1076 VISUAL SENSATIONS. [BOOK in. 



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 characters 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 develop- 

 ment 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 shew 

 any sign of atrophy, and the part of the optic tract which does 

 not degenerate is the inferior commissure connecting the two 

 median corpora geniculata. Obviously these parts are associated 

 with functions of the brain other than those of sight. The lateral 

 corpora geniculata, the pulvinar and the anterior corpora quadri- 

 gemina, are, we may repeat, alone to be regarded as the chief 

 central parts in which the optic nerves end. We may also repeat 

 that owing to the peculiarity of the optic decussation each optic 

 nerve thus finds its endings in both sides of the brain. 



While the optic chiasma is, as we have seen, helping to form 

 the floor of the third ventricle, it gives off fibres to the posterior 

 perforated spot. Some of these have been supposed to pass 

 directly in the wall of the ventricle to the nucleus of the third 

 (oculo-motor) nerve, and to serve as a channel for afferent impulses, 

 causing constriction of the pupil ; but to this we shall return in 

 dealing hereafter with the movements of the pupils. 



670. Though the above three bodies are undoubtedly the 

 chief endings of the optic nerve, three primary visual centres, if we 

 may so call them, it is also believed that some fibres of the optic 

 tract, making connections with neither of these three bodies, pass 

 by the crus cerebri straight to certain parts of the cerebral hemi- 

 sphere (Fig. 133, d) ; but this fourth ending is by no means so 

 clearly established as are the other three. 



And undoubtedly the main connection of the cerebral hemi- 

 sphere with the optic tract is not a direct one, but an indirect 

 one, through the three bodies in question. We said, 633, that 

 fibres proceeding from the occipital cortex and reaching the 

 thalamus through the hind limb of the internal capsule formed 



