248 HUMAN PHYSIOLOGY 



centres are supposed to be connected with each other and with the 

 sensory areas chiefly by association and commissural fibres, and to 

 contain relatively few corona fibres connecting it with the lower 

 parts of the brain. The larger part of the fibres from the corona 

 radiata are supposed to proceed to the sensory and motor areas. 

 Moreover, the individual sensory areas are supposed not to be con- 

 nected with each other by the association fibres, but only with the 

 association centres. 



Nothing is known concerning the nature of the psycho- 

 physical processes which underlie psychical phenomena. 

 Up to the present time the investigations of these processes 

 have been limited to their duration. 



Reaction time is the time elapsing between the beginning 

 of the stimulation of a sense organ and a most rapidly 

 executed muscular movement, e.g. of a finger. Both these 

 times are registered. 



The measurements of the reaction time are : 



For optical stimulation 0.150.22 second. 



" auditory " o. 12-0. 18 " 



11 tactile " 0.09-0.19 " 



" taste " o. 16-0.22 " 



The reaction time is smaller for areas which are more frequently 

 stimulated, e.g. the yellow spot, the tip of the finger, than for 

 areas less frequently stimulated, as the periphery of the retina, skin 

 of the arm. It is also dependent upon the degree of attention and 

 practice, and upon the psychical attitude. Individual peculiarities 

 also influence the reaction time. 



When a very accurate registration of time must be made, for in- 

 stance by a person noting the passage of a star across the thread 

 of a telescope, the reaction time must be taken into consideration. 

 The individual variations of the reaction time are brought into 

 account by astronomers as "personal equation." 



The more complex the psychical processes which intervene 

 between the sense stimulation and the reaction, and the 

 longer the time necessary for reflection, the greater will be 

 the length of time between the beginning of stimulation and 

 the reaction. 



The elucidation of the psychical .phenomena themselves (sensa- 

 tion, thought, volition, attention, memory, etc.) is the object of 

 Psychology. 



