REFLEX ACTIONS. 



157 



itself. According to Snyder, the time interval lies between 0.0113 

 and 0.015 sec, while Hoffmann's results give an interval of 0.019 

 to 0.024 sec. The calculations of both observers indicate that the 

 time is sufficient for a reflex, and much too long for a direct excita- 

 tion. In the case of the Achilles jerk, Hoffmann finds that it may 

 be liberated by electrical stimulation of the n. tibialis and that 

 under these circumstances there is first a deflection of the galvano- 

 meter, due to direct stimulation of the gastrocnemius through 

 its motor nerve, and this is followed later by a second deflection, due 

 to reflex stimulation. This latter accords in time interval with the 

 Achilles jerk, and gives a new proof that the phenomenon is a 

 genuine reflex. In view of these facts it would seem to be safe 

 to conclude that the knee-kick and similar phenomena are reflexes 



Fig. 71. — Lombard's figure to indicate the daily rhythm in the extent of the knee- 

 jerk and the effect of mental stimuli. The ordinates (0-110) represent the extent of the 

 kick in millimeters. Each dot represents a separate kick, while the heavy horizcHital lino 

 gives the average extent for the period indicated. 



but reflexes in which a single nerve impulse is sent out from the 

 Gord, causing a simple contraction in the muscle affected. 



Conditions Influencing the Extent of the Knee-jerk. — The 



effect of various normal conditions upon the knee-jerk has been 

 studied by a number of observers, particularly by Lombard.* The 

 results are most interesting in that they indicate very clearly that 

 the irritability of the spinal cord varies with almost every marked 

 change in mental activity. During sleep the jerk disappears 



♦Lombard, "The American Journal of Psychology," 1887, p. 1. See 

 also article "Knee-jerk" (Warren), "Wood's Ref. Handbook of Med. Sci* 

 ences." 



