666 



FUNCTIONS OF THE SPINAL CORD. [CH. XLVII. 



primary circuit is made, and the chronograph moves. As soon as D feels 

 the shock he opens It, the current is thus broken, and the chronograph lever 

 returns to rest. Measure the time between the two movements of the 

 chronograph (,i), 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 o'2 second, but is increased in : 



The Dilemma. The primary circuit is arranged as before. Lead the 



Fig. 492. Reaction, time. 



wires 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 ', applied to different parts of D's body (Fig. 492). 



Arrange previously that D is to open JB, when one part is stimulated, but 

 not the other, C adjusting the reverser unknown to D. Under these cir- 

 cumstances the reaction time is longer. 



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



Fig. 493. The Dilemma. 



pain, etc., varies a good deal ; the condition of the subject of the experiment 

 is also an important factor. This, however, is really a practical branch of 

 psychology, and has recently been much worked at by students of that 

 science. 



Special Centres in the Cord. In addition to the general function of 

 reflex action, the grey matter also exercises control over certain special actions. 



Olio-spinal centre ; this centre controlling the dilatation of the pupil is 

 situated in the lower cervical region, reaching as far down as the origin of 

 the first to the third thoracic nerve. 



Centres for defcecation, micturition, erection, and parturition, are situated 

 in the lumbar enlargement of the cord. 



Subsidiary raso-motor centres are scattered through the grey matter, the 

 principal vaso-motor centre being situated in the bulb. 



