220 



Prof. C. S. Sherrington. Reciprocal [Nov. 13, 



to say, astride of the median longitudinal plane of the body, and a somewhat 

 analogous case is that of the reciprocal innervation exerted by cortex cerebri 

 on certain symmetrical muscle pairs in, for instance, the musculature of the 

 eyeballs.* Stimulation of a point in the right cortex, while causing left 

 external rectus to contract, causes right external rectus to relax, and con- 

 versely inhibits left internal rectus while exciting contraction of right 

 internal rectus. 



But in the case of such symmetrical muscle-pairs, though some circum- 

 stances and some afferents deal with the two members of the pair by 

 "reciprocal innervation," it is equally clear that some deal with them 

 by " identical innervation." Thus with the two vastocrurei, right and left, 

 though most of the limb-afferents influence the two reciprocally, the genito- 

 crural nerve influences them identically ,f namely, excites concurrent 

 contraction of both muscles, and it is clear that in their natural use both 

 the muscles are sometimes thrown into contraction together and thrown 

 out of contraction together, as happens in the gallop and in standing and 

 sitting. Similarly with the lateral recti of the eyeballs, stimulation of 

 certain brain points and certain voluntary acts which cause ocular con- 

 vergence exert an identical influence on the two internal recti exciting both 

 together, and similarly an identical influence on the two external recti 

 inhibiting both. The relation of reciprocal innervation to the symmetrical 

 muscle-pairs differs, therefore, from its relation to antagonistic muscles, in 

 so far that in the former case it is only one of the ordinary modes of 

 innervation obtaining for the muscle-pair, whereas in the latter case there is 

 little evidence at present that reciprocal innervation of the muscle-pair is at 

 all commonly under normal circumstances replaced by identical innervation. 



Such symmetrical muscle-pairs present, therefore, the problem that some- 

 times they are co-ordinated by reciprocal innervation and sometimes by 

 identical innervation. In the present observations it has been sought to see 

 whether by experimental means in the purely reflex preparation they can 

 be made sometimes to contract together or to relax together and at other 

 times to behave reciprocally, the one member of the pair contracting con- 

 comitantly as the other relaxes. 



II. Change from Reciprocal Innervation to Identical in Symmetrical 



Extensors. 



When an afferent nerve of one hind limb is stimulated in the double 



(right and left) vastocrureus preparation (decerebrate cat), reciprocal innerva- 



* 'Koy. Soc. Proc.,' 1893, vol. 52, p. 333. 

 t ' Journ. Physiol.,' 1910, vol. 40, p. 53. 



