The cerebellum is composed of white and gray matter, the latter on the surface (Fig. 143, _III_). The folds of its surface present thus a contrivance for increasing the amount of gray matter. The white matter forms a central mass from which tracts extend into the folds. The whole mass of white matter has thus in section (more particularly in a longitudinal section of the vermis) the appearance of a tree, whence the name =arbor vitæ= (Fig. 143). The =fourth ventricle= (Fig. 141, _h_; Fig. 143, _m_) is the cavity of the original hindbrain. It begins caudad at the clava (Fig. 141, _d_) as a widening and continuation of the central canal of the spinal cord and extends craniad, becoming wider and passing ventrad of the cerebellum. It becomes narrower craniad and ends at the posterior corpora quadrigemina (_p_), where it becomes continuous with the aqueductus cerebri (Fig. 141, _o_; Fig. 143, _j_). The cavity is shallow and is encroached upon dorsally by the vermis of the cerebellum (Fig. 143). The floor of the cavity is known as the =fossa rhomboidea= (Fig. 141, _h_). It is formed by the continuation of the gray matter which surrounds the central canal of the cord. It is marked by a median longitudinal groove. At its widest part are seen two considerable tracts of white fibres (=striæ medullares=) which pass from near the median line laterad and extend into the auditory (eighth) nerves. The floor caudad and craniad of these striæ is marked by a number of elevations and depressions. Similarly situated elevations in the human brain differ from one another slightly in color, are made up of gray matter, and are the centres of origin of most of the cranial nerves. Their homologues in the cat appear not to have been determined. The side walls of the fourth ventricle (Fig. 141) are formed by the following in order, beginning caudad: the =clava= (_d_), the =corpus restiforme=, =brachium pontis= (_l_), =brachium conjunctivum= (_k_), and =caudal corpora quadrigemina= (_p_) (=colliculi inferiores=). The roof (Fig. 143) is formed caudad by a thin layer of non-nervous matter which is closely associated with the pia mater. This thin layer is known as the =velum medullare posterius= (_n_). It connects the dorsal surface of the medulla with the caudal border of the cerebellum. The pia mater covering this portion of the roof is vascular and is folded in toward the floor of the ventricle, forming the =choroid plexus= of the fourth ventricle. In the middle the roof of the fourth ventricle is the cerebellum, while craniad the roof is the =velum medullare anterius= (Fig. 143, _l_). This is a thin layer just craniad of the cerebellum, connecting it with the corpora quadrigemina, and attached laterally to the brachia conjunctiva. Here the fourth ventricle narrows craniad and becomes continuous with the slender =aqueductus cerebri= (Fig. 143, _j_) (aqueduct of Sylvius). The narrowed portion of the brain is frequently known as the =isthmus rhombencephali=.