RECIPROCAL INNERVATION 815 



from the volitional center in the cerebrum. The same result may be 

 obtained by electric stimulation of the center for eye movements on the 

 cerebral cortex. 



The most important details concerning the mechanism of reciprocal 

 innervation have been obtained by studying the flexion reflex in a spinal 

 animal which has completely recovered from shock. In such an animal 

 the tonus of the extensor muscles of the knees is well marked. This 

 tonus is maintained by afferent impulses transmitted to the spinal cord 

 from receptors situated in the muscles, and its degree of intensity can 

 be estimated by the briskness of the knee-jerk, which, it will be remem- 



Fig. 212. Record from myograph connected with the extensor muscle of the knee. During 

 the time marked by the lower signal, the skin of the opposite foot was stimulated, thus causing 

 the crossed extension reflex. While still maintaining this stimulation, faradic shocks were ap- 

 plied to the skin of the foot of the same side (as indicated by the upper signal), with the result 

 that immediate inhibition of the contracted extensor occurred. (From Sherrington.) 



bered, is elicited by tapping the patellar tendon, and consists of a sud- 

 den extension movement at the knee joint. By observing the briskness 

 of the knee-jerk we are therefore enabled to form an estimate of the 

 tonicity of the extensor muscles; and if after doing so we throw the 

 flexors which are their antagonists into activity by eliciting the flexion 

 reflex, the knee-jerk will be found much less active. If we prevent the 

 flexors from acting on the knee, joint and the leg is held in an extended 

 position, irritation of the skin of the leg will cause the flexion of the 



