106 EXPERIMENTAL PHYSIOLOGY. 



This should be tried on a number of subjects. After observing 

 the extension in each subject, try the effect of REINFORCEMENT 

 by the following methods: 



1. Clenching the hands together vigorously. 



2. Strong sensory stimulation such as pulling the hair. 



3. Mental effort as solving a problem. 



The magnitude of the response seems to depend on the tone 



of the muscle. 



Tone depends on the condition of the nervous system, therefore 

 the knee jerk is an important means of ascertaining certain patho- 

 logical conditions of the central nervous system. 



REACTION TIME IN MAN. 



Akin to reflex latent time is the so-called REACTION TIME in man , 

 that is the time which elapses between the application of a stimulus 

 and a prearranged voluntary reaction. 



Experiment 48. In this experiment the apparatus is set up as in 

 Fig. 31. With the operator's key A open, and the subject's key 

 B closed, the operator closes, and the subject opens B when- 

 ever he feels the stimulus. The signal connected with the keys 

 writes on a quickly revolving drum on which a time tracing in 

 lOOths of a second is also inscribed. Determine the average 

 reaction time. Now increase the stimulus to find whether the 

 reaction time changes. What are the additional factors in- 

 volved in reaction time as compared with reflex time? How do 

 different subjects vary? 



