FUNCTIONS OF THE CEREBRAL HEMISPHERES 453 



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 psychic area, the auditory- 

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

 indicated in Fig. 224, p. 431. / 



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 put 

 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 a hungry 

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

 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. 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 recognize the difference between middle C and middle 

 ( 1 , 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 C# 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 

 ;nul 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 



