788 VISUAL SENSATIONS. [BOOK m. 



place in the optic tract, after removal of the retina ; it seems 

 rather a degeneration from pure want of functional activity, as 

 if the cells in the corpus geniculatum are unable to act as they 

 ought to do on the arrival of visual impulses along the optic 

 fibres, if their ties with the occipital cortex are broken. 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 sup- 

 plied by the three bodies of which we are speaking, and a sec- 

 ondary 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 interpre- 

 tation 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 experi- 

 ment is conducted on an animal. We can estimate the motor 

 effect quantitatively, we can measure and record the contrac- 

 tion of the muscles ; 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 experi- 

 ments instituted by disease ; but the many advantages thus 

 secured are often more than counterbalanced by the diffuse 

 characters, or the complex concomitants of the lesion. In 

 dealing with sensory effects we must expect and be content 

 for the present with conclusions less definite and more uncer- 

 tain even than those gained by the study of motor effects. 



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

 able 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, regarded as a whole, with its central as well 

 as its peripheral parts, by referring to certain defects of vision 

 due to changes in the eye itself. The eye may fall into such 

 a condition, that the mind can only appreciate, and that to a 

 varying degree, the difference between light and darkness ; the 

 mind is aware that the retina (or it may be part of the retina) 

 is being stimulated to a less or greater degree, but cannot per- 

 ceive that one part of the retina is being stimulated in a differ- 

 ent way from another part ; a sensation of light is excited, but 

 not a set of visual sensations corresponding to the sets of pen- 



