STRUCTURE OF THE MESENCEPHALON 



190 



in the grey matter of the floor of the ventricle but nearer the middle line, 

 and the seventh from a nucleus which lies in the formatio reticularis, 

 and from which the fibres of the nerve pass backwards to the floor of the 

 ventricle, then longitudinally upwards for a short distance, finally 

 bending forwards and downwards and emerging between the transverse 

 fibres at the sides of the pons. 



At the upper part of the pons (fig. 233) the fourth ventricle narrows 

 considerably towards the Sylvian aqueduct, and behind and on either 



rf; 



FIG. 234. SKCTIONS THROUGH THE ORIGIN OF THE FOURTH NERVE, a 



A, transverse section at the place of emergence of the nerve-fibres. B, oblique section 

 carried along the course of the bundles from the nucleus of origin to the place of emer- 

 gence. Aq, Sylvian aqueduct, with its surrounding grey matter ; IV. the nerve-bundles 

 emerging; IV. decussation of the nerves of the two sides; IV". a round bundle passing 

 downwards by the side of the aqueduct to emerge a little lower down ; n.lV. nucleus of 

 the fourth nerve. I, fillet; s.c.p., superior cerebellar peduncle; d.V. descending root of 

 the fifth nerve; pi, posterior longitudinal bundle ; r, raphe. 



side of it two considerable masses of longitudinal white fibres make their 

 appearance. These are the superior crura of the cerebellum (s.c.p.}, 

 and they tend as they pass upwards gradually to approach the middle 

 line (fig. 234 A), across which in the region of the posterior pair of the 

 corpora quadrigemina they pass, decussating with one another, to the 

 formatio reticularis of the opposite side (fig. 235, A). 



FIG. 235. OUTLINE OF TWO SECTIONS ACROSS THE MESENCEPHALON. (Natural size.} 



A, through the middle of the inferior corpora quadrigemina. B, through the middle of the 

 superior corpora quadrigemina. cr, crusta ; s.n. substantia nigra ; /, tegmentum ; s, 

 Sylvian aqueduct, with its surrounding grey matter ; e.g. grey matter of the corpora qua- 

 drigemina ; l.g. lateral groove ; pj. posterior longitudinal bundle ; d.V. descending root of 

 the fifth nerve ; s.c.p. superior cerebellar peduncle ; /, fillet; n.lll. its nucleus ; ///. third 

 nerve. The dotted circle in B indicates the situation of the tegmental nucleus. 



In sections across the mesencephalon (fig. 235), the upward con- 

 tinuity of the parts which have thus been described in the lower parts 

 of the nerve-centres, can still in great measure be traced. 



