236 



THE ESSENTIALS OF HISTOLOGY. 



considerably towards the Sylvian aqueduct, and behind and on either 

 side of it two considerable masses of longitudinal white fibres make 

 their appearance. These are the superior crura of the cerebellum (s.c.p.) 



FIG. 266. SECTIONS THROUGH THE ORIGIN OF THE FOURTH NERVE. (Schwalbe.) f. 



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; dV, descending root of 

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



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

 line (fig. 266, 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 (figs. 267, A, 268). 



The antero-lateral ascending tract of the spinal cord is continued up 

 through the ventral part of the pons Varolii lateral to the pyramid- 

 bundles, but at about the level of the exit of the fifth nerve its fibres 

 begin to pass obliquely towards the dorso-lateral part of the pons, 

 where the superior cerebellar peduncle is emerging from the cerebellar 

 hemisphere. The tract in question now curves over the lateral aspect 

 of this peduncle, and then takes a sharp backward turn, passing over 



FIGS. 267, 268. OUTLINE OF TWO SECTIONS ACROSS THE MESENCEPHALON. 

 (Natural size.) 



267, through the middle of the inferior corpora quadrigemina. 208, through the middle of 

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

 Sylvian aqueduct, with its surrounding grey matter ; c.q. , grey matter of the corpora quad- 

 rigemina ; l.g., lateral groove ; p.L, posterior longitudinal bundle ; d. V., descending root of 

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

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



