150 . PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



man, from 0.025 to 0.073 sec. Moreover, we have no definite basis 

 upon which to estimate what should be the time required if the 

 act were a genuine reflex. For the act of winking in man Exner 

 estimated a total time of 0.0578 sec., but May hew obtained a 

 smaller figure 0.0471 sec. In the lower animals the results have 

 also been uncertain. Applegarth, making use of a dog with a 

 severed spinal cord, obtained for the time of the knee-jerk an 

 interval of 0.014 to 0.02 sec. Assuming a velocity of 100 ft. or 

 more per second, this would allow sufficient time for the impulse 

 to travel to the cord and back provided there was no delay in 

 the nerve centers. Waller and Gotch, using the rabbit, found the 

 time to be only 0.008 to 0.005 sec., that is, just about the 

 latent period of a muscle contraction and too short a time for 

 a reflex. It is evident that more facts are necessary before a 

 positive statement can be made upon this point. In favor of 

 the reflex theory attention may be called to the fact that in 

 some cases a crossed reflex is obtained affecting the muscles of 

 the other leg. This apparently undoubted reflex shows that an 

 efficient sensory impulse has reached the cord, and, according to 

 our knowledge of reflexes, the effect in such cases should always 

 be most marked on its own side. It would seem to be unjustifiable 

 in these cases to suppose that the effect on the same side is not 

 reflex while on the opposite side it is reflex. 



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. 66. It would seem from his experiments 

 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. See 

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

 ences," second edition, 1902. 



