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INTEKNAL STEUCTUKE OF THE PONS. 567 



Dm the cerebellum to the nuclei pontis ; but there is some doubt concerning the 

 istence of any such fibres. The brachium pontis thus may contain both efferent and 

 :erent cerebellar fibres ; but no fibres pass continuously through the pons from one 

 achium pontis into the other. 



| Certain of the transverse fibres of the pons turn backwards and enter the dorsal or 

 ;gmental part of the pons, but the precise connexions of these are doubtful. 



Corpus Trapezoideum. This name is applied to a group of transverse fibres 

 ,[iich traverse the lower part of the pons (Fig. 498). They are quite distinct from 

 .ose which have been just described as entering the brachium pontis, and they lie 

 the boundary between the dorsal and basilar parts of the pons, but encroaching 

 nsiderably into the ground of the former. They arise from the cells of the 

 rminal nucleus of the cochlear division of the acoustic nerve, and constitute a 

 act which establishes certain central connexions for that nerve. They will be 

 ore fully described when we treat of the cerebral connexions of the acoustic nerve. 



Pars Dorsalis Pontis (Dorsal or Tegmental Part of the Pons). On the dorsal 

 rface of the tegmental part of the pons there is spread a thick layer of gray 

 atter, covered with ependyma, which forms the floor of the upper or pontine 

 it of the fourth ventricle. Beneath this the median raphe of the medulla 

 longata is continued up into the pons, so as to divide its tegmental part into two 

 i mmetrical halves. 



In the inferior part of the pons, immediately beyond the medulla oblongata, the 

 stiform body is placed on the lateral side of the dorsal part (Fig. 498). In trans- 

 rse sections through the pons it appears as a large, massive oval strand of fibres 

 'rich inclines backwards into the cerebellum, and thus leaves the pons. 

 itween the restiform body and the median raphe the tegmental part of the pons 

 composed of substantia reticularis, continuous with the same material in the 

 : idulla oblongata. Thus, arcuate or transverse fibres, curving in towards the raphe, 

 : d also longitudinal fibres, are seen breaking through a mass of gray matter which 



supies the interstices of the intersecting fibres. To the naked eye the formatio 

 :;,icularis presents a uniform gray appearance, but its constituent parts are 

 : sealed by low powers., of the microscope in properly stained and prepared 

 ;:3cimens. Embedded in this substantia reticularis are various clumps of compact 

 jiy matter and certain definite strands of fibres. These we shall describe as we 

 ;'ss from the restiform body medially towards the median raphe. 



1 (1) Spinal Root of the Trigeminal Nerve and its Nucleus. Close to the medial side 

 ( the restiform body, but separated from it by the vestibular root of the acoustic 

 irve as it proceeds backwards through the pons, is seen a large crescentic group 

 < coarse transversely divided bundles of fibres. This is the tractus spinalis (O.T. 

 ynal root) of the trigeminal nerve; and applied to its medial concave side is 

 i, small mass of gray matter, which is the direct continuation upwards of the 

 f bstantia gelatinosa. 



(2) The nucleus of the facial nerve comes next. It is sunk deeply in the 

 <rsal part of the pons and lies close to the transverse fibres of the corpus 

 I .pezoideum. It is a conspicuous, obliquely placed, ovoid clump of gray matter. 



om its lateral and dorsal aspect the root-fibres of the facial nerve stream back- 

 A rds and medially towards the gray matter on the floor of the fourth ventricle. 



ssing forwards between this nucleus and the trigeminal sensory nucleus a solid 

 irve-bundle may be observed. This is the facial nerve, traversing the pons 

 1 Yards its place of emergence from the brain. 



[3) Immediately medial to the facial nucleus, but placed more deeply in the 

 t'jmental part of the pons, is the superior olivary nucleus. It lies in a bay formed 



by the transverse fibres of the corpus trapezoideum. These fibres curve 



imd its ventral aspect, and many of them may be observed penetrating into its 



tance. In man, it is a very small mass of gray matter, and presents little 



emblance to the inferior olivary nucleus, except in the size and shape of its 



c,istituent cells. In sections through the part of the pons where it attains its 



j size, it appears in the form of two, or it may be three, small isolated 



is of gray matter. It is intimately connected with the acoustic fibres, and 



* ablishes manifold connexions between them and the nuclei of other nerves. 



37 c 



