1080 VISUAL SENSATIONS. [BOOK in. 



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 angular 

 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 movements so 

 produced being such as are ordinarily connected with vision. It 

 will not be profitable to enter here into the details concerning 

 the exact topography of the excitable parts or of the special 

 characters of the movements so called forth. But it is important 

 to note that these movements are unlike the movements excited 

 by stimulation of the appropriate motor area in as much as 

 their occurrence is far less certain, they need a stronger stimulus 

 to bring them out, when evoked they are feeble, being easily 

 antagonized by appropriate stimulation of the motor area, and 

 they have a much longer latent period. They are not due to any 

 indirect stimulation of the motor area, through "association" 

 fibres connecting the spot stimulated with the motor area or 

 otherwise, since they persist after removal of the motor area. 

 Movements of this kind may also be witnessed in the dog. They 

 are obviously the result of impulses transmitted in some direct 

 manner from the cortex to some parts below, and may be taken 

 as an indication that the parts of the cortex in question are in 

 some way connected with vision. The exact manner however 

 in which they are brought about is at present obscure. The 

 explanation of their genesis which is frequently offered, namely, 

 that the stimulation so affects the cortical grey matter as to give 

 rise to visual sensations, and that the movements express these 

 sensations, does not seem satisfactory. For, if it be possible that the 

 gross changes which the electric current sets going in the cortical 

 grey matter can reproduce the psychical events which take place 

 in that grey matter in the normal action of the brain, we should 

 expect stimulation of any and every part of the cortex to call 

 forth some movement or other, since it cannot be doubted that 

 every part of the cortex is in some way or other engaged in 

 psychical operations, and that every psychical phase tends to 

 express itself in movement. Whereas outside the motor region, 

 with the exceptions we are now discussing, the cortex is, as we 

 have seen " inexcitable," and even within the motor region itself 

 the excitable substance is scattered, with increasing segregation 

 as we advance along the animal scale, among inexcitable substance. 

 When we speak of the region, or substance as inexcitable, we do 

 not mean that the electric current produces no effect ; we only 

 mean that the effect is not manifested by movement ; the real 

 difference between the excitable motor region and the inexcitable 

 rest of the cortex is probably that in the several motor areas the 



