REFLEX ACTIONS. 



159 



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

 cord, 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 

 and in mental conditions of a restful character its extent is relatively 

 small. In conditions of mental excitement or irritation, on the 

 contrary, the jerk becomes markedly increased. Lombard ob- 

 served also, in his own case, a' daily rhythm, which is represented 

 in the chart given in Fig. 71. It would seem from his experiments 



April OtH 1887 



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 horizontal line 

 gives the average extent for the period indicated. 



that the extent of the knee-jerk is a sensitive indicator of the 

 relative state of irritability of the nervous system: "The knee- 

 jerk is increased and diminished by whatever increases and di- 

 minishes the activity of the central nervous system as a whole." 

 This general fact is supported, especially as regards mental activity, 

 by observations on other similar mechanisms, such, for instance, 

 as the condition of the nervous centers controlling the bladder. 



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

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



See 

 Sci< 



ences. 



