602 



THE NEEVOUS SYSTEM. 



a column of loosely arranged pear-shaped unipolar cells which are placed in the 

 extreme lateral part of the gray matter surrounding the aquaeductus cerebri. As 

 this root is traced downwards it gradually increases in size by the accession of new 

 fibres, and it assumes a crescentic form in transverse section (Figs. 501, p. 569 ; 

 532, p. 601 ; 533 ; and 534, p. 603). In the inferior part of the mesencephalon it 

 lies on the medial side of the brachium conjunctivum ; and the trochlear nerve, on its 

 way to the surface, runs downwards in its concavity and on its medial aspect. In 

 the superior part of the pons it continues its course downwards on the lateral and deep 

 aspect of the gray matter in the floor of the fourth ventricle. Finally, reaching the 

 level of the nuclei of the trigeminal nerve, the fibres of the mesencephalic root turn 

 forwards and are said to join the sensory part (Johnston) of the trigeminal nerve. Otto 

 May and Horsley, however, confirm the usual description, viz., that it passes into the 

 motor root; but, according to them, it cannot be traced beyond the semilunar ganglion. 



It is customary to de- 

 scribe this mesencephalic root 

 as belonging to the motor 

 division of the trigeminal 

 nerve; but Johnston has re- 

 cently questioned this and 

 claimed : (1) that its fibres 

 become associated at their exit 

 from the central nervous 

 system with the sensory, and 

 not with the motor, root ; (2) 

 that its nucleus develops in 

 the alar and not in the basal 

 lamina ; and (3) that the 

 pear-shaped unipolar cells, 

 from which its fibres arise, 

 conform to the sensory and 

 not to the motor type. 



The reason why its sensory 

 nature has not been suspected 

 hitherto is no doubt the fact 

 that its fibres arise not in 

 FIG. 533. SECTION THROUGH THE INFERIOR COLLICULUS AND THE some ganglion outside the 

 TEGMENTUM OF THE MESENCEPHALON AT THE LEVEL OF THE central nervous system like 



(Sg). PA F ND LEnS * ^^ NEKVE other sensory nerves/ but 



from cells in the tectum 



mesencephali. If Johnston's view is correct, the neural crest in the mesen- 

 cephalic region must have been drawn into the neural tube during development 

 and given rise to this sensory nucleus of origin (not a terminal nucleus) within 

 the central nervous system. 



Otto May and Sir Victor Horsley have shown that the mesencephalic root is 

 a mixture of ascending and descending fibres, but there is no evidence to show 

 that the latter may not be sensory like the former. Nothing is known of their 

 peripheral distribution. 



Nervus Trochlearis. The trochlear nerve supplies the superior oblique muscle 

 of the eyeball. It emerges from the brain, on its dorsal aspect, at the superior 

 part of the anterior medullary velum, immediately below the lower border of the 

 inferior colliculus (Fig. 517, p. 583). The nucleus from which it arises is a small 

 oval mass of gray matter, placed in the ventral part of the central gray matter, at 

 the level of the superior part of the inferior colliculus. The close association of this 

 nucleus with the medial longitudinal bundle has already been referred to. It is 

 sunk deeply in a bay which is hollowed out on the dorsal and medial aspect of 

 that tract. The nerve has a course of some length within the mesencephalon. The 

 axons of the cells leave the lateral aspect of the nuclear mass, and curve backwards 

 and laterally in the central gray matter until they reach the concave medial surface 

 of the mesencephalic root of the trigeminal nerve. Here they are gathered together 



Decussation of lateral lemniscus fibres 



Aquseductus cerebri 



Central gray matter 



Nucleus of inferior 

 colliculus 



Inferior brachium 



Mesencephalic root 

 of trigeminal nerve 



Nucleus of trochlear 

 nerve 



Medial longi- 

 tudinal bundle 



Lateral lemniscus 



Decussation of the 

 brachia conjunctiva 



Medial lemniscus 



