1078 



HUMAN ANATOMY. 



ridges are produced by the underlying pyramidal tracts in their journey through 

 the pons from the cerebral peduncles to the medulla. The transverse striation 

 indicates the general course of the superficial fibres towards the cerebellum. 



The lateral surface, continued from the ventral without interruption, above 

 is rounded and sloping and separated from the cerebral peduncles by a distinct 

 furrow. Below, it passes insensibly into the middle cerebellar peduncle, into which 

 the lower and lateral part of the pons is prolonged. Whilst the superficial striation 

 in a general way follows the contour of the pons, a broad band (fasciculus obliquus 

 pontis) from the upper part of the ventral surface sweeps obliquely backward and 

 downward and overlies the more horizontally directed middle and lower fibres. 



The free portion of the dorsal surface of the pons contributes the upper half 

 of the floor of the fourth ventricle and is, therefore, not visible until the roof of that 

 cavity is removed. Above the middle peduncle, the sides of the pons are blended 

 with the overlying superior cerebellar peduncles, which, in conjunction with the 

 intervening superior medullary velum, complete dorsally the ring of tissue sur- 

 rounding the narrowed superior end of the fourth ventricle. 



INTERNAL STRUCTURE OF THE PONS VAROLII. 



Viewed in transverse sections the pons is seen to include two clearly defined 

 areas, the ventral and the dorsal (Fig. 933). The ventral part (pars basilaris ) 

 presents a characteristic picture in which the large pyramidal tracts are covered in 



FIG. 933. 



Abducent fibres 



Inferior cerebellar 



Idiinc.le 

 Facial fibres 



Substantia gelatinosa 



Spinal root of V 



Facial nucleus 



Trapezoidal fibres 



Superior olive 



Pyramidal tracts 



Inferior cerebellar peduncle 

 Nucleus Post, long Nucleus 



of VI fasciculus of VI 



x ^^^HKitinerging facial fibres 



'/^Vcstibular fibres 



TT 



Spinal root of V 

 .Olivary peduncle 



_ Superior olive 



Formatio reticularis 

 -, oftegmentum 



Transverse fibres 



Transverse section of pons at level I, Fig. 910; showing general subdivision into ventral and dorsal (tegmental) 

 areas and nuclei of sixth and seventh nerves. X 3. 



and excluded from the surface by a conspicuous layer of superficial transverse fibres 

 ( stratum supiTiiciale pontis ), that laterally sweep backward into the cerebellar peduncle 

 and are traversed by the root-fibres of the >c\i nth ami eighth nerves. The pyra- 

 mids no longer appear as compact fu Ids, but are broken uj> into smaller bundles by 

 the transverse strands of ponto-cereU liar fibres. This subdivision becomes more 

 marked at higher levels of the pons (Fig. 936), in which the interweaving of the 

 longitudinal and transverse bundle.-, produces a coarse feltwork (stratum complexum). 

 At the upper border of the pons, the scattered pyramidal bundles become once more 

 collected into two compact strands, which are continued into the central part of the 

 crusta <>f the cerebral peduncle. The dorsal limit of the ventral field is occupied 

 In a well marked deeper layer of transverse fibres (stratum profundum pontis). A 

 considerable am< Hint of gray matter, collectively known as the pontile nucleus 



