THE DEEP CONNEXIONS OF THE CEEEBEAL NEEVES. 603 



.nto one or two round bundles, which, bending sharply, turn downwards at a right 

 ingle and descend on the medial side of the trigeminal root. When the region 

 Delow the inferior colliculus is reached, the nerve makes another sharp bend. This 

 ime it turns medially, enters the superior end of the anterior medullary velum, in 

 vhich it decussates with its fellow of the opposite side. Having thus crossed the 

 nedian plane, the trochlear nerve emerges at the medial border of the brachium 

 jonjunctivum. The course pursued by the trochlear nerve within the central 

 l^ray matter may be traced by examining in succession Fig. 533 ; Fig. 534 ; 

 Fig. 502, p. 570; and Fig. 512, p. 577. 



Nervus Oculomotorius. The oculomotor nerve supplies the levator palpebrse 

 juperioris, all the ocular muscles, with the exception of the superior oblique and 

 }he lateral rectus, and also two muscles within the eyeball, viz., the sphincter iridis 

 ind the musculus ciliaris. The nucleus of origin is placed in the ventral part of 

 .;he central gray matter subjacent to the superior colliculus (Fig. 521, p. 587). In 

 .ength it measures from 5 to 6 

 .nm. Its inferior end is par- 

 tially continuous with the 

 lucleus of the trochlear nerve, 

 whilst its superior end extends 

 .ipwards for a short distance 

 Beyond the mesencephalon 

 .nto the gray matter on the 

 i|ade wall of the third 

 /entricle. Its relation to the 

 ,nedial longitudinal bundle is 

 iven more intimate than that 

 )f the trochlear nucleus. It 

 .s closely applied to the 

 .lorsal and medial aspect of 

 }his strand ; many of its cells 

 >ccupy a position in the in- 

 ,-ervals between the nerve- 

 Bundles of the tract, and some 

 ',ven are seen on its ventral or 

 negmental aspect. The axons 

 >f the nuclear cells leave the FIG. 534. SECTION THROUGH THB INFERIOR COLLICULUS AND THE 



Central gray 

 atter 



Aquseductus 

 cerebri 



Mesencephalic 

 root of tri- 

 geminal nerve 

 Trochlear 

 nerve leaving 

 nucleus 



Medial 



longitudinal 



bundle 



Decussation of 

 the brachia 

 conjunctiva 



TEGMENTUM OF THE MESENCEPHALON AT THE LEVEL OF THE 

 INFERIOR PART OF THE NUCLEUS OF THE TROCHLEAR NERVE 

 (Orang). 



mcleus in numerous bundles, 

 i vhich describe a series ' of 

 I'.urves as they proceed for- 

 vards through the medial longitudinal bundle, the tegmentum, red nucleus, and 

 nedial margin of the substantia nigra, to emerge finally from the brain-stem along 

 he bottom of the sulcus oculo-motorius on the medial aspect of the basis pedunculi. 

 The cells of the oculomotor nucleus are not uniformly distributed through- 

 put it. They are grouped into several more or less distinct collections or 



slumps, some of which possess cells which differ in size and appearance from the 

 )thers. These cell-clusters are very generally believed to possess a definite relation 

 ;o the several branches of the nerve and the muscles which they supply. Perlia 

 ecognises no less than seven such cell-clusters in each nucleus, with a small median 

 mcleus placed accurately on the median plane, and from which fibres for both nerves 



Spring. Whilst the majority of the fibres in the oculomotor nerve arise from the 

 iell-groups which lie on its own side of the median plane, it has been satisfactorily 

 i established that a certain proportion of its fibres are derived from the nucleus of 

 yhe opposite side, thus forming a crossed connexion and giving rise to a median 

 lecussation. These crossed fibres are supposed by some to supply the medial rectus 

 nuscle ; and we have seen that there is reason to believe that the part of the nucleus 



Tom which these fibres are derived stands in connexion through the medial loiigi- 

 , -udinal fasciculus with the abducens nucleus from which proceeds the nerve of supply 

 , or the lateral rectus muscle. The harmonious action of the medial and lateral 



ecti in producing the conjugate movements of the eyeballs is thus explained. 



