THE TEGMENTUM OF THE CRUS CEREBRI AND ITS GANGLIA. 443 



The grey matter in the interior of the corpora quadrigemina, 

 like that on the internal surface of the optic thalamus, belongs 

 to the grey matter of the central cavities ; as the grey matter 

 surrounding the aqugeductus, it forms the continuation of the 

 lining of the third ventricle. The description of this grey 

 matter, which is to be distinguished from the ganglia of the cor- 

 pora quadrigemina, must first be given, because its presence 

 and its intimate connection with the former renders the signi- 

 ficance of the corpora quadrigemina in one respect extremely 

 clear. The central grey substance of the cavities around the 

 aquaeductus Sylvii is limited posteriorly by the decussation of 

 the medulla of the corpora "quadrigemina, and anteriorly by the 

 transverse section of the posterior longitudinal fasciculi (figs. 

 249, 250 A', Xp, B*,L). Its transverse section in the upper pair 

 of the corpora quadrigemina is surrounded laterally by sharply 

 defined marginal fibres that run into the tegmentum, whilst in 

 transverse sections of the inferior pair of the corpora quadri- 

 gemina its limits are not less sharply defined by a chain of trans- 

 verse sectional areas of fasciculi gradually developed in the wall 

 of the aquseduct itself, and external to the posterior longi- 

 tudinal fasciculi ; these are the descending roots of the fifth 

 nerve (fig. 250, 5'). The various forms presented at different 

 heights by the transverse section of the aquseduct with its 

 lining of columnar epithelium have been described by Gerlach. 

 If by means of a transverse line passing in the nates in front 

 of, and in the testes through, the aquseduct/the ring of central 

 grey substance surrounding it be divided into two semi-circles, 

 the cells distributed through the posterior semi-circle will be 

 found to be small, having a long diameter of only 25 JJL and a 

 breadth of 5 /a, whilst those in the anterior portion of the sec- 

 tion are larger, having a length of 30 50 JJL and a breadth of 

 15 25 ILL. The larger cells are in part more sparsely scat- 

 tered, but in part also form a compact mass ; as the latter they 

 constitute, in the neighbourhood of the nates, bounded by fibres, 

 which will be immediately described, the oculo-motor and 

 trochlear nerve nucleus, strictly so called, which lies behind 

 the posterior longitudinal fasciculi, and is lodged in an exca- 

 vation in the upper half of the testis. In a wider sense belong 

 also to the same nucleus the scattered formations which are 



