STRUCTURE OF THE MESENCEPHALON. 



237 



its dorsal aspect to enter the middle lobe of the cerebellum in the 

 superior medullary velum. 



Mid-brain or Mesencephalon. In sections across the mesencephalon 

 (figs. 267, 268, 269), the upward continuity of the parts which have 

 thus been described in the lower parts of the nerve-centres, can still 

 in great measure be traced. 



FIG. 269. SECTION ACROSS THE MID-BRAIN THROUGH THE INFERIOR PAIR OF 

 CORPORA QUADRIGEMINA. (Magnified about 3J diameters.) 



Sy, aqueduct of Sylvius ; c.ctr., central grey matter of the aqueduct ; n.III.lV., group of 

 cells forming part of the conjoined nucleus of the third and fourth nerves ; c.q.p., one 

 of the posterior corpora quadrigemina ; gr, median groove separating it from that of 

 the opposite side; str.l., stratum lemnisci (layer of the fillet), forming its superficial 

 layei- ; /, upper fillet ; /', lateral fillet ; d'. V, descending root of fifth nerve ; p.l.b., pos- 

 terior longitudinal bundle; f.r.t., foi-matio reticularis tegmenti ; d, d', decussating 

 fibres of tegmenta ; s.c.p., superior cerebellar peduncle; p.p., pes pedunculi (crusta); 

 s.n., substantia nigra ; g.i.p., interpeduncular grey matter. 



The Sylvitin aqueduct (fig. 269, Sy), with its lining of ciliated epithe- 

 lium, represents the central canal of the cord and the fourth ventricle 

 of the medulla oblongata. In the grey matter which surrounds it (central 

 grey matter) there is seen in all sections of the region a group of large 

 nerve-cells lying anteriorly on each side of the middle line, close to the 



