EXTENSOR REFLEXES AND INHIBITION 119 



is suppoi-ted in a nearly vertical posture with the left hip flexed almost to 

 a right angle (PI. IX, fig. 8). 



Apply a pair of electrodes (the bare standard-carried pattern or preferably 

 the glass-sheathed pattern) to the prepared left sciatic nerve, as in exerc. 

 XVIII, § V (PI. IX, fig. 8). 



IV. Note the posture of the left knee-joint ; it is maintained in a greater Obs 87. 



or less degree of extension. If the extension is not well marked it can be ^g®j.'eactfon^' 

 improved by gently raising the stump below the knee, while steadying the 

 thigh with the other hand, thus giving the knee a more extended posture ; on 

 releasing the stump the extended posture thus passively given is more or less 

 maintained (plastus tonus, the ' shortening reaction '). 



V. Steadying the thigh with one hand, so as to help the clamped drill- Obs. 88. 

 head in preventing the thigh-posture from being displaced, depress the end of jng^ea^ctfon^'^' 

 the leg-stump firmly but slowly, thus flexing the knee. At a certain degree 



of pressure the knee will be found to yield, and when released will retain 

 approximately the degree of flexion imposed on it. This is the ' lengthening 

 reaction ' already observed, § I, but it is now observed in the isolated vasto- 

 crureus muscle, and thus shown to be a proprioceptive reaction of that muscle 

 itself. It is called the 'lengthening reaction' because in it the postural 

 (tonic) length of the muscle is increased. 



Elicit the knee-jerk. Note that it has the characters met with in exerc. 

 XVI, obs. 75. 



VI. Use the ' shortening reaction ' to give the knee a posture of Obs. 89. 

 extension, and while it is in the extended posture, i.e. at an angle of something ?®^®f^. 

 more than 90°, set in operation the primary circuit of the induction coil with pogture. 

 the magnetic interrupter, and open the secondary circuit's bridge-key, thus 

 faradizing the central stump of the left sciatic nerve. Note that this stimu- 

 lation causes the leg-stump to drop into flexion. This is due to reflex 

 inhibition of the postural tone of the left vastocrureus muscle. Close the 

 bridge-key. The cessation of the stimulus may be followed by a return of the 

 extension of the knee (post-inhibitory rebound) or it may not. Rebound is 



m ore likely to occur when the pre-existing tonus of the preparation is strong 

 and the stimulus is fairly strong but brief, i.e. not more than Y\ 



VII. Place preparation fully supine, and raise the right leg, disturbing as Obs. 90. 

 little as possible the electrodes on the left sciatic. Adjust a pair of electrodes, ^®^®^. 

 belonging to the other induction apparatus, to the right sciatic nerve. CAttach contraction. 



