en. XLVIL] 



REACTION TIME 



675 



Their usefulness in the normal state is very well put by Starling in 

 the following words: "Every joint is protected by inextensible 

 ligaments and by muscles. A sudden strain on a ligament will 

 rupture some of its fibres, and perhaps injure the joint surfaces. An 

 ordinary reflex contraction could not prevent this, for the mischief 

 would be done before the reaction could take place. But the central 

 nervous system keeps the muscles awake, so that they themselves 

 may react to any sudden increase in the tension by an equally 

 sudden contraction which saves the joint before the central nervous 

 system has had time to become aware of the strain." 



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

 pied in the centre 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. 490) 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), 

 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 (7, is opened. The key 

 J5, under the control of D, is closed. The electrodes E are applied to some part of 

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

 As soon as D feels the shock he opens J5, the current is thus broken, and the 

 chronograph lever returns to rest. Measure the time between the two movements 

 of the chronograph (1), 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. Lead the wires 



FIG. 490. Reaction time. 



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

 each pair of end screws, attach a pair of electrodes E and E' t applied to .different 

 parts of >'s body (fig. 491). 



Arrange previously that D is to open B, when one part is stimulated, but not 

 the other, G adjusting the reverser unknown to D. Under these circumstances the 

 reaction time is longer. 



