VISUAL AND OTHER SENSATIONS. 699 



optic tract at the chiasma. Clinical histories teach the same lessons as these 

 experiments on animals ; lesions limited to the occipital lobe have for a 

 symptom hemiopia ; and this is said to be especially the result of mischief 

 limited to the apex of the occipital lobe, that is, to the cuneus. But experi- 

 ments on monkeys have been made in which destruction of one angular 

 gyrus has produced, not hemiopia, but crossed blindness or crossed ambly- 

 opia, that is to say, has affected the whole of the retina of one eye, and that 

 the crossed eye, the eye of the same side not being, or being supposed not to 

 be, at all affected ; similar results have also been stated to follow upon 

 removal of one occipital lobe. And a few clinical cases have been recorded 

 in which disease, especially of the angular gyrus, seemed to affect the vision 

 of the 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. 148 with Fig. 152.) 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 experi- 

 mental results we may once more insist, and the warning applies perhaps 

 with particular force to these experiments on vision, on the danger of con- 

 founding those immediate effects of operative interference, which are of the 

 nature of "shock" in the wide sense of that word, with those pure "de- 

 ficiency " 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 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 hemi- 

 sphere. 



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

 been arrived at by experiment or by clinical observation are duly weighed, 

 it will be felt that while the evidence for the occipital lobe, especially the 

 cuneus, being concerned in the matter is convincing, we cannot in the pres- 

 ent 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. 



586. 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 sensa- 

 tion?" 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 



