THE BRAIN AND ITS MEMBRANES. 721 



The ventral longitudinal fibres are placed just dorsal to, and are concealed 

 from below by, the transverse fibres of the ventral surface just described. They 

 are the direct continuations of the fibres of the pyramid. Each of these pyram- 

 idal bundles soon after entering the pons breaks up into smaller bundles which 

 are intersected by certain transverse fibres (see below). Superiorly, they are 

 continued upward into the crusta of the mid-brain. These fibres lie on each side 

 of the middle line, and cause a corresponding bulging of the tuber annulare. 

 Thus is produced the median groove (sulcus basilaris) for the basilar artery. As 

 they pass upward through the pons these fibres are somewhat increased in number 

 from being reinforced by fibres derived from the nerve-cells in the ventral trans- 

 verse fibres (see below.). 



The dorsal longitudinal fibres are separated by quite an interval from the pre- 

 ceding. This interval is filled in by transverse fibres, especially the trapezium 

 (see below). They are continued upward from the formatio reticularis of the 

 medulla, and among them are especially to be noted the ascending root of the fifth 

 nerve, the fillet, and the posterior longitudinal bundle. 



The Transverse Fibres. These comprise ventral and dorsal, and must not be 

 confounded, especially the former, with the superficial transverse fibres of the 

 ventral surface (tuber annulare}, already described. These transverse fibres now 

 under discussion belong to the " deep portion " of the pons, dorsal to those of the 

 ventral surface. 



The ventral transverse fibres of the deep portion of the pons intersect the 

 bundles of the pyramidal fibres (see above), and then curve dorsally and join 

 with those of the ventral surface to make up the middle peduncle of the cerebellum. 

 These transverse fibres, taken together, form a much thicker layer than the super- 

 ficial set, and contain much gray matter between them. Across the median line, 

 intersecting or dorsal to the pyramidal bundle, they meet and interlace with 

 those coming from the opposite side. Furthermore, all of these fibres do not join 

 the middle peduncle, many of them joining the nerve-cells, which are situated in 

 the gray matter (nuclei pontis) of this layer. From these cells other fibres are 

 given off which proceed to the pyramidal bundles (see above). 



The dorsal transverse fibres of the deep portion of the pons, especially in its 

 lower half, are collected into a distinct mass called, from its shape, the trapezium. 



The trapezium is situated just dorsal to the pyramidal bundles, and its fibres 

 proceed laterally on each side, tapering as they go, until they reach the cells (with 

 which they become connected) of the accessory (ventral) auditory nucleus (Fig. 488), 

 and, through this, the lateral root of the auditory nerve. Some of the fibres of the 

 trapezium are connected with the cells of the superior olivary nucleus (see below), 

 which lies just dorsally on each side, and others pass to the fillet. 



The Septum or Raphe. This is the upward prolongation of the medullary 

 raphe. It is found in that portion of the pons which is dorsal to the trapezium, 

 and does not extend to the ventral surface except at the upper and lower extrem- 

 ities of the pons. At these places certain of the raphe fibres pass out of the 

 median line, and then bend laterally to join with and become part of the upper 

 and lower margins, respectively, of the tuber annulare. It follows, therefore 

 (see page 720), that some of the fibres of the upper margin of the tuber annulare 

 actually encircle the corresponding crus cerebri. 



The Gray Matter of the Pons. This may be arranged as follows : 



(a) The nuclei pontis, which are small masses of gray matter, containing small 

 multipolar nerve-cells, found scattered between the bundles of the ventral trans- 

 verse fibres (see above), and also to a less extent, between those of the tuber 

 annulare. Some of the fibres of the latter may have an arrangement i. e. inter- 

 lacing and taking origin from these nuclei similar to that already described as 

 occurring in many of the fibres of the ventral transverse set. 



(b) Gray Matter of the Formatio Reticularis. This formatio, as before stated, 

 lies dorsal to the trapezium. Its gray matter comprises, first, its own gray 

 matter /'. c. small reticularly arranged masses with nerve-cells, exactly similar to 



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