THE CENTRAL NERVOUS SYSTEM 749 



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 occi- 

 pital lobe is concerned with vision in the right halves of the 

 two retinae (Figs. 271 and 258). Now, under normal condi- 

 tions 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 rota- 

 tion 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. 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 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. It has been stated 

 that after complete removal of the occipital lobes in young monkeys, 

 the power of vision, lost for a time, is gradually regained, the growth 

 of new nerve-cells and nerve-fibres having made good the deficiency 

 (Vitzou). 



Auditory Centre. On the outer surface of the temporo- 

 sphenoidal lobe, in the hinder portion of the first and second 

 temporal convolutions, lies an area associated with the 

 sense of hearing. Stimulation in the region of the first 

 temporal convolution may cause the animal to prick up its 

 ear on the opposite side. Destruction of this area on both 

 sides is followed by complete and irremediable loss of hear- 

 ing. If it is destroyed only on one side there is partial deaf- 

 ness of the opposite ear, and also to some extent of the ear 

 on the same side. This is gradually recovered from. If it is 

 destroyed on the left side there is also the peculiar condition 



