466 THE NERVOUS SYSTEM. 
floor of the fourth ventricle becomes narrower, and other objects appear in the 
tegmental substance, 
The superior cerebellar peduncle (brachium conjunctivum) is a very con- 
spicuous object, in sections, through the middle and upper parts of the pons. In 
transverse section it presents a semilunar outline, and as it emerges from the 
cerebellum it lies immediately on the outer side of the fourth ventricle, towards 
which its concave aspect is turned (Fig. 345). Its dorsal border is joined with the 
corresponding peduncle of the opposite side by the thin lamina of white matter, 
termed the superior medullary velum, whilst its ventral border is sunk to a small 
extent in the dorsal part of the pons. As it is traced upwards it sinks deeper and 
deeper into the pons until it becomes completely submerged, with the exception 
of the posterior border to which the superior velum is attached. It now hes on 
the outer side of the tegmental or reticular substance of the pons, and this position 
it maintains until the mesencephalon is reached (Fig. 346). 
About half-way up the pons the nuclei of the trigeminal or fifth cranial nerve 
inark a very important stage in its tegmental portion. These nuclei are two in 
number on each side, viz. a large oval terminal nucleus for certain of the sensory 
fibres of the nerve and a nucleus of origin equally conspicuous for certain of the 
motor fibres (Fig. 345). The sensory nucleus lies close to the outer surface of the pons, 
deeply sunk in its tegmental part, and in the interval between the submerged 
anterior border of the superior cerebellar peduncle and the ventral part of the pons. 
The motor nucleus is placed on the inner side of the sensory nucleus, but somewhat 
nearer the dorsal surface of the pons. At this level the spinal root of the fifth 
nerve disappears by joining the fibres of the sensory portion. The two roots of 
the fifth nerve traverse the ventral part of the pons on their way to and from the 
region of the nuclei. 
Above the level of the nuclei of the trigeminal nerve a new tract of fibres 
comes into view. This is the mesencephalic root of the fifth nerve, as it descends 
to join the emerging fibres of the motor part of the fifth nerve. It is a small 
bundle of nerve fibres, semilunar in cross section, which lies close to the inner side 
Valve of Vieussens 
_—. Gray matter on floor of ventricle IV. 
Upper end of ventricle IV. 
AN 
Tesencephalic root of the fifth nerve Long SS .. —~ — Superior cerebellar peduncle 
V5 deo 
Posterior longitudinal bundle —_ SS sae ae . ie “i \, Laterall fillet 
Formatio reticularis 
Commencing decussa- 
tion of superior 
cerebellar peduncles 
——Mesial fillet 
Nucleus of lateral fillet 
Fifth nerve 4 
Fic. 346.—SEcTION THROUGH THE Upper Part oF THE PONS VAROLIL OF THE ORANG, ABOVE THE 
LEVEL OF THE TRIGEMINAL NUCLEI. 
of the superior cerebellar peduncle and on the outer and deep aspect of the gray 
matter on the floor of the fourth ventricle (Figs. 346 and 347). 
On a slightly deeper plane than the mesencephalic root of the fifth nerve, 
between it and the posterior longitudinal bundle, and in close relation to the gray 
inatter on the floor of the ventricle, is the collection of pigmented cells which con- 
stitute the substantia ferruginea. 
4 
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