608 THE HUMAN BODY. 



stimulus, that would not prove a volitional conscious use of 

 them ; we see quite similar phenomenon when there is nothing 

 purpose-like in the movement. Many dogs reflexly kick vio- 

 lently the hind leg of the same side when one flank is tickled. 

 If this leg be held and the tickling continued, very frequently 

 the opposite hind leg will take on the movements, which it 

 never does in ordinary circumstances. This is quite compar- 

 able to the frog's use of its other leg under the circumstances 

 above described, but here it would be obviously absurd to talk 

 of a volitional source for such a senseless movement. 



Reflex Time. This is the time elapsing between the stimu- 

 lation of a sensory surface and the resulting reflex contraction 

 of a muscle. It contains, of course, several elements the 

 time taken in the origination and afferent course of the nerve 

 impulse, the time occupied in the centre, and that in the 

 eiferent nerve-fibres, and the period of latent excitement of 

 the muscles. Since the rate of travel of nerve impulses and 

 the time of latent excitement are known with tolerable ac- 

 curacy they can be estimated; and their sum subtracted from 

 the whole time gives the time taken up in the central organ. 

 This, as might be expected, when we consider the highly 

 complex nature of the processes required to produce a co- 

 ordinated reflex movement, is very much greater than the 

 time occupied in traversing an equal length of nerve trunk. 

 An electric shock given to one eyelid causes a reflex wink of 

 both, and by suitable apparatus the time lapsing between 

 stimulation of one eyelid and movement of the other can be 

 measured. It is about .0660 sec. ; the calculated time for the 

 passage of the afferent impulse to the centre in the gray matter 

 of the fourth ventricle and of the efferent to the orbicularis 

 muscle of the other eyelid, or the period of latent excitation, 

 is about .0160 sec., leaving .0500 sec. for the central processes. 

 Reflex time varies considerably. It is longer for more com- 

 plicated reflex movements; also the strength of the stimulus 

 has an influence ; if one toe of a decapitated frog be immersed 

 in very dilute acid the time which elapses before it is with- 

 drawn is greater than when the acid is a little stronger. 



