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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



i 



FIG. 170. Record of the knee-kick of a demented 

 patient. The knee was tapped at regular intervals of 

 five seconds. While the patient was asleep and all 

 about was quiet, no response was obtained ; after such 

 an irresponsive period the sound of some one walking 

 on the floor below caused at A a series of kicks which 

 gradually diminished ; the same at B. At C two taps 

 with a pencil and a distant locomotive-whistle produced 

 a longer series. The arrow indicates the direction in 

 which the record is to be read (Noyes). 



Noyes. 1 It is found that under given conditions, the variations in the 

 extent of the kick can be referred to variations in the excitability of that 

 portion of the spinal cord from which the fibres controlling the muscles take 



their origin, namely, the second, 

 third, and fourth lumbar seg- 

 ments. 



In the same individual under 

 constant conditions and for short 

 periods of time, the knee-kick 

 may be fairly constant in its ex- 

 tent, but the normal extent for 

 different individuals may vary 

 widely, all the way from those 

 cases in which this reaction is nor- 

 mally absent to those in which it is 

 normally very large. In the same 

 individual there are also variations 

 from day to day, variations com- 

 parable for instance to those in the 

 condition of athletes whose capacity 

 for performing a given feat is, as 

 we know, by no means constant. 



Experimentally the most marked variation which is observed in the extent 

 of the knee-kick occurs when the patient passes from the waking to the 

 sleeping state, or vice versa. The regulated blow of a hammer automatically 

 released, and striking the same point of the tendon, will produce little or no 

 reaction when the patient is asleep, whereas in full wakefuluess the reaction 

 may be very evident. Figures 170, 171 illustrate such variations. 



Attention was first directed to this peculiar reaction for the reason that in 

 some degree it could be used to test the physiological condition of the spinal 

 cord, it being found that the knee-kick was usually abolished in those condi- 

 tions in which the lumbar portion of the cord is damaged or its connections 

 with the higher centres interrupted, whereas it was much exaggerated in those 

 conditions in which disturbance in the higher centres tended to cause excessive 

 stimulation of the cord. As soon, however, as the reaction was studied with 

 greater care in normal persons, it became evident that the condition of this 

 part of the spinal cord was subject to remarkable fluctuations, and that these 

 fluctuations depended in a measure on circumstances which could be controlled. 

 For example, there are here given (Fig. 171) six records showing respectively 

 the increase in the extent of the knee-kick after the subject was suddenly 

 awakened ; on repeating Browning's Poem, " How they brought the good 

 news from Ghent to Aix ; " as the result of talking ; in consequence of the 

 crying of a child in the next room ; and immediately after swallowing. The 

 point here insisted upon and for which illustration is sought by the accom- 

 1 American Journal of Psychology, 1892. 



