1084 VISUAL SENSATIONS. [BOOK in. 



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, their 

 movements being 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 had been wholly or 

 almost wholly removed. And, though this is a matter at present 

 outside exact knowledge, and though it is perhaps possible for 

 simple afferent impulses to determine even complex movements 

 without the intervention of ' consciousness,' we are probably justi- 

 fied 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 propa- 

 gated along the fibres of the optic radiation, constitute something 

 quite different from the impulses along the optic tract or nerve. 



Judging from the analogy of the motor region we may probably 

 assume that in vision the cortical events are psychical in nature, 

 and that the function of the optic radiation is to furnish what we 

 may call crude visual sensations for further psychical elaboration. 



Nor need this view compel us to suppose that injury to, or 

 removal of the cortex must produce only psychical blindness or 

 psychical impairment of vision, though this point has probably 

 not been sufficiently held in view during the various experiments, 

 sufficient care not having been taken to determine how far the 

 blindness was purely psychical. Bearing in mind the degeneration 

 following upon lesions of the occipital cortex, and the far-reaching 

 effects of any operation on the brain, we may suppose that injury 

 to the cortex affects the lower centres as well : and some of the 

 transient impairment of vision, on which we have just dwelt, may 

 perhaps be explained as the effect of the cortical injury on the 

 lower centres. 



Although the matter is thus in many of its details at present 

 outside our exact knowledge, we may probably conclude that in 

 the complex act of complete vision, while part, especially the more 

 psychical part, is carried out in the cortex, more particularly of 

 the occipital region, part is accomplished in the lower centres, 

 the tegmental masses. As to the several functions of the three 

 masses, we know almost absolutely nothing. Electric stimulation, 

 and it is said, mechanical stimulation also, of the anterior corpora 

 quadrigemma in mammals, or the optic lobes in lower animals 

 calls forth movements of the eyes, and of various parts of the 

 body; and removal of them causes blindness and in some cases 

 loss of coordination of movements. Our knowledge on these 

 points is not very exact ; but from the above facts as well as from 

 the connections of the anterior corpora quadrigemina with the 

 parts of the brain behind we may possibly suppose that these 

 bodies are more especially concerned with the part visual impulses 



