VISUAL AND OTHER SENSATIONS. 693 



attached to and forms part of the floor or ventral wall of that ventricle. 

 In a view of the basal or ventral surface of the brain the diverging optic 

 tracts are seen to separate the anterior perforated space and lamina cinerea 

 in front from the posterior perforated space, tuber cinereum with the in- 

 fundibulum, and corpora albicantia behind, all these being parts of the 

 floor of the third ventricle. From the gray matter in this floor, fibres 

 forming what is sometimes spoken of as Meynert's commissure, belonging 

 neither to the optic nerves nor to the inferior commissure, join the optic 

 tracts, eventually leaving them to pass to the pes. Hence the whole of the 

 optic tract is by no means derived from the optic nerve ; the fibres just 

 mentioned and the inferior commissure form parts of the optic tract not 

 connected with the retina. 



Each optic tract crosses obliquely, being in crossing firmly attached to 

 the ventral surface of the crus cerebri of the same side (Fig. 131, (?), and 

 is soon lost to view, being covered up by the temporo-sphenoidal lobe of 

 the hemisphere. When this is removed the tract is seen to sweep dorsally 

 round the crus toward the dorsal aspect, and as we have already ( 543) 

 said, to become connected on the further side of the crus with the two cor- 

 pora geniculata, lateral and median. We may say at once that the median 

 corpus geuiculatum has no connection with that part of the tract which is 

 derived from the optic nerve, and is not concerned in vision, but is con- 

 nected with that part of the tract, sometimes called the median part, which 

 goes to form the inferior commissure. We may confine our attention to 

 that part of the tract which consists exclusively of fibres coming from the 

 retinas of the two eyes, for it is this part, and this part only, which is con- 

 cerned in vision. 



582. This ends in three main ways, as shown diagrammatically in 

 Fig. 156 In the first place, part of the tract ends in the lateral corpus 

 geniculatum (GL.\ formed of alternating layers of white and gray matter, 

 the gray matter containing in some parts large nerve-cells, and in others 

 small nerve-cells. In these cells of one kind or another, many of the fibres 

 appear to end. In the second place, a very large number of fibres passing 

 the corpus geniculatum on its ventral and lateral surfaces spread out into 

 the pulvinar (PV.). In the third place, others in considerable number, 

 taking a more median direction, reach the anterior corpus quadrigeminum 

 (AQ.). These two sets also, like the first, end apparently in the nerve-cells 

 of the respective bodies. Thus, the really optic fibres of the optic tract 

 end in one of three collections of gray matter, the lateral corpus genicula- 

 tum, the pulvinar, or the anterior corpus quadrigeminum. Further, we 

 have reasons for thinking that a considerable part at all events of the gray 

 matter of these three bodies is associated with and, in a certain sense, 

 dependent on the fibres of the optic nerves ; the reasons are as follows : We 

 know that when a nerve-fibre is cut away from its trophic centre it degen- 

 erates ; but the division and the loss of the peripheral degenerating portion 

 has no obvious effect on the trophic centre ; when a spinal nerve, for in- 

 stance, is divided below the spinal ganglion, though the nerve below the 

 section degenerates, the ganglion and the piece of nerve in connection with 

 it remain very much as before ; we have it, however, in our power to bring 

 about changes of a deeper and wider character, a cessation of growth 

 amounting to atrophy, by operative interference with nervous structures 

 before they are fully developed. Thus, in an adult animal, a section of an 

 optic nerve or removal of the eye leads to degeneration in the optic nerve 

 and optic tract ; the optic fibres have their trophic centre in certain cells of 

 the retina, of which we shall speak in treating of vision, and cut away from 

 that centre they degenerate ; by this means the nature of the optic decussa- 



