CHAP. n. J THE BRAIN. 1083 



whole of the crossed eye. (It must be remembered that the 

 angular gyrus of man corresponds to a part only of the whole 

 angular gyrus of the monkey. Cf. Fig. 125 with Fig. 129.) Some 

 authors have, in accordance with this, put forward the theory 

 that the occipital lobe serves as a cortical centre for the optic 

 tract of its own side only, and so for one half of each retina, while 

 in front of this on the angular gyrus is a centre in which both 

 optic tracts are represented. But the clinical histories bearing 

 on this point cannot be regarded as wholly satisfactory ; and with 

 reference to the experimental results we may once more insist, 

 and the warning applies perhaps with particular force to these 

 experiments on vision, on the danger of confounding those imme- 

 diate effects of operative interference, which are of the nature of 

 'shock' in the wide sense of that word, with those pure 'deficiency' 

 phenomena which are alone the outcome of the loss of the part 

 removed. It is difficult to resist the conclusion that much of the 

 transitory blindness which is observed in these experiments 

 belongs to the former category, that the effect is transient because 

 it is of the nature of shock and not because the loss of faculty is 



t/ 



supplied by some other cortical area being subsequently substituted 

 for the one removed. In the dog, injury to the frontal region of 

 the cortex unaccompanied by any secondary mischief in the 

 occipital region, has led to impaired vision ; and this was probably 

 an instance of ' shock,' for we have no other reason to connect the 

 frontal region of the cortex with vision. We must be very 

 careful in drawing the conclusion that, because an operation 

 produces transient blindness, the part operated on has a direct 

 share in vision ; and we may well hesitate to accept the view that 

 the whole retina is represented in the crossed hemisphere. 



In conclusion we may say that, when all the many results 

 which have been arrived at by experiment or by clinical obser- 

 vation are duly weighed, it will be felt that while the evidence 

 for the occipital lobe, especially the cimeus, being concerned in the 

 matter is convincing, we cannot in the present state of our 

 knowledge, dogmatically exclude the angular gyrus, and that 

 hence the only clear and consistent statement which can be made 

 with any confidence is the broad and simple one that the hind 

 region of the cortex is in some way intimately concerned in 

 vision. 



673. Such an attitude becomes all the more necessary 

 when we ask ourselves 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 essential 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 



