700 THE BRAIN. 



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 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. 



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 func- 

 tion of the optic radiation is to furnish what we may call crude visual sensa- 

 tions 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 ex- 

 plained 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 accom- 

 plished in the lower centres, the tegmental masses. As to the several func- 

 tions of the three masses, we know almost absolutely nothing. Electric 

 stimulation, and, it is said, mechanical stimulation also, of the anterior cor- 

 pora quadrigemina 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 move- 

 ments. Our knowledge on these points is not very exact; but from the 

 above facts as well as from the connections of the anterior corpora quad- 

 rigemina with the parts of the brain behind we may suppose that these 

 bodies are more especially concerned with the part visual impulses play in 

 determining the coordination of movements. We must remember, however, 

 that all three masses are connected with the cortex, and probably all three 

 play a part in vision even of the highest psychical kind. 



Sensations of Smell. 



587. In many animals in whom the sense of smell is acute, a portion 

 of the cortex, known as the " pyriforrn lobe " or " hippocampal lobule," and 

 which is anatomically continuous with the front end of the hippocampal 

 gyrus (the part to which the name uncinate gyrus is often restricted), ac- 

 quires relatively large dimensions. This and the anatomical relations just 

 mentioned would lead us to suppose that a part of the cortex which is con- 

 tinuous with the front end of the hippocampal gyrus is in some way con- 

 nected with smell. The argument from comparative anatomy, however, is 

 one which must be used with caution ; since, besides the great difficulty of 



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