ELEMENTARY EXPERIMENTAL PHYSIOLOGY 217 



vibrations per second, is connected with two Daniell cells and with the 

 chronograph C. By means of either of the two Du Bois keys, K x and 

 K 2 , the chronograph can 

 be short circuited. The 

 key Kj is closed and K 2 

 is open; the tuning fork 

 is set vibrating, but does 

 not affect the chrono- 

 graph. The subject, 

 whose reaction time is 

 to be determined, is told s$n& U (U 

 to listen for the sound xXX^^L 



of the opening of the FlG . 205 ._ Diagram of the apparatus for the determination 

 key Kj and to close the of ^action time, 



key K 2 directly he hears the sound. When the key K x is opened the 

 chronograph vibrates in unison with the tuning-fork and the vibrations 

 are recorded upon a revolving drum ; the closure of the key K 2 by 

 the subject of the experiment brings the chronograph to rest. The 

 number of vibrations recorded upon the drum gives the reaction 

 time for sound in yj^ths of a second. 



The total reaction time in this experiment is composed of (1) the 

 time taken by the sound to reach the ear ; (2) the time taken for the 

 reception of the stimulus by the sensory endings of the auditory nerve 

 and the transmission of the nervous impulse to the sensory area ; (3) 

 the time for the transmission to the higher centres so that volitional 

 impulses may be started in the cerebral motor centres ; (4) the time for 

 the propagation of those motor impulses to the nerve cells of the spinal 

 cord ; (5) the time required for the generation of impulses in these cells 

 and their passage down the motor nerves to the muscles of the hand ; 

 and (6) the latency of the contraction of those muscles. 



The reaction time for sound is about 0*150 second, for light O'l 95 

 second, and for touch about 0-145 second. 



CHAPTER LXI. 



THE RATE OF DISCHARGE OF NERVOUS IMPULSES FROM THE 

 CENTRAL NERVOUS SYSTEM. 



THE rate at which nervous impulses can be discharged by the central 

 nervous system can be investigated in the frog by exciting the nerve 

 cells by means of a drug such as strychnine and recording the resulting 

 incomplete tetanus ; or in man by the record of the contraction of a 



