THE FOURTH VENTRICLE 125 



the brachium pontis. In fact, it is customary to take the exit of this nerve as 

 marking the point of junction of the pons with its brachium. The nerve has two 

 roots which lie close together: the larger is the sensory root, or portio major; 

 the smaller is the motor root, or portio minor (Fig. 88). 



The posterior surface of the pons forms the rostral part of the floor of the 

 fourth ventricle, along the lateral borders of which there are two prominent 

 and rather large strands of nerve-fibers, the brachia conjunctiva (Figs. 88, 89). 



The brachia conjunctiva or superior cerebellar peduncles lie under cover of 

 the cerebellum. As they emerge from the white centers of the cerebellar hemi- 

 spheres they curve rostrally and take up a position along the lateral border of 

 the fourth ventricle. They converge as they ascend and disappear from view 

 by sinking into the substance of the mesencephalon under cover of the inferior 

 quadrigeminal bodies. Each consists of fibers which connect the cerebellum 

 with the red nucleus, a large gray mass situated within the midbrain ventral to 

 the superior colliculus of the corpora quadrigemina. The interval between the 

 two brachia conjunctiva, where these form the lateral boundaries of the fourth 

 ventricle, is occupied by a thin lamina of white matter, the anterior medullary 

 velum (Fig. 85). This is stretched between the free dorsomedial borders of the 

 two brachia and forms the roof of the rostral portion of the ventricle. Caudally 

 it is continuous with the white center of the cerebellum. The fibers of the 

 trochlear nerves decussate in the anterior medullary velum and emerge from its 

 dorsal surface (Fig. 89). As they run through the velum they produce a raised 

 white line which extends transversely from one brachium to the other. 



THE FOURTH VENTRICLE 



The lozenge-shaped cavity of the rhombencephalon is known as the fourth 

 ventricle. It lies between the pons and medulla oblongata, ventrally, and the 

 cerebellum dorsally, and is continuous with the central canal of the closed por- 

 tion of the medulla, on the one hand, and with the cerebral aqueduct on the 

 other (Fig. 84). On each side a narrow curved prolongation of the cavity ex- 

 tends laterally on the dorsal surface of the restiform body. This is known as 

 the lateral recess (Figs. 89, 90). It opens into the subarachnoid space near the 

 flocculus of the cerebellum; and through this lateral aperture of the fourth ven- 

 tricle (foramen of Luschka) protrudes a small portion of the chorioid plexus 

 (Fig. 90). There is also a median aperture (foramen of Magendie) through the 

 roof of the ventricle near the caudal extremity. By means of these three open- 

 ings, one medial and two lateral, the cavity of the ventricle is in communica- 



