24 Foster, Physical Basis of Psychical Events. 



proceeds direct as one of the so-called 'optic radiations' to 

 the cortex of the occipital lobe, or, what appears to be the 

 more ordinary, if not the only course, links itself to another 

 unit, another nerve cell, also lying within the corpus geni- 

 culatum, it being the axon of this second cell which is 

 prolonged as a fibre of the optic radiations. Possibly 

 even still another unit may be intercalated in the chain, 

 within the corpus geniculatum. 



In any case, a differentiation, probably a large and 

 yet subtle one, takes place in this corpus geniculatum, and 

 the nervous impulses or nervous changes which are 

 produced along the optic radiations are of a very different 

 nature from those which travelled along the fibres of the 

 optic nerve and tract. 



The fibres of the optic radiations having reached the 

 cortex in the occipital region of the brain (we may leave 

 on one side for the present the topographical limitations 

 of the region), link themselves to cells in that cortex, and, 

 so far as mere visual sensations are concerned, the final 

 differentiation takes place here. For, as I said at the 

 beginning of this lecture, the integrity of the occipital 

 cortex, and this part alone of the cortex, is essential for 

 the development of visual sensations. Remove, destroy, 

 or injure this part of the cortex, and this part only of the 

 cortex, indeed, this part only of the whole nervous system, 

 and blindness, or imperfect vision, is the result. Remove, 

 destroy, or injure any other part of the nervous system, 

 save and except this occipital cortex, and the nervous 

 visual mechanism of which it forms the head, the develop- 

 ment of psychical visual sensations, if affected at all, is 

 affected in an indirect manner only. 



The visual mechanism is a chain of nervous units 

 beginning with the cone (or rod) and ending in the 

 occipital cortex. The psychical event which we call a 

 visual sensation is the culmination of a series of difteren- 



