VISUAL AND OTHER SENSATIONS. 697 



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, inasmuch as their occurrence is far less cer- 

 tain, 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 con- 

 necting 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 trans- 

 mitted 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 gray 

 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 gray matter 

 can reproduce the psychical events which take place in that gray 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 in- 

 creasing 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 current, playing upon the beginnings of 

 the pyramidal fibres, is able to inaugurate simple motor impulses or some- 

 thing like them, whereas elsewhere the molecular changes induced by the 

 current are too confused to reach their normal expression. There can be no 

 doubt, of course, that molecular changes in this or that part of the brain, 

 set going by processes other than actual visual impulses along the optic 

 nerves, may give rise to visual sensations ; and, as we shall see in dealing 

 with the senses, the subject of such "subjective" sensations is unable to 

 distinguish them from sensations of "objective" origin ; but it is at least 

 unlikely that the coarse disturbances started by a tetanizing current should 

 take such a definite form. Moreover, the view in question is disproved by 

 the experimental result that the same movements are brought about when 

 the cortex is pared away and the electrodes are applied to the subjacent 

 white matter. This result suggests the existence of efferent tracts or bundles 

 of a special kind, differing from those of the pyramidal kind, though like 

 them making connections with the ocular and other muscles; we have, 

 however, as yet no other evidence of such tracts existing. 



585. The results of removal of the cortex support the same general 



