932 THE CENTRAL NERVOUS SYSTEM 



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

 360 and 379). 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 cen- 

 tralis, the area of most distinct vision, has aroused especial interest. 

 It has been asserted that a circumscribed area in the region of the cal- 

 carine 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 dis- 

 tributed 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 to the structurally differentiated area 

 (visuo -psychic area of Campbell), as the lower centre corresponds to 

 his structurally differentiated visuo-sensory area (Figs. 376, 377). 



Auditory Centre. On the outer surface of the temporo-sphenoidal 

 lobe, mainly in the first temporal convolution, lies an area asso- 

 ciated with the sense of hearing. Stimulation in the region of the 

 first temporal convolution may cause the animal to prick up its ears 

 on the opposite side. Destruction of this area on both sides is 

 followed by complete and irremediable loss of hearing. If it is 

 destroyed only on one side, there is partial deafness of the opposite 



