THE CENTRAL NERVOUS SYSTEM 859 



eyes and head are turned to the left that is, there is conjugate 

 deviation to the opposite side. In the higher monkeys the eye 

 movements can be elicited only from the extreme posterior apex 

 of the occipital lobe and from its calcarine region, and then not 

 easily. The movements differ from those produced by stimula- 

 tion of the area for eye movements in the frontal lobe. They 

 are not so certain, their latent period is longer, and a stronger 

 stimulus is required to evoke them. It cannot be doubted that 

 the occipital region is concerned in vision, and it is a very natural 

 suggestion that the movements are the result of visual sensations 

 in the excited occipital cortex. The right occipital lobe is con- 

 cerned with vision in the right halves of the two retinae (Figs. 342 

 and 363). Now, under normal conditions, a visual image would 

 be cast on the two right retinal halves by an object placed 

 towards the left of the field. The movements of the head and 

 eyes to the left may therefore be plausibly explained as an 

 attempt to look at, and a rotation towards, the supposed object. 



The pathological evidence is very clear that disease of the occipital 

 lobe, especially of the cuneus, a triangular area on its mesial surface, 

 causes hemianopia in man. A limited lesion may even be associated 

 with an incomplete hemianopia, and cases have been recorded in 

 which colour hemianopia (blindness of the corresponding halves of 

 the two retinae for coloured objects) co-existed with normal vision for 

 white light. The precise limits of the occipital visual area are still 

 disputed. It probably occupies, in addition to the cuneus, the 

 lingual lobule and a portion of the external aspect of the occipital 

 lobe. The question of the projection of the retina upon the visual 

 cortex i.e., the question whether each retinal area is represented 

 in a definite cortical area has given rise to much debate. The 

 representation of the fovea centralis, the area of most distinct 

 vision, has aroused especial interest. It has been asserted that a 

 circumscribed area in the region of the calcarine fissure is the centre 

 for the fovea (Henschen). But it is totally opposed to this view 

 that extensive lesions of the occipital cortex, even on both sides, do 

 not, except in rare cases, cause total blindness in the foveal region, 

 although peripheral vision is destroyed. On the other hand, in no 

 case has a purely cortical lesion been found associated with blindness 

 confined to the fovea (Monakow) . The fibres of the optic radiation 

 which are on the path from the fovea are accordingly distributed 

 diffusely to the visual cortex. Sometimes dimness of vision in the 

 whole of the opposite eye (crossed amblyopia), and not hemianopia, 

 is caused by a lesion of the occipital cortex. It seems impossible 

 to explain this and other facts without postulating the existence of 

 more than one visual centre ; and it has been supposed that in the 

 angular gyrus and the neighbouring region a higher visual centre 

 exists which is connected with the lower occipital centres for the 

 two halves of the opposite eye. Thus, the right angular gyrus 

 would be in connection with the part of the right occipital cortex 

 which has to do with vision in the nasal half of the left eye, and with 

 the part of the left occipital cortex which has to do with vision in 

 the temporal half of that eye. This higher centre, which perhaps 

 functions as a storehouse of visual memories, probably corresponds 



