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Reciprocal Innervation and Symmetrical Muscles. 



By C. S. Sherrington, F.E.S., Professor of Physiology, University of 



Liverpool. 



(Received November 13, 1912,— Read January 23, 1913.) 

 (From the Physiology Laboratory, University of Liverpool.) 



I. Introduction. 



If we attempt to decipher the biological meaning of reciprocal innervation 

 its various instances when marshalled together say plainly that one of the 

 functional problems which it meets and solves is mechanical antagonism. 

 Where two muscles have directly opposed effect on the same lever, " reciprocal 

 innervation " is the general rule observed by the nervous system in dealing 

 with them, and this holds whether the reciprocal innervation is peripheral 

 as with the antagonists of the arthropod claw, or is central as with 

 vertebrate skeletal muscles. Also where one and the same muscle is 

 governed by two nerves influencing it oppositely, reciprocal innervation 

 seems again the principle followed in the co-ordination of the two opponent 

 centres, as has been shown by Bayliss* in his observations on vasomotor 

 reflexes. 



But the distribution and occurrence of reciprocal innervation extend 

 beyond cases of mere mechanical antagonism. The reflex influence exerted 

 by the limb-afferents on symmetrical muscle-pairs such as right knee-extensor 

 and left is reciprocal .f Thus right peroneal nerve excites the motoneurones 

 of left vastocrureus, and concomitantly inhibits those of the right. The 

 reflex inhibition of the one is concurrent with, increases with increase, and 

 decreases with decrease of, the excitatory effect on the other. Here the 

 muscles are not in any ordinary sense antagonistic ; not only do they not 

 operate on the same lever, but they are not even members of the same limb, 

 nor do they belong even to the same half of the body. They are, however, 

 actuated conversely in the most usual modes of progression — the walking and 

 the running step — though not always in galloping. 



Similarly with other symmetrically paired limb muscles,! the limb afferents 

 when tested for their reflex effect on such twin muscles commonly exert an 

 opposite and reciprocal effect on the members of the pair. Here the 

 bifurcation of the afferent path which leads to the reciprocal effect sits, so 



* ' Eoy. Soc. Proc.,' 1908, B, vol. 80, p. 539. 

 t ' Journ. Physiol.,' 1898, vol. 22, p. 398. 

 % ' Eoy. Soc. Proc.,' 1905, B, vol. 76, p. 286. 

 VOL. LXXXVI. — B. B 



