FUNCTIONS OF THE CEREBRAL HEMISPHERES 517 



This conclusion is in accordance with our experience on the latent 

 period of muscle, the greater part of which is due to changes occurring 

 in the nerve-endings, i.e. in the synapses between motor nerve and 

 muscle. The greater the number of synapses involved in any given 

 reaction, i.e. the greater the complexity of the reaction, the longer ^will 

 be the period which elapses between the moment of application of the 

 stimulus and the moment at which the response takes place. Espe- 

 cially is this the case when the complex meshwork of neurons of the 

 cerebral hemispheres is involved, or when the occurrence of the reaction 



FIG. 234. Arrangement of apparatus for determination of reaction time. 



(ALCOCK and ELLISON.) 



E, coil ; E, exciting electrodes ; F, tuning-fork ; A, B, keys ; s, T, electro- 

 magnetic signals ; D, drum. 



is associated with the conscious processes of sensation and volition. 

 In the latter case the determination of the reaction time has the added 

 interest that it gives information as to the time-relations of the psychical 

 processes which are the representation in consciousness of the physio- 

 logical changes occurring in the neurons of the central nervous system. 



Many methods are employed for the measuring of the reaction 

 time of conscious processes. In most methods the application of 

 the stimulus is arranged so as to close the circuit of a current which 

 flows through an electro- magnet activating a lever which writes 

 on a rapidly moving blackened surface. The reaction of the individual 

 who is the subject of experiment is arranged so that the resulting 

 movement activates a key by which the same current is opened. We 

 thus obtain a tracing on the blackened surface showing the moment 



