488 PHYSIOLOGY CHAP. 



is the prolongation of that which lines the floor of the fourth 

 ventricle. In addition to many scattered cells, the grey matter of 

 the aqueduct contains the cell columns which give origin to the 

 roots of the 3rd, 4th, and the descending root of the 5th nerve 

 (Fig. 208). 



The posterior corpora quadrigemina consist almost entirely of 



TH.IV 



py- 



FIG. 243. Transverse section across mid-brain, through inferior corpora quadrigemina. Magnified 

 about 3 diameters. From a photograph. (Schafer.) grr., dorsal quadrigeminal groove (sulcus 

 longitudinalis) ; c.q.p., corpus quadrigeminum postering; str.l., stratum lemnisci ; c.gr., central 

 grey matter ; n.III, IV, oculo-motor nucleus ; d.V., descending root of 5th nerve ; p. I. b. , posterior 

 longitudinal bundle; f.r.t., formatio reticularis tegmenti ; d, d', decussating fibres of teginentuin; 

 s.c.p., decussating fibres of superior cerebellar peduncles; /, upper fillet; /', lower or lateral 

 fillet; p.p., pes pedunculi ; s.n., substantia nigra ; g.i.p., interpeduncular grey matter; Sy, 

 Sylvian aqueduct. 



grey matter (Fig. 243). The cells which they contain are in 

 connection with the endings of the fibres of the lateral fillet, 

 which arise from the nucleus cochlearis of the auditory nerve 

 on the opposite side. In correspondence with this intimate 

 relation of the posterior corpora quadrigemina with the nucleus 

 of (the cochlear nerve, it is in mammals only which have a 

 well -developed auditory apparatus that the posterior corpora 

 quadrigemina appear as distinct prominences. Other vertebrates, 



