THE ROLE OF REFLEX INHIBITION 595 



innervation, nor does it exemplify fully what usually happens 

 in the elbow muscles when the act of flexion of that joint is 

 executed naturally. If we voluntarily hold the arm semi-flexed 

 nothing is easier than to make both flexor and extensor contract 

 together ; by simply feeling the hardness of the muscles through 

 the skin we can assure ourselves of that fact. This concurrent 

 contraction of both antagonists seems impossible to obtain 

 under normal circumstances as a simple reflex by stimulation 

 of any single point or any single nerve. But it can be obtained 

 when we produce pure reflex reflexion of the joint in another 

 way. While some afferent channels regularly produce reflex 

 flexion of the joint, others just as regularly produce reflex 

 extension of it. If two afiferents, one producing flexion of the 

 joint, the other extension, be stimulated simultaneously it is 

 not difficult to find a grade of conjoint stimulation producing 

 the flexion of the joint. In this flexion there is, if suitable 

 strengths of stimuli are chosen, some contraction of the extensor 

 muscle as well as of the flexor muscle (fig. 4) ; but the con- 

 traction of the flexor predominates and the joint is therefore 

 flexed. What happens is instructively studied by the myograph. 

 If the reactions of the muscles in response to either of the 

 single stimuli are compared with their reaction under the 

 double stimulus, the motor centre for the flexor is shown in 

 the latter case to be under a two-fold influence ; similarly 

 the extensor centre also under a two-fold influence. Of the 

 two afiferents concurrently stimulated, that one which if stimu- 

 lated alone causes flexion of the joint excites the motor centre 

 of the flexor and inJiibits that of the extensor ; and the other 

 afferent, which if stimulated alone causes extension of the 

 joint, excites the motor centre of the extensor and inhibits 

 that of the flexor. When both are stimulated simultaneously 

 with appropriate intensity the discharge from the flexor motor 

 centre represents the algebraic sum of the opposed excitation 

 and inhibition which the two afiferents exert on it, and the 

 discharge from the extensor motor centre similarly represents 

 the algebraic sum of its opposed excitation and inhibition 

 (fig. 3). If the intensity of the stimuli be suitably chosen, both 

 flexor and extensor motor-centres discharge, but the discharge 

 of neither is as great as it would be were the antagonistic 

 influences not present. 



An interest attaching to this double reciprocal innervation 



