420 



PHYSIOLOGY 



CHAP. 



In transverse sections of the brain stem at the level of the 

 pons, the ventral part, or pons proper, must be distinguished from 

 the dorsal part or tegmentum, which is the continuation of the 

 bulb. The former contains the transverse fibres which pass to the 

 middle peduncles ; the most superficial lie over the pyramids, the 

 deeper pass partly between the pyramidal fibres, partly dorsal to 

 them ; reaching the middle line they decussate with the fibres from 

 the other side (Fig. 219). The grey matter of the pous contains 



a.V 



O.S. 



FIG. 219.--Sec.tion across lower part of pons. (Stilling and Sehwalbe.) jii/, pyramidal bundles con- 

 tinued up from medulla ; po, transverse fibres of pons passing from middle cms of cerebellum, 

 before (/in-) and behind (/'O 1 ) chief pyramidal bundles ; t, deeper transverse fibres, constituting 

 trapezium ; the grey matter between the transverse fibres is not represented in this or in the 

 following figures ; r, raphe ; o.s., superior olivary nucleus ; n.V, bundles of ascending roots of 

 5th nerve, enclosed by prolongation of grey substance of Rolando; VI, 6th nerve; TO. VI, 

 its nucleus; VII, facial nerve; Vll.a, ascending portion of facial root; TO. VII, its nucleus; 

 VIII, superior root of auditory nerve ; TO. VIII, part of nucleus of Deiters ; V, section of vein. 



small multipolar nerve-cells scattered among the superficial and 

 deeper bundles (Fig. 220). 



The dorsal part of the pons represents the continuation of the 

 formatio reticularis and grey matter of the bulb, but it also con- 

 tains a more definite and compact mass of grey matter, known as 

 the superior olivary nucleus, as well as the nuclei of the 5th, 6th, 

 and 7th cerebral nerves. 



The cerebellum or dorsal part of the mesencephalon occupies 

 the posterior fossa of the skull : its median portion forms the roof of 

 the fourth ventricle (Fig. 221). Between the two superior peduncles 

 this roof is completed by the velum medullare superius, or valve 



