THE BRAIN AND ITS MEMBRANES. 723 



smaller of which, in each case, goes to the nucleus, while the other takes a dis- 

 tinct course, differing for the two roots, thus: The "non-nuclear" division of the 

 motor root passes upward as a distinct bundle through the dorsal part of the pons 

 and into the mid-brain, where its fibres terminate in a group of large nerve-cells 

 situated in the gray matter on the side of the aqueduct of Sylvius. This is the 

 so-called descending root of the fifth nerve The "non-nuclear" division of the 

 sensory root is the so-called ascending root of the fifth nerve, already sufficiently 

 described. 



Floor of the Fourth Ventricle (Fig. 423). 



A? already stated, the floor of the fourth ventricle is made up of the mesial 

 portions of the dorsal surfaces of, the pons Varolii above and upper half of the 1 

 medulla oblongata below. It is lozenge- or diamond-shaped ; that is to say, it is 

 composed of two triangles, with their bases opposed to each other. Hence it is 

 often called the fossa rhomboidaUs. 



The lower triangle is formed by the divergence of the clavae of the funiculi 

 graciles and the restiform bodies. These columns pass upward and outward at an 



Pnst. corp. quad. 



Wge - ' 



sup. ped. of cerdt. 



ip.Jovea 



^ 



Fun. tun. 

 Fun.grac. 



FIG. 423. Floor of the fourth ventricle. (Henle.) 



acute angle, leaving by their divergence a triangular space which forms the lower 

 half of the floor of the fourth ventricle. In like manner the upper triangle is 

 formed by the divergence of the superior peduncles of the cerebellum. These, 

 traced downward, as they emerge from beneath the corpora quadrigemina of the 

 mid-brain, are almost in ontact by their inner margins, but they gradually 

 diverge, passing downward, backward, and outward, to reach the cerebellum, thus 

 enclosing a triangular space which forms the upper half of the floor of the fourth 

 ventricle. 



The floor presents four angles. The upper angle reaches as high as the upper 

 border of the pons ; it presents the lower opening of the aqueduct of Sylvius, by 

 which this ventricle communicates with the third ventricle. The lower angle is 

 the angle of divergence of the clavse, and is about on a level with the lower 

 end of the olivary body. It presents a minute opening, the aperture of the 

 central canal of the spinal cord. 



The two lateral angles are situated each at an end of the conjoined bases of 

 the triangles. The distance between them is the widest part of the floor. Each 

 lateral angle is also the point of the "coming together "of the superior and 

 inferior peduncle (restiform body) just as they pass into the cerebellum. 



In the median line of the floor is a longitudinal groove which extends between 



