VISUAL AND OTHER SENSATIONS. 695 



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 preliminary obser- 

 vations may prove of use. 



In the first place, as we have previously urged, the interpretation 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 concomitants of the lesion. In dealing with sensory effects we 

 must expect and be content for the present with conclusions less definite 

 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 move- 

 ments 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 perceive that one part of the retina is being stimulated 

 in a different way from another part ; a sensation of light is excited, but not 

 a set of visual sensations corresponding to the sets of pencils of luminous 

 rays, which, reflected or emanating from external objects in a definite order, 

 are falling upon the eye. The eye again may fall into another condition, in 

 which such sets of visual sensations are excited, but on account of dioptric 

 imperfections or for other reasons the several sensations are not adequately 

 distinct ; the mind is aware through the eye of the existence of " things," 

 but cannot adequately recognize the character of those things ; the visual 

 images are blurred and indistinct. And a large number of gradations are 

 possible between the extreme condition in which only those objects which 

 present the strongest contrast with their surroundings are visible to a condi- 

 tion which only just falls short of normal vision. Imperfections of this kind, 

 of varying degree, may result from failure, not in the peripheral apparatus, 

 not in the retina or optic nerve or other parts of the eye, but in the central 

 apparatus ; the retinal image may be sharp, the retina and the optic fibres 

 may be duly responsive, but from something wrong in some part or other of 

 the brain the visual sensations excited by the visual impulses may fail in 

 distinctness, and that in varying degree ; imperfections of vision, whether of 

 central or peripheral origin, in w r hich visual sensations fail in distinctness, 

 are generally spoken of under the not wholly unexceptionable name of 

 amblyopia. 



If one optic nerve be divided, total blindness of one eye will result ; but 

 if one optic tract be divided, it follows from what has been said above that 

 half-blindness in the corresponding halves of both eyes will result. If, for 

 instance, the right optic tract (Fig. 154, Op. T.) be divided, the left visual 



