THE CENTRAL NERYOUS 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 vitae (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 (/), 

 where it becomes continuous with the aqueductus cerebri (Fig. 

 141, o ; 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 (striae medul- 

 lares) which pass from near the median line laterad and extend 

 into the auditory (eighth) nerves. The floor caudad and 

 craniad of these striae 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 (/), brachium con- 

 junctivum (), and caudal corpora quadrigemina (p) (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 (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 



