922 CENTRAL NERVOUS SYSTEM 



sions, or the later stages of spinal shock, the knee jerk is much more 

 brisk in character because the contraction of the muscles is unimpeded by 

 the tone of their antagonists. The return of the leg to its original posi- 

 tion is very rapid since there is no tonic prolongation of the contraction 

 of the extensors, and the shank tends to bounce up and down in the 

 absence- of any constraining tonic action on the part of the muscles 



Fig. 230. A, tracing of the knee-jerks of a normal man. B, tracings of the knee-jerks of the 

 hypotonic leg of a man with a cerebellar injury of eight years' duration. The records should be 

 read from right to left. (From Holmes.) 



(Fig. 230). Under these conditions the return is a passive act produced 

 by the weight of the leg, and not by a compensating flexor contraction, 

 for if the limb is supported on a bed when the knee jerk is elicited, it 

 shows no tendency to flex again after the extension has been produced. 

 Impulses from other parts of the nervous system may prevent the 

 occurrence of the tendon jerks by preventing the afferent impulses set 



