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‘ 
DEEP ORIGIN OF THE TROCHLEAR NERVE. 499 
DEEP ORIGIN OF THE CRANIAL NERVES WHICH ARISE WITHIN THE 
MESENCEPHALON. 
Two of the motor cranial nerves, viz. the oculo-motor and the trochlear nerves, 
as well as the mesencephalic root of the trigeminal nerve, obtain origin within the 
mesencephalon. The nuclei from which they spring are all situated within the 
gray matter of the Sylvian aqueduct. 
Mesencephalic Root of the Trigeminal Nerve (radix descendens).—The fibres 
of this root arise from a column of large, sparsely-arranged cells, which extends 
throughout the entire length 
of the mesencephalon. These 
cells lie in the outer part of 
the Sylvian gray matter, close 
to the tegmentum. The axons 
which emerge from the cells _ yucieus of inferior 
run downwards close to the avadrigeminal body ~ 
outer surface of the Sylvian Inferior brachium - 
gray matter in the form of  Mesencephalic root 
a small, gradually -increasing pega? 
facts Inthe lower part of Nucleus of fourth 
the mesencephalon this tract Dostadanieeee 
assumes a crescentic outline, tudinal bundle 
and ultimately comes to lie on Lateral fillet 
the inner aspect of the superior Decussation of the 
cerebellar peduncle (Fig. 346, ““Peror cenbenst 
p. 466). The further course of 
the mesencephalic root of the ; 
fifth nerve through the upper ( 
part of the pons, to the point Mose 
where it joins the emerging ie eel 
motor root of the trigeminal p= Euan lilt ai alias Ge 
nerve, has already been traced Fic. 371.—Srotion THROUGH THE INFERIOR QUADRIGEMINAL Bopy 
Decussation of lateral fillet fibres —— 
—— 
Sylvian aqueduct~ 
(p 485) AND THE TEGMENTUM OF THE MESENCEPHALON AT THE LEVEL 
OF THE MIDDLE ParT OF THE NUCLEUS OF THE TROCHLEAR 
Trochlear or Fourth NERVE (ORANG). 
Nerve (nervus trochlearis).— 
The trochlear nerve supplies the superior oblique muscle of the eyeball. It 
emerges from the brain, on its dorsal aspect, at the upper part of the superior 
medullary velum, immediately below the lower border of the inferior quadri- 
geminal body (Fig. 375, p. 502). The nucleus from which it arises is a small 
oval mass of gray matter, placed in the ventral part of the Sylvian gray 
matter at the level of the upper part of the inferior quadrigeminal body. The 
close association of this nucleus with the posterior longitudinal bundle has already 
been alluded to. It is sunk deeply in a bay, which is hollowed out on the dorsal 
and inner aspect of that tract. The nerve has a course of some length within the 
mesencephalon. The axons of the cells leave the outer aspect of the nuclear mass, 
and curve backwards and outwards in the Sylvian gray matter until they reach the 
concave inner surface of the mesencephalic root of the trigeminal nerve. Here they 
are gathered together into one or two round bundles, which, bending sharply, turn 
~downwards at a right angle and descend on the inner side of the trigeminal root. 
When the region below the inferior quadrigeminal body is reached, the nerve 
makes another sharp bend. This time it turns inwards, enters the upper end of 
the superior medullary velum, in which it decussates with its fellow of the opposite 
side. Having thus crossed the mesial plane, the trochlear nerve emerges at the 
inner border of the superior cerebellar peduncle. The course pursued by the fourth 
nerve within the Sylvian gray matter may be traced by examining in succession 
Fig. 372; Fig. 367, p. 496; Fig. 368, p. 496; and Fig. 347, p. 467. 
Oculo-motor or Third Nerve (nervus oculo-motorius).—The oculo-motor nerve 
supplies the levator palpebrz superioris, all the ocular muscles, with the exception 
of the superior oblique and the external rectus, and also two muscles within the 
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