B16 . THE CENTRAL NERVOUS SYSTEM 



disconnected hamstring muscles simultaneously with a visible relaxation 

 of the extensors I Pig. 212 I. If the Leg is held properly, this relaxation may 

 be marked enough to cause a slight flexion at the joint; and in any case, 

 if the knee-jerk is regularly elicited by equal taps applied to the patellar 

 tendon, it will be found that, while the flexion is being produced, the 

 knee-jerk is very much less than normal, if not entirely absent, thus in- 

 dicating that the tone of the extensor muscles is diminished. This ex- 

 periment is very striking when performed on a decerebrate animal, in 

 which ;1S Ave shall see, the extensor muscles of the limb are in a per- 

 manent state of hypertonicity (Fig. 213). 



Before it is permissible to conclude that this reciprocal inhibition is a 

 necessary event in the movement of a joint, we must however show that 

 it occurs at exactly the same time as the flexion of the antagonist. Sher- 

 rington has succeeded in doing this in a considerable variety of experi- 



,-Ant Crural N. 

 (Femoralis) 



Sciatic N. 

 (Isch'tadicus) 



lig. 213. — Diagram showing the muscles and nerves concerned in reciprocal innervation. (After 



Sherrington.) 



ments, one of which we may cite here. If, in a spinal dog, the tendons 

 of the flexor muscles of the knee joint of one hind limb and the ex- 

 tensor tendons of the opposite limb are cut, then the former limb will 

 be unable to flex properly, but will nevertheless exhibit reciprocal 

 inhibition of the intact extensor muscle, while the latter limb will flex, 

 but require passive extension to bring it back to its old position. If 

 suitable stimuli are simultaneously applied to the skin of both legs and 

 the movements of the isolated muscles recorded, the onset of inhibition 

 of the intact extensor of the one leg and the contraction of the flexors 

 of the opposite leg will be found to agree with regard to latent periods, 

 strength of required stimulus, summation and indeed all the other phys- 

 iological properties of reflex action. 



Reciprocal innervation can also be demonstrated by stimulating the 

 central end of suitable afferent nerves — that is, certain afferent nerves 



