Manchester Memoirs, Vol. xlii. (1898), No. \%, 25 



tiations which the simpler events begun in the cone 

 undergo as they pass through the successive links of the 

 chain. 



What can be said about the several differentiations ? 

 Some writers, influenced by the experience that in 

 monkeys and some other animals removal of the occipital 

 cortex immediately produces total blindness, the absence 

 of all visual sensations, seem from their writings to hold 

 the view that the psychical feature is introduced into the 

 whole business at the cortex, and at the cortex alone. 

 Such writers use expressions which seem to imply that 

 all the events which take place in the mechanism, in the 

 retina, optic nerve and lower bodies are purely physical 

 changes, unaccompanied by any psychical development, 

 and that it is only when these purely nervous changes 

 reach the cortex that they suddenly put on psychical 

 characters, as if up to the cortex what was going on was 

 mere invisible heat which in the cortex suddenly burst out 

 into visible flame. 



But if we turn to lower animals, such as rabbits, birds, 

 and the like, we find that careful observers have found 

 what appears to be unassailable evidence that these animals 

 are influenced in a remarkable and complex manner by 

 retinal impressions, even after the occipital cortex, and 

 indeed the whole of both cerebral hemispheres have been 

 removed. In such animals, while in this condition, some- 

 thing takes place in the cerebral structures still left in the 

 corpora quadrigemina or optic lobes, for instance, which 

 influences and guides the movements of these animals 

 in a manner strikingly analogous to the guidance of 

 movements by visual sensations. So strong is the 

 analogy that the onus probandi must be considered as 

 resting on those who maintain that the events taking 

 place in the visual mechanism in this condition are of a 

 wholly different character from those taking place when 

 the mechanism is entire. 



