FUNCTIONS OP THE CEREBRAL HEMISPHERIC 453 



saliva; and if the animal be provided with a salivary fistula the extent of the emotion 

 of appetite may be gauged in cc. of saliva flowing from the fistula. It is found in 

 such an animal that a flow of saliva may be excited, not only by the sight or adminis- 

 tration of food, but also by any other event which has become associated as the result 

 of experience with the taking of food. Thus we may use this method in order to deter- 

 mine the sensitiveness of the animal's perception of pure tones. Thus if we wish to 

 know whether the animal can recognise the difference between middle C and middle 

 Cjf, as produced by tuning-forks, we can for some days or weeks allow him to hear both 

 these sounds frequently but always follow up one of them, say C, by giving him a piece 

 of meat. After a time it is found that not only can he distinguish between the two 

 sounds, but that he has a memory of the absolute pitch, so that whenever the note 

 middle C is sounded or any note differing from it by not more than 8 d.v. per second, 

 there is a flow of saliva from the fistula, whereas the note Cjf is heard without producing 

 any response. Such an acquired reaction is designated by Pawlow, a ' conditioned 

 reflex ' and the method has been applied by him to study the association between the 

 most widely different impressions and the condition which we can regard as appetite 

 and which is associated psychically with the idea of food. 



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 experience the more fitted will these acts be to his 

 maintenance amid the ever-changing stresses of the environment. In this 

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

 profit also from accumulated racial experience. The increased complexity 

 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 complexity 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 impressions. 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 hemi- 

 spheres. 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 

 everyday 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 



