452 PHYSIOLOGY 



spending 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 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 

 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 some seat of sense impressions which will be 

 predominant, and a train of ideas may be specially visual, or auditory, or 

 tactile. It is therefore not surprising that in the immediate neighbourhood 

 of the cortical areas which receive the endings of the sensory tracts associa- 

 tion areas are developed which may be labelled according to the sense-organ 

 with which they are most nearly in relation. Thus we may speak of the 

 visual- sensory and the visual association, or psychical area, the auditory- 

 sensory and the auditory- psychical, and so on. The limits of these areas are 

 indicated in Fig. 224, p. 432. 



Conditioned reflexes. Until recently, our study of the processes of association 

 and therewith all the higher functions of the cerebral hemispheres was chiefly carried 

 out in man, and in most cases by the introspective method. Even when carried out 

 on other men, it was chiefly by using speech as an index to the introspective experi- 

 ences of those who were being investigated. During the last few years a method has 

 been introduced by Pawlow, for investigating the higher cerebral functions by an 

 objective method which is capable of very wide application. When an animal which 

 is hungry is shown food, we say that ' its mouth waters,' i.e. there is a secretion of 



