CHAP, ii.] THE BRAIN. 1079 



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 may be linked on to a chain 

 of psychical 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 

 ultimate psychical effect a whole series of events intervene : and 

 we may take it for granted that the chain may be broken or spoilt 

 at any of its links, at the later as well as at the earlier ones. 

 We may therefore consider it possible that the break or damage 

 may occur at the links by which the fully developed visual 

 sensation joins on to psychical operations. We may suppose 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 influenced 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 sensations being blurred or blunted, or to some failure 

 in the psychical appreciation of the sensations ; and in most cases 

 such an analysis 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. 



671. Since we have in this matter to trust so much to 

 analogies with our own experience, we may turn at once to the 



