510 PHYSIOLOGY 



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 consciousness at the same time as the 

 consciousness of the luminosity itself, and this location of the stimu- 

 lation 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, &c. These qualities are founded on past experiences 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 connection 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, and man. 

 Although as we ascend to man there is an absolute increase in the 

 amount 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 co-operation of a large number of 

 distant portions of the brain substance, in most of them there will be 



