THE CENTRAL NERVOUS SYSTEM. 349 
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 vite (Fig. 143). 
The fourth ventricle (Fig. 141, 4; Fig. 143, m) is the 
cavity of the original hindbrain. It begins caudad at the clava 
(Fig. 141, @) as a widening and continuation of the central 
canal of the spinal cord,and extends craniad, becoming wider 
and passing ventrad of the cerebellum. 1t becomes narrower 
craniad and ends at the posterior corpora quadrigemina (J), 
where it becomes continuous with the aqueductus cerebri (Fig. 
141, 0; Fig. 143, 7). 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, %). 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 (strie medul- 
lares) which pass from near the median line laterad and extend 
into the auditory (eighth) nerves. The floor caudad and 
craniad of these strie 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 (@), 
the corpus restiforme, brachium pontis (/), brachium con- 
junctivum (4), and caudal corpora quadrigemina (/) (col- 
liculi 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 (z). 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 
