720 



FUNCTIONS OF THE SPINAL CORD 



[CH. XLIX, 



Reaction Time in Man. The term reaction time is applied to the time occu- 

 pied in the central nervous system in that complex response to a pre-arranged 

 stimulus in which the brain as well as the cord comes into play. It is sometimes 

 called the personal equation. It may be most readily measured by the electrical 

 method, and the accompanying diagram (fig. 444) will illustrate one of the numerous 

 arrangements which have been proposed for the purpose. 



In the primary circuit two keys (A and B) are included, and a chronograph (1), 

 arranged to write on a revolving cylinder (fast rate). Another chronograph (2), 



FIG. 444. Reaction time. 



marking l-100ths of a second, is placed below this. The experiment is performed 

 by two persons C and D. The key A, under the control of C, is opened. The key 

 B, under the control of D, is closed. The electrodes E are applied to some part of 

 D's body. C closes A. The primary circuit is made, and the chronograph moves. 

 As soon as D feels the shock he opens J3, the current is thus broken, and the 

 chronograph lever returns to rest. The time between the two movements of the 

 chronograph (1) is measured by means of the time-tracing written by chronograph 

 (2). From this, the time occupied by transmission along the nerves has to be 

 deducted, and the remainder is the reaction time. It usually varies from 0'15 to 0*2 

 second, but is increased in : 



The Dilemma. The primary circuit is arranged as before. The wires from 

 the secondary coil lead to the middle screws of a reverser without cross wires. To 

 each pair of end screws, a pair of electrodes E and E' pass ; these are applied to 

 different parts of D's body (fig. 445). It is arranged previously that D is to open 

 B, when one part is stimulated, but not the other, C adjusting the reverser unknown 

 to D,. Under these circumstances the reaction time is longer. 



The reaction time in response to various kinds of stimuli, sound, light, pain, 

 etc., varies a good deal; the condition of the subject of the experiment is also an 



FIG. 445. The Dilemma. 



important factor. This, however, is really a practical branch of psychology, and 

 has recently been much worked at by students of that science (see also p. lyi). 



