1909.] 



Innervation of Antagonistic Muscles. 



263 



Under this concurrent stimulation of both nerves the extensor muscle at once 

 contracts and the contraction of the flexor muscle continues, but in much 

 ■diminished degree. About two seconds later the stimulation of the contra- 

 lateral afferent is discontinued, that of the ipselateral remaining as before. The 

 •contraction of the extensor at once dies out, and the flexor resumes the higher 

 degree of contraction, approximately, which it exhibited before the stimulation 

 of the contralateral nerve set in. Finally the stimulation of the ipselateral 

 afferent itself is withdrawn ; the contraction of the flexor ceases forthwith, 

 and the relaxed extensor then exhibits a rebound contraction, regaining its 

 tonus, and exhibiting, indeed, a greater tonic shortening than it possessed 

 before it was subjected at all to inhibition.* It is obvious from the record 

 that the flexor muscle and the extensor, during the concurrent stimulation, 

 are in contraction both of them at the same time, and that the height of con- 

 traction of the flexor is less under the twofold stimulation than under that 

 ■of the ipselateral afferent alone. That is evident not only from comparison 

 with the flexor contraction in the first and third parts of Obs. II itself, but 

 from comparison with Obs. I, where the behaviour of the two muscles under 

 the stimulation of ipselateral afferent alone with the same intensity of 

 stimulus as in Obs. II is seen. Observations I, II, and III are succes- 

 sive observations taken at one minute interval. In Obs. I the ipselateral 

 afferent alone was stimulated, and in Obs. Ill the contralateral afferent 

 alone. Eeturning to Obs. II, although there is clear evidence from it that 

 the contraction of the flexor during the concurrent stimulation of the 

 .antagonist nerves is an algebraic resultant of excitation and inhibition values, 

 what is not so evident is that the concurrent contraction of the extensor 

 similarly exhibits an inhibitory component. That this is, however, really the 

 case is quite clear on comparing Obs. II with Obs. III. In Obs. Ill the 

 contralateral afferent alone was stimulated. This stimulation has no obvious 

 effect on the flexor muscle (see above, Section I) ; on the extensor it has the 

 ■ effect of exciting a contraction. The intensity of stimulus applied to the nerve 

 was unaltered from that of Obs. II. But the height of contraction evoked is 

 much greater in Obs. Ill than in Obs. II. The height of contraction has, of 

 course — this being a tonus preparation of the extensor — to be reckoned, not 

 from the particular grade of contracted state which the tonic muscle 

 happens to be exhibiting at the moment of excitation, but from the base line 

 of full relaxation, such as is revealed under the inhibitions in Obs. I and II. 

 The greater intensity of the extensor's contraction in III than in 

 Obs. II proves that in Obs. II the extensor's contraction is less than it is under 

 .the same intensity of stimulus applied to the contralateral afferent alone and 

 * Sherrington, ' Eoy. Soe. Proc.,' Note IX, 1906. 



