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THE DEEP CONNEXIONS OF THE CRANIAL NERVES. 481 
fifth nerve, which is placed on its inner side. Its fibres come to an end in three 
nuclei of termination, which are situated in the dorsal part of the pons and medulla, 
viz. (1) the principal nucleus or dorsal nucleus; (2) the nucleus of the descending 
root; and (3) the nucleus of Deiters. 
The principal nucleus (Figs. 334, p. 463, and 357, p. 480) is a large diffuse 
nuclear mass, which lies in the floor of the fourth ventricle subjacent to the 
surface district known as the area acustica. It is situated, therefore, in both the 
pons and the medulla to the outer side of the fovea superior and the fovea 
inferior. In section it is prismatic in outline, and crossing its surface immedi- 
ately under the ependyma of the ventricle are the strize acustice. 
When the nervus vestibularis, as it traverses the brain, reaches the inner aspect 
of the dorsal portion of the restiform body, a very large proportion of its fibres 
turn vertically downwards in separate bundles and form the descending root of 
the vestibular nerve (Figs. 341, p. 458; 345, p. 460; 544, p. 463; 357, p. 480). 
This proceeds through the lower part of the pons into the medulla, in which it 
may be traced as far as the level of the decussation of the fillet. Associated with 
the descending root there is a column of gray matter, with nerve-cells strewn 
sparsely throughout it. This is the nucleus of the descending root, and the fibres 
end in fine arborisations around these nerve-cells. 
The fibres of the vestibular nerve end mainly in the nucleus of Deiters. This 
nucleus is composed of a number of large and conspicuous multipolar nerve-cells, 
which are scattered amidst the bundles of the descending root of the vestibular 
nerve. As it is traced upwards into the pons the nucleus gradually inclines back- 
wards, and finally it occupies a place in the lateral wall of the fourth ventricle. It 
attains its greatest development at the level of the emerging part of the facial nerve, 
and this upper part is sometimes termed the nucleus of Bechterew. 
Central Connexions of the Cochlear Nerve.—The cochlear nerve is brought 
into connexion with the inferior quadrigeminal body, and the corpus geniculatum 
internum of the opposite side by the fibres of the corpus trapezoides and the lateral 
fillet. But this connection is not direct ; the chain is composed of several separate links 
or neurons superimposed one over the other. 
The fibres of the cochlear nerve end in the ventral cochlear nucleus and in the tuber- 
culum acusticum. From the cells of these nuclei two tracts arise, viz. a ventral tract, 
composed of the fibres of the corpus trapezoides, and a dorsal tract, which is represented 
by the strize acusticee. 
The corpus trapezoides (Figs. 357 and 358) is formed of the axons of the cells of the 
ventral cochlear nucleus, as well as certain of the axons of the cells of tuberculum 
acusticum. Many of these fibres end in the superior olive, whilst others are added to the 
tract from the cells of that nuclear mass. So constituted, the trapezial fibres cross the 
mesial plane and decussate with the corresponding fibres of the opposite side. Reaching 
the opposite superior olivary nucleus a further interchange of fibres takes place, and 
almost immediately after this the strand turns upwards and becomes the lateral fillet 
(Figs. 346, p. 466; 347, p. 467). But still another nucleus is interposed in its path, viz. 
the nucleus of the lateral fillet. Here some fibres are dropped, whilst from the nuclear 
cells others are acquired, and the lateral fillet then proceeds onwards without further 
interruption until it reaches the inferior quadrigeminal body and the corpus geniculatum 
internum, in which its fibres end. It is probable, however, that some likewise extend 
nto the superior quadrigeminal body. 
But the lateral fillet also includes the fibres of the striz acustice of the opposite 
side. These fibres arise from the cells of the tuberculum acusticum, and arrange them- 
selves in the conspicuous bundles which sweep round the dorsal aspect of the resti- 
form body and proceed inwards across the floor of the fourth ventricle, immediately 
beneath the ependyma (Fig. 333, p. 447). Reaching the middle line they dip forwards 
into the substance of the medulla, and, crossing the mesial plane, they join the lateral 
fillet. 
It is well to remember that the connexion between the terminal cochlear nuclei and 
the inferior quadrigeminal body is not altogether with that of the opposite side, as the 
foregoing description and the diagram (Fig. 357) might lead one to infer. A few fibres 
pass directly to the inferior quadrigeminal body of the same side, but none to the corre- 
sponding corpus geniculatum internum ; the connexion with the latter is entirely crossed 
(Ferrier and Turner). 
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