THE TEGMENTUM OF THE CRUS CEREBRI AND ITS GANGLIA. 439 



The internal corpora geniculata in Mammals keep pace with the 

 development of the two pairs of corpora quadrigemina with which 

 they are connected. The internal corpus geniculatum develops in 

 harmony with the size of the tegmentum, not alone on account of its 

 relation to the corpora quadrigemina, but because it attaches itself, 

 especially by its fasciculi associated with the arm of the inferior cor- 

 pus quadrigeminum, to the ganglia of origin of the tegmentum, which 

 relation, however, requires further elucidation. 



The most internal fasciculi of the tractus opticus encircle 

 partly the crusta of the crus cerebri, and partly interweave with 

 its most external fasciculi (fig. 248). I am unacquainted with 

 their ultimate distribution. I can only with certainty refer 

 to the illusory appearance which, on a superficial examina- 

 tion, makes it appear that these tracts connect themselves, as 

 Burdach has stated, with the closely adjoining substantia 

 nigra. They really always cease abruptly near or behind it. 



The region of the corpora quadrigemina is situated internally 

 to the so-called origin of the optic nerve, forming a coalesced 

 mass of three stages, superimposed upon one another around 

 an excentric cavity situated at a high level, which is the aquse- 

 ductus Sylvii (fig. 249, A). The corpora quadrigemina extend 

 over the aquseduct, their central portion constituting its roof. 

 Immediately anterior to the aquseduct lie the tegmenta of 

 the crura cerebri, fusing in the middle line in a raphe (R), the 

 under surface of which forms the lamina perforata posterior 

 (La). Laterally on each side of the latter extends the trans- 

 verse section of the most inferior stage of the quadrigeminal 

 region, viz., the crusta. The corpora quadrigemina are com- 

 posed of a superior and inferior pair of ganglia, that together, 

 and with their brachia (which are the fasciculi of the corona 

 radiata entering them from the cortex), occupy a transverse 

 position in the cerebrum. This, however, is not strictly cor- 

 rect in regard to the inferior brachia, because these are pressed 

 forwards as axes of revolution by the cushion of the optic 

 thalamus (fig.. 243, Bi). The ganglia of the corpora quadri- 

 gemina lie imbedded between a superficial (fig. 249, BsXp) 

 and a deep medullary layer (figs. 249, XpTe; 241, m'). 

 The superficial represents a kind of zonular layer, resembling 



