78G VISUAL SENSATIONS. [Book hi. 



sivety of fibres coming from the retinas of the two eyes, and it 

 is this part, and this part only, which is concerned in vision, 

 ends in three main ways, as shewn diagrammatically in Fig. 138. 

 In the first place part of the tract ends in the lateral corpus 

 geniculatum (6r.L). In the second place, a very large number 

 of fibres passing the corpus geniculatum on its ventral and lat- 

 eral surfaces spread out into the pulvinar (PV^. In the third 

 place others, in considerable number, taking a more median direc- 

 tion, reach the anterior corpus quadrigeminum (A$) These 

 three sets end apparently in connection with 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 quad- 

 riffeminum. Further, we have reasons for thinking that a con- 

 siderable part at all events of the grey matter of these three 

 bodies, especially of the first two, 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 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 decussation in animals, and 

 indeed in man, has been ascertained. But if the eyes be removed 

 (removal of both eyes being desirable on account of the charac- 

 ters of the optic decussation), in a new-born animal, not only 

 do both the optic nerves and the greater part of both optic 

 tracts cease to be further developed and degenerate, but the 

 bodies mentioned above, the two lateral corpora geniculata, the 

 pulvinar on each side, and the two anterior corpora quadrigemina 

 do not fully develope ; certain parts of them undergo atrophy. 

 The development of these nervous structures seems therefore 

 to be largely dependent on their functional connection with the 

 eyes by means of the optic tracts and nerves. 



The same method confirms the view expressed above that 

 the median corpus geniculatum has no connection with vision. 

 When the eyes of new-born animals are extirpated neither the 

 median corpora geniculata nor the posterior corpora quadrigemina 



