452 PHYSIOLOGY 



imagined that a simple sensation would ensue as the result of local stimula- 

 tion, say of the visual centre on one side. Our knowledge of the properties 

 of the systems of neurons composing the central nervous system would teach 

 us that no excitatory process could remain confined to one portion of the 

 brain, but must diverge in many directions. It is true that excision of the 

 occipital lobes on one side causes blindness to objects in the opposite half, 

 of the field of vision. This is however merely a result of localisation of the 

 end of visual fibres, and the same effect can be brought about by division of 

 the right optic tract, or damage to the right half of both retinae. 



On the other hand, an appeal to our own experience shows that no 

 sensation can be regarded as simple, i.e. as following merely stimulation of 

 visual fibres or visual centres. Thus the sensation of a luminous point 

 has connected with it not only luminosity but also colour and intensity. 

 Moreover the apparent position of the luminous point comes into conscious- 

 ness at the same time as the consciousness of the luminosity itself, and this 

 location of the stimulation involves muscular impressions from the eyeballs 

 and an association between certain points on the retina and certain corre- 

 sponding muscular movements of the eye muscles, of the head and neck, and 

 even of the body and arm movements which would be necessary to bring 

 the image of the spot on to the fovea centralis and to approach the whole 

 body to the site of the stimulating object. 



As the visual sensation becomes more complex, the associated sensations 

 and experiences which it evokes become more numerous. Thus the image 

 of a chair falling on the retina excites a long train of nervous processes. At 

 once we become aware not only of a visual impulse but of an object which 

 possesses colour, extension, or size in three dimensions, solidity, hardness, 

 distance or position in space, etc. These qualities are founded on past ex- 

 periences visual, muscular, and tactile. Moreover we are at once aware 

 of the uses of the chair, and of its name both spoken and written, a mental 

 activity connoting revival of higher visual and auditory sensations. The 

 higher in the scale of intelligence, the greater is the development of the 

 cerebral hemispheres and the more extensive are the associations arising in 

 connexion with any single sense impression. 



Besides the portions of the brain which send out the motor paths and 

 which receive the endings of the sensory paths, there may be whole regions 

 taken up by the interconnecting neurons which subserve the association of 

 the activities of all parts of the cerebral hemispheres, and the higher the 

 animal is in the scale of intelligence the larger must be the relative amount 

 of brain substance set apart for these functions of association. This is very 

 evident if we compare the brain of three animals, such as the dog, the ape 

 a n< 1 man. Although as we ascend to man there is an absolute increase in the 

 a UK >unt of brain substance involved say in the motor areas or in the sensory 

 areas the increase is very small as compared with that in those portions 

 of the brain which give no response on stimulation, and in man these 

 ' silent ' parts of the brain form the greater part of the cerebral cortex. 

 Although every phase of cerebral activity, every conscious event, involves 



