THE TEGMENTTJM OF THE CRUS CEREBRI AND ITS GANGLIA. 423 



than the latter. Still, like the tractus itself, it follows the 

 inner margin of the lamina. 



In sagittal sections (fig. 247, il') the ganglion presents the 

 form of a sickle with the concavity forwards. Luys states that 

 the ganglia of opposite sides come into contact in the median 

 line, which I have not observed to occur, and thinks the roots 

 of the optic nerves arising in them decussate in the tuber cine- 

 reum, which is certainly incorrect, since the fine medullated fasci- 

 culi (and not as Foville maintains, grey roots,) immediately turn 

 outwards into the nervus opticus. The specimens that show most 

 clearly that there are non-decussating optic fasciculi may there- 

 fore be obtained from this region of their origin. Appreciating 

 fully the difficulties of settling this question from sections, I do 

 not feel myself qualified, in regard to the remaining portion of 

 the nervus opticus, either to deny the complete decussation 

 stated by Biesiadecki to exist in the chiasma, nor, from the 

 results of my own examinations, altogether to coincide with 

 his view. 



The fact of the existence of a connection between the optic 

 nerve and the central grey substance of the ventricles by no 

 means justifies its comparison with the peripheric nerves that 

 arise from the latter. In place of maintaining that there is an 

 analogy between its chiasma and the chiasma of the olfactory 

 nerves, and between the retina and the olfactory bulb, and 

 of regarding the optic fibres as a peripheric link of the projec- 

 tion system (a view directly contradicted by the character of 

 these fibres, which are fine, non-medullated, and like those of 

 the central white substance of the brain), it is more reasonable 

 to regard the root in question as an upper link of the projec- 

 tion system, and to see its central extremity in the grey concen- 

 tric substance of the retina, which is allied to the cortex of the 

 cerebrum, and its peripherie extremity in the basal optic 

 ganglionfoom which presumably tracts that are still unknown, 

 running in the central grey matter of the ventricle, extend to 

 some peripheric organ perhaps to the musculature of the eye. 



2. Immediately behind the basal optic ganglion the tuber 

 cinereum includes a commissure (fig. 245, A), the fasciculi of 

 which turn backwards within the central grey substance of the 

 ventricle, though their ultimate destination is still unknown. 



