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Prof. C. S. Sherrington. Reciprocal [Mar. 25, 



stimulation applied to inhibitory afferents may simply mean that we fail by 

 artificial inhibitory stimuli to induce a reflex inhibition of such a kind as 

 a natural inhibitory tonus would be, just as we fail by artificial excitatory 

 stimuli to induce a reflex contraction of such a grade as is the natural 

 excitatory tonus. In light of this consideration the probability appears that 

 just as there exists a natural excitatory tonus, e.g., the state of the extensors 

 and of some adductors in decerebrate rigidity, so likewise there is a natural 

 reflex inhibitory tonus, but that artificial stimuli such as we have at command 

 at present fail to reproduce it. It may be that the relative slackness of the 

 flexor muscles, in other words their exclusion from excitatory tonus, in 

 decerebrate rigidity is the expression of an inhibitory tonus. Decerebrate 

 rigidity is, as I have shown elsewhere,* to be regarded as a postural reflex, 

 namely, the reflex of standing. If the flexors are under an inhibitory tonus 

 in that reflex condition while the extensors are under an excitatory tonus, it 

 becomes clear that reciprocal innervation applies in this static postural reflex 

 as well as in the ordinary reflexes which execute movements and changes of 

 posture. 



In the myograph records (fig. 8) which illustrate failure to obtain balance 

 between the natural reflex tonus of the extensor and the artificially excited 

 reflex excitation there occurs a further feature. The rate of elongation of 

 the tonic muscle under the weak artificially excited inhibition continues 

 about the same from start to finish, even when the starting point is with the 

 muscle fairly fully shortened and the end point is with the muscle fully 

 relaxed. In other words, the degree to which the reflex inhibition over- 

 balances the natural excitation causing the tonic contraction seems practically 

 the same throughout. But the intensity of the stimulation of the inhibitory 

 afferent in these observations was kept the same from start to finish ; hence 

 the inference is that the intensity of the natural reflex excitation causing the 

 tonic contraction is about the same throughout. That is to say, the intensity 

 of the excitatory tonus is practically the same when it keeps the extensor 

 muscle so shortened as to maintain the knee fully extended as when the 

 muscle under the tonus maintains the knee at an angle of e.g. 60°. In other 

 words, the intensity of the reflex tonus of the extensor may be practically 

 the same while the muscle maintains postural lengths widely different. 

 Thus the same conclusion is reached as has been arrived at by other 

 observations which I have published before.f 



IV. — The extensor muscle of the knee (vastocruretts) and the flexor muscle 



* ' Integrative Action of the Nervous System,' London and New York, 1906. 

 t Sherrington, 'Koy. Soc. Proc.,' Note XII, August, 1908 ; and ' Quart. Journ. of Expt. 

 Physiol.,' loc. cit. 



