720 THE NERVOUS SYSTEM. 



greater than that of the medulla. There are four surfaces to the pons superior, 

 inferior, ventral, and dorsal; the two latter are free. The superior and inferior 

 surfaces are seen only on section, the former being attached, by direct continuation 

 of fibres, to the mid-brain, while the latter is similarly attached to the medulla. 



The ventral surface is markedly convex from side to side ; it rests upon the 

 grooved dorsal surface of the dorsum sellce of the body of the sphenoid bone. It 

 presents along the middle line a longitudinal groove, wider in front than behind, 

 which lodges the basilar artery. This surface consists entirely of a rather thick 

 layer of well-marked transversely running fibres, extending across the median 

 groove from side to side. The lowermost fibres slightly overlap the upper ends 

 of the pyramids and cross over the extreme upper end of the anterior median 

 fissure. The uppermost fibres, similarly but to a greater extent, overlap the 

 lower part of the ventral surface of the mid-brain (crura cerebri or crustae). 

 Hence it follows that this surface has an upper and lower curved free margin, 

 and each somewhat rounded and distinct from the medulla on the one hand and 

 the crura cerebri on the other, and that these margins have nothing to do with 

 the respective attached surfaces of the pons. Furthermore, after crossing the 

 middle line the superior fibres bend downward and the inferior upward. The 

 middle fibres are exactly transverse ; hence their extremities are overlapped by 

 those of the other two sets. The extremities of all these sets of fibres are seen, 

 in horizontal section, on the dorsal aspect of the pons, and they here form, 

 together with other transverse fibres coming from the deep part of the pons, a 

 large rounded bundle of fibres on each side, which is directed dorsally into the 

 cerebellum, and is known as the middle peduncle of the cerebellum of the corre- 

 sponding side. Owing to its prominence this surface is often called the tuber 

 annulare. 



The dorsal surface of the pons is almost flat, and, though free, is concealed 

 from above by the cerebellum. It is divided into a mesial and two lateral por- 

 tions. Each lateral portion is raised up somewhat from the mesial, and is seen 

 to be a rather broad, flat band of white fibres. These bands are not parallel, but 

 .converge from below upward. Superiorly, the fibres of each are continued into 

 the mid-brain ; inferiorly, they pass into the cerebellum. These bands are the 

 superior peduncles of the cerebellum. Besides being raised from the mesial por- 

 tion, each of these peduncles overhangs it a little by its inner margin. Between 

 the inner margins of these peduncles stretches a delicate layer of Avhite matter 

 (valve of Vieussens) roofing over the following : 



The mesial portion of the dorsal surface of the pons is the upper half of the 

 floor of the fourth ventricle. Like the lower half (see page 711), it is triangular 

 in shape, but its apex is upward. Its base corresponds to that of the lower or 

 medullary half. As these two portions of the floor of the ventricle run into each 

 other without any line of demarkation, it follows that the entire floor of the fourth 

 ventricle is rhomboidal or diamond-shaped. The widest part of the floor is the 

 line of union of the two bases of the triangles, and, if this line is continued 

 ventrally, it will be found to run close along the lower free margin of the tuber 

 annulare (ventral surface of pons). The floor, as a whole, will be described after 

 the description of the pons is completed. 



Relations of the Cerebellar Peduncles to Each Other. If the cerebellum be 

 removed from the pons and medulla by cutting through the three peduncles on 

 each side close to the pons and medulla, it will be found that the cut ends are 

 all grouped together in an area immediately external to the widest part of the 

 floor of the ventricle. In this group the cut end of the middle is external to the 

 cut ends of the superior and inferior peduncles, which here are in contact (see 

 Fig. 416). 



Deep Portion of the Pons. This is comprised between the dorsal and ventral 

 surfaces. It is made up of both longitudinal and transverse fibres and gray 

 matter. The longitudinal and transverse fibres in each lateral half of the pons 

 are arranged in two groups, ventral and dorsal. 



