694 THE BRAIN. 



tion 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 cha- 

 racters of the optic decussation) in a newborn animal, not only do both the 

 optic nerves and the greater part of both optic tracts cease only to be fur- 

 ther 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 develop ; 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 quadrigemiua show any signs of atrophy, and the part of 

 the optic tract which does not degenerate is the inferior commissure con- 

 necting the two median corpora geuiculata. Obviously these parts are asso- 

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

 corpora geniculata, the pulviuar, and the anterior corpora quadrigemina, are, 

 we may repeat, alone to be regarded as the chief central parts in which the 

 optic nerve ends. 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. 



583. 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 hemisphere (Fig. 156, 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 hemisphere with 

 the optic tract is not a direct one, but an indirect one, through the three 

 bodies in question. A number of fibres proceeding from the occipital cortex 

 and reaching the thalamus through the hind limb of the internal capsule 

 formed what was called the " optic radiation." These fibres beginning (or 

 ending) in the cortex of the occipital region, end (or begin) (Fig. 156, op. rad.} 

 to a large extent in the pulvinar and in the lateral corpus geniculatum, but 

 also in the anterior corpus quadrigeminum, reaching it by the anterior 

 brachium ( 547). When even in a grown animal the occipital cortex is 

 destroyed, not only these fibres, but also parts of the pulvinar and external 

 corpus geniculatum, undergo degeneration, and there is some change in the 

 anterior corpus quadrigeminum. When the same cortex is destroyed in a 

 newborn animal the same parts atrophy ; and in such cases the optic tract 

 and nerve, which are but little affected by the operation in the adult animal, 

 are also involved in the atrophy. We may add that removal of both eyes 

 in the newborn animal is said to lead, besides the atrophy of the three bodies 

 in question, to diminished occipital lobe due to lack of white matter. We 

 may therefore conclude that in the complex act of vision two orders of cen- 

 tral apparatus are involved ; we may speak of two kinds of centres for 

 vision the primary or lower visual centres supplied by the three bodies of 

 which we are speaking, and a secondary or higher visual centre supplied by 



