60 THE MEDULLA OBLONG ATA AND PONS VAROLII. 



fibres which have passed into the raphe as floras rectae or as superficial or deep arched 

 fibres, and in it have altered their direction and become longitudinal. The obliquely 

 crossing fibres are the deep arched fibres which enter or emerge from the raphe. 

 Others, however, seem to come from the nuclei of the nerve-roots, and these may 

 pass more directly across as commissural fibres either into the reticular formation or 

 into the pyramid of the other side, in either case becoming longitudinal. The nerve- 

 cells of the raphe are multipolar cells, those in the middle being chiefly spindle- 

 shaped. The latter are connected with fibrse rectas (Clarke), whilst the more laterally 

 situated ones, at least those near the anterior median fissure, are connected with 

 some of the superficial arched fibres. 



Internal structure of the pons Varolii. Sections of the pons are greatly 

 modified by the appearance of the transversely coursing fibres between the two halves 

 of the cerebellum which have already been noticed. These occupy the whole of the 

 ventral portion and enclose and conceal from view the bundles of the pyramids, which 

 can be traced upwards into and through the pons from the medulla oblongata. 



Between the bundles of fibres of this ventral portion of the pons grey matter 

 with small multipolar nerve-cells is everywhere found (nuclei pontis (fig. 49, n.p.}). 

 It is probable that many of the transverse fibres terminate in this grey matter and 

 are through it connected in some way with the longitudinal fibres of the pyramidal 

 tract ascending through the pons ; but the transverse fibres do not appear to turn 

 upwards, for they are smaller than the longitudinal fibres. 



The posterior or dorsal portion of the pons is chiefly constituted by a continua- 

 tion upwards of the reticular formation and of the grey matter of the medulla 

 oblongata. As in the latter, there exists here also a median septum or raphe, which 

 is similar in structure to that of the medulla oblongata. It does not extend through 

 the ventral half, being obliterated, or nearly so, by the great development of the 

 obliquely and transversely passing fibres, except near the upper and lower borders 

 where the superficial transverse fibres of the pons turn in at the middle line ; and 

 especially at the upper border where bundles of the same fibres encircle the crura 

 cerebri as they emerge from the pons. 



In the reticular formation, in addition to the scattered and reticulaiiy arranged 

 grey matter with nerve-cells everywhere met with, there are one or two more 

 important collections which lie embedded in this formation and from which nerve- 

 fibres arise. One of these is the superior olivary nucleus, another is the nucleus of 

 the seventh or facial nerve, and others give origin to portions of the fifth nerve. 



The superior olivary nucleus (fig. 50, o.s.) is a collection of small nerve-cells 

 which lies dorsal to the outer part of the trapezium, in what would correspond (as 

 indicated by the passage outwards of the roots of the sixth and seventh nerves) to a 

 prolongation of the lateral area of the medulla oblongata. In man it is very much 

 smaller than the inferior olive, to which it does not present much resemblance in 

 form, although in structure and in the size of its cells there is a close similarity. In 

 some animals, however, it is larger, and has a distinctly sinuous outline. From it, 

 as above mentioned (p. 47), fibres pass into the trapezium ; it may be connected 

 through these with the accessory auditory nucleus of the opposite side (p. 55). 

 Running upwards on the mesial side of the superior olive between this and the fillet 

 is a bundle of fibres which has been termed the central tract of the tcgmentum, but its 

 connections have not yet been satisfactorily ascertained. 



Besides its connection with the contra-lateral accessory auditory nucleus through the corpus 

 trapezoides, the upper olive is connected, according to Bechterew, (1) to both the posterior 

 (inferior) corpora quadrigemina through the lower fillets, (2) to the cerebellum through the 

 restif orm body, (3) with the nucleus of the sixth nerve (and perhaps, also, with that of the third 

 and fourth nerves) through the posterior longitudinal bundle, and (4) with the spinal cord through 

 fibres passing down the anterior column. 



