Chap, ii.] THE BRAIN. 791 



movements so produced being such as are ordinarily connected 

 with vision. It will not be profitable to enter here into the de- 

 tails 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 move- 

 ments 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 ' associa- 

 tion ' 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 not clear. 



§ 499. The results of removal of the cortex in the monkey 

 also shew clearly that the hind region of the cortex is specially 

 connected with vision, though there has been and indeed still is 

 much discordance among the various observers both as to the 

 particular results and especially as to their interpretation. 



Many observers have found that removal of the occipital lobe 

 on one side, the region marked 'vision ' in Figs. 123, 124, caused 

 hemiopia, the effect on the visual fields being a crossed one ; 

 when the right lobe for instance was removed there was blind- 

 ness in the left visual fields, that is in the right halves of the 

 retinas of both eyes ; in other words the visual impulses passing 

 along the right optic tract failed to produce their usual effect, so 

 that the animal disregarded objects on its left-hand side. We 

 may remark that the decussation of the optic nerves in the monkey 

 is very similar to that in man. When both occipital lobes were 

 removed, total blindness resulted. Such a result seems perhaps 

 at first sight, not only in accordance with the anatomical connec- 

 tions spoken of above, but also simple and satisfactory. Visual 

 impulses originating in the corresponding halves of the two re- 

 tinas travel along the appropriate optic tract to the primary vis- 

 ual centres of one side, and thence pass by the optic radiation to 

 the occipital lobe of the same side (for there is no decussation 

 along the whole line save at the optic chiasma), where they give 

 rise to visual perceptions. But even a little consideration will 

 raise difficulties. The eyes are preeminently organs of bilateral 

 use ; and if, when we are looking with the two eyes at an object 

 whose image occupies parts of the two halves of each retina, 

 the impulses generated in the corresponding halves of the two 

 retinas, affect exclusively the one side of the brain only, there 



