IV_) which is clearly a slightly modified continuation of the spinal cord; this is the =myelencephalon= or =medulla oblongata=. Just craniad of this, rising high above it, is an irregularly lobulated rounded portion, the =cerebellum= (_III_). This is produced as a thickening of the wall of the original tube; it forms the dorsal part of the =metencephalon=. 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= (_II_), 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= (_I_) 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 transverse tract, the =pons= (_i_); 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 hindbrain 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=. In this portion of the brain the continuation of the central cavity is a narrow canal which receives the name =cerebral aqueduct= (=aqueductus cerebri=) or aqueduct of Sylvius. [Illustration: FIG. 138.--VENTRAL SURFACE OF THE BRAIN.