CHAP. IL] THE BRAIN. 1075 



the optic nerve, the fibres just mentioned and the inferior com- 

 missure 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. 108 C, 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, 

 towards the dorsal aspect, and as we have already ( 630) said 

 to become connected on the farther side of the crus with the two 

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

 the median corpus geniculatum has no connection with that part 

 of the tract which is derived from the optic nerve, and is not 

 concerned in vision, but is connected 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 

 concerned in vision. 



669. This ends in three main ways, as shewn diagrammati- 

 cally in Fig. 133. In the first place part of the tract ends in the 

 lateral corpus geniculatum (GL), formed of alternating layers of 

 white and grey matter, the grey 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 (PF). 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 grey matter, the lateral corpus geniculatum, the pulvinar, and 

 the anterior corpus quadrigeminum. Further, we have reasons 

 for thinking that a considerable part at all events of the grey 

 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 degenerates ; but the division, and the loss of 

 the peripheral degenerating portion, has no obvious effect on the 

 trophic centre ; when a spinal nerve, for instance, 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 

 adul^ animal, a section of an optic nerve or removal of the eye 



