CHAP, ii.] THE BRAIN. 1077 



what was called the ' optic radiation.' These fibres beginning 

 (or ending) in the cortex of the occipital region, end (or begin), 

 (Fig. 133, 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 ( 634). 

 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 new-born 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 

 new-born animal is said to lead, besides the atrophy of the three 

 bodies in question, to a diminished occipital lobe due to lack of 

 white matter. We may therefore conclude that in the complex 

 act of vision two orders of central 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 the 

 cortex in the occipital region of the cerebrum. And experimental 

 results accord with this view. 



Before we proceed to discuss those results, one or two pre- 

 liminary observations may prove of use. 



In the first place, as we have previously urged, the interpreta- 

 tion of the results of an experiment in which we have to judge 

 of sensory effects, are far more uncertain than when we have to 

 judge of motor effects, that is of course when the experiment is 

 conducted on an animal. We can estimate the motor effect 

 quantitatively, we can measure and record the contraction of the 

 muscle ; but in estimating a sensory effect we have to depend on 

 signs, our interpretation of which is based on analogies which may 

 or may not be misleading. We are on safer ground when we can 

 appeal to man himself in the experiments instituted by disease ; 

 but the many advantages thus secured are often more than 

 counterbalanced by the diffuse characters, or the complex con- 

 comitants of the lesion. In dealing with sensory effects we must 

 expect and be content for the present with conclusions less defi- 

 nite and more uncertain even than those gained by the study of 

 motor effects. 



In the second place, in dealing with vision, it will be desirable 

 to know the meaning which we are attaching to the words which 

 we employ. By blindness, that is ' complete ' or ' total ' blindness, 

 we mean that the movements and other actions of the body are in 

 no way at all influenced by the amount of light falling on the 

 retina. Of partial or incomplete or imperfect vision, using the 

 word vision in its widest sense, there are many varieties ; and we 

 may illustrate some of the defects of the visual machinery, re- 



