1912.] 



Rhythmic Activity of the Nervous System. 



285 



these cases there is really a compounding of two antagonistic excitations — 

 for there must be a background of maintained extension, as in the decerebrate 

 condition, or as in the experiments in the low spinal preparation in which 

 the mechanical stimulation of the spinal cord produced the maintained 

 extensor contraction. 



Even in the case of the simple reflex there may be a compounding of two 

 antagonistic excitations apart from any background present. This may be 

 the case if the afferent nerve can activate (excite) both centres (extensor and 

 flexor), although not with equal effect. Such a condition may occur if the 

 nerve split in the spinal cord and one part goes to each centre to excite it. 

 Or it may occur if the afferent nerve stimulated in these experiments 

 contains fibres some of which pass to one of the antagonistic centres and 

 some to the other. 



That such a double reciprocal innervation may occur even with a simple 

 stimulus — that is, a stimulus applied to a single afferent nerve — seems to be 

 shewn by the fact that sometimes contraction of both muscles may occur 

 during the process of the stimulus. 



Another point which seems to lend support to this view is the occurrence 

 of flexor and of extensor " rebound contraction after excitation." If rebound 

 be a phenomenon associated with a state of inhibition it must seem that 

 even in the efferent centre of the contracting muscle there is a certain 

 factor of inhibition produced by the peripheral stimulus. 



It may be stated here that the corresponding phenomenon " flexor (or 

 extensor) rebound relaxation after inhibition," which had not been observed 

 when the above phenomenon was described,* has since been seen by the 

 author in the case of the extensor muscle. An ipsilateral stimulus which 

 evoked flexor contraction, and along with that a well-marked extensor 

 relaxation, at the cessation of its stimulation was followed by an additional 

 extensor relaxation. 



VIII. Theories oj Rhythmic Activity. 



It is not intended here to discuss these in detail. It may be remarked, 

 however, that certain evidence given in this paper seems to be directly 

 opposed to three well-known theories. 



In the first place the occurrence of relaxation in a rhythmic phenomenon 

 may markedly precede the occurrence of the synchronous contraction of the 

 antagonistic muscle. *f This phenomenon would not be possible if the state 

 of inhibition was conditioned by a phenomenon of " drainage " as suggested 



* ' Quart. Journ. of Exp. Physiol.,' 1911, vol. 4, p. 331. 

 t ' Eoy. Soc. Proc.,' 1911, B, vol. 84, p. 308. 



