Chap, ii.] THE BRAIN. 793 



view, that the macula of each eye unlike the rest of the retina is 

 represented in both cerebral hemispheres. But this is not very 

 satisfactory. Certain cases on the other hand have been met 

 with, which like some of the experiments on monkeys point to 

 at least some share being taken by the angular gyrus. 



Many experiments have been made on the dog, and the results 

 obtained have been interpreted by some observers as shewing that 

 in this animal not only is an area in the occipital region of the 

 cortex specially connected with vision, but even that particular 

 parts of the area correspond to particular parts of the field of 

 vision, the several parts of the retina being as it were projected 

 on to the cortex. We need not enter here upon the details of 

 this view which needs some special exposition since in the dog 

 vision is much less binocular than in man ; it will become desir- 

 able to do so should evidence be forthcoming that a similar pro- 

 jection obtains in man : but, though some clinical histories have 

 been held to indicate this, others are opposed to it. In short 

 though it is clear that the occipital cortex is concerned in vision, 

 our present knowledge does not afford a consistent view of the 

 exact way in which it is concerned. 



§ 500. We may perhaps say a few words on the question 

 what is it which actually takes place in the cortex during 

 vision ? Are we to conceive of it as if a visual impulse set 

 going along the fibres of the optic tract underwent no essen- 

 tial change until it reached the cortex, as if it there suddenly 

 developed into a ' visual sensation ' ? We can hardly suppose 

 this. Between the cortex and the optic tract, the lower visual 

 centres, the tegmental masses, intervene ; and we can hardly 

 suppose that interference with these bodies produces the same 

 effect on vision as simple section of the optic tract. We have 

 seen in a previous section that the frog and the bird certainly, 

 and according to some observers also the rabbit, are in the 

 absence of the cerebral hemispheres not totally blind, since 

 their movements appear to be guided by retinal impressions ; 

 and cases are recorded of the dog being obviously still guided 

 in some measure by retinal impressions after the occipital lobes 

 and indeed the greater part of the brain had been removed. This 

 is a matter of no little difficulty ; it is perhaps possible for sim- 

 ple afferent impulses to determine even complex movements 

 without the intervention of 'consciousness,' and we may be jus- 

 tified in speaking of the effects of light on a brainless animal as 

 being mere instances of ' mechanical ' reflex action ; still we are 

 probably justified in assuming that the simple visual impulses, 

 travelling along the fibres of the optic tract, undergo important 

 transformations in the tegmental masses, and that the changes 

 which are propagated along the fibres of the optic radiation, 

 constitute something quite different from the impulses along 

 the optic tract or nerve. We may perhaps assume that in 



