6% THE BRAIN. 



fields of both eyes will be blotted out. The same condition will be brought 

 about by failure in the optic tract at its central ending, provided of coursi? 

 the mischief be confined to the ending of the one tract. Such a half-blind- 

 ness or half-vision is spoken of as hemianopsia or hemianopia or kemiopia ; 

 the words left and right are generally used in reference to the visual field ; 

 thus, left hemiauopsia is the blotting out of both left visual fields through 

 failure of the right optic tract. 



If, instead of the whole optic nerve being divided, certain bundles only 

 were cut across, partial blindness would be the result, a portion of the visual 

 field would be blotted out, and mischief limited to a few bundles of one optic 

 tract would lead to corresponding blots in the corresponding halves of the 

 visual fields of both eyes. 



Further, an affection of half the retina or of a limited area in the retina 

 might occur of such a character as to lead, not to complete, but to partial 

 blindness, to a hemi-amblyopia or to a partial amblyopia. The part of the 

 retina so affected might be central, or peripheral, or a quadrant, or any patch 

 of any size, form, and relative position. And we may further imagine it at 

 least possible that mischief in the brain might be so limited as to produce 

 any of the above partial effects, though the retina, optic nerve, and optic 

 tracts all remained intact. 



The above visual imperfections we have illustrated by changes in the 

 peripheral apparatus, but there is a kind of imperfection which we may still 

 call a visual imperfection, though it is of purely central origin. In a normal 

 state of things a visual sensation excited in the brain is or maybe linked on 

 to a chain of physical events ; we often, then, speak of it as a visual idea. 

 When we see a dog the visual sensation, or rather the group of sensations 

 making up the visual perception of the dog, does not exist by itself, apart 

 from all the other events of the brain ; it joins and affects them, and among 

 the events which it so affects may be and often are psychical events; the 

 visual perception "enters into our thoughts" and modifies them. Between 

 the visual impulse, as it travels along the optic nerve or tract, and its ulti- 

 mate psychical effect, a whole series of events intervene ; and we may take 

 it for granted that the chain may be broken or spoiled at any of its links, alt 

 the later as well as at the earlier ones. We may therefore consider it possi- 

 ble that the break or damage may occur at the links by which the fully 

 developed visual sensation joins on to psychical operations. We may sup- 

 pose that an object is seen and yet does not affect the mind at all, or affects 

 it in an abnormal way. 



These foregoing considerations emphasize the difficulty and uncertainty of 

 interpreting the visual condition of an animal which has been experimented 

 upon. When, for instance, after an operation, an animal ceases to be influ- 

 enced in its previous normal manner by the visual effects of external objects, 

 a most careful psychical analysis is often necessary to enable us to judge 

 whether the newly introduced disregard of this or that object is due to the 

 mere visual sensation being blurred or blunted, or to some failure in the 

 psychical appreciation of the sensations ; and in most cases such an analy.sis 

 is beyond our reach. The greatest caution is needful in drawing conclusions 

 from experiments of this kind, especially from such as appear to have been 

 hastily carried out or hastily observed ; and we must be content here to 

 dwell on some of the broader features only of the subject. 



584. Since we have in this matter to trust so much to analogies with our 

 own experience, we may turn at once to the monkey as being more instruc- 

 tive than any of the lower animals. We have already said that electrical 

 excitation of the occipital cortex behind the motor region may produce 

 movements, but that these movements are in character different from those 



