FUNCTIONS OF THE CEREBRAL HEMISPHERES 511 



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 association 

 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. 222, p. 489. 



THE FUNCTION OF SPEECH 



The acts of a conscious individual, i.e. one possessing cerebral 

 hemispheres, are determined by his experience. The wider the 

 range of past sense impressions which can be called up and taken 

 into the chain of processes involved in any reaction the more, that 

 is to say, the individual weighs his acts in the light of past expe- 

 rience the more fitted will these acts be to his maintenance amid 

 the ever-changing stresses of ihe environment. In this guiding of 

 behaviour by experience man, as well as the higher mammals, may 

 profit also from accumulated racial experience. The increased com- 

 plexity of the neural processes concerned in every reaction of the body, 

 and the excessive lost time brought about by the intercalation of one 

 neuron after another in the chain of the excitatory process, would 

 finally counteract the advantages derived from the growth in com- 

 plexity of the brain, were it not that, as a result of education or training, 

 short cuts are laid down, by means of which reactions adapted to the 

 maintenance of the individual can be carried out immediately, without 

 thought and without correlated calling up of numberless sense impres- 

 sions. Education in fact consists in laying down these ' short cuts ' 

 which, as habits, are the basis of the behaviour of the animal. The 

 more complex the central mechanism and the wider the range of 

 environmental change to which adaptation is necessary, the longer 

 must be the time involved in this process of road-making within the 

 cerebral hemispheres. The behaviour of man is therefore a product 

 of many years' training, during which time he is in a state of subjection 

 and unfit, from the absence of habit, to maintain himself as a unit 

 in the human community. The neural short cuts of habit are, 

 however, only of advantage to the individual in dealing with those 

 events which are of every-day occurrence. Every novel circumstance 

 must involve a revival of past sense impressions and a calling up 

 of activities of the most diverse portions of the brain in order 

 to arrive at the safest or most advantageous mode of action 

 adapted to the circumstances. Here again the complexity of the 

 process would, by the very delay involved, put a stop to a further 



