790 VISUAL SENSATIONS. [BOOK in. 



agine 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 sen- 

 sation, 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 

 4 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 sensa- 

 tion 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. 



498. Since we have in this matter to trust so much to an- 

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

 monkey, as being more instructive 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 caused by 

 stimulation of the motor region itself. In the monkey stimulation 

 of parts of the occipital region, the occipital lobe and the angu- 

 lar gyrus for instance, may give rise to movements of the eyes, 

 of the eyelids, and of the head, that is of the neck, all the 



