1913.] arising from Rivalry of Antagonistic Reflexes. 243 



tonus, and r. muscle shows a slight return to greater tone than it had during 

 the influence of r. nerve's stimulation. 



Although the results, therefore, appear curiously simple, the conditions 

 underlying them are a complex of various factors. Without pretending to 

 offer any full analysis of these, some of them may be here briefly adverted to. 



1. Influence of the Laterality of the Stimulus. 



Decisive points here are (1) that reciprocal innervation holds for the 

 symmetrical muscles, (2) that with extensors the direction of reciprocity 

 is excitation contralateral and inhibition ipsilateral, with flexors conversely, 

 and (3) that the rhythmic stepping here concerned arises, proceeds, and ceases 

 with commencement, continuance, and withdrawal of the exercise of double 

 reciprocal innervation. 



In such observations as shown in figs. 1, 4, and 9, we may term the stimuli 

 according to their sequence " pre- current," "added," and "remaining." It 

 will be seen from the figures that the added stimulus when extensor muscles 

 are used always causes, as its first effect, the contraction phase of the contra- 

 lateral muscle's reaction, and the relaxation phase of the ipsilateral muscle's 

 reaction. That is, it causes, as its first effect, the extension phase of the step at 

 the contralateral knee and the flexion phase of the step at the ipsilateral knee. 

 This is in agreement with the rule observed in the case of stepping provoked 

 by direct faradisation of the spinal cord. As noted previously,* I found that 

 regular stepping of the hind limbs is readily provoked in the spinal animal 

 by unipolar faradisation applied with a stigmatic electrode directly to a 

 certain area of the distal cut face of the cord in the cervical region. If 

 the electrode be applied in the right lateral half of the cord the stepping 

 begins with flexion phase of right hind limb, and extension phase of step 

 in left hind limb. 



It might have been supposed that the reflex stepping produced by the 

 double reciprocal action of symmetrical r. and 1. afferents upon the knees 

 r. and 1. symmetrical extensor muscles must of necessity be bilateral. 

 Experiment shows, however, that that is, in fact, not the case. The reflex 

 stepping so obtained, although usually bilateral, is sometimes unilateral. Thus, 

 figs. 4 and 5 are from the same experiment at short interval. In the former 

 the stepping is bilateral, in the latter it is confined to the left muscle. The 

 observation in fig. 4 was obtained with values of stimuli right nerve 15 cm. 

 left nerve 17 cm. as measured on scale distances of the two induction apparatus. 



* 'Journ. Physiol.,' 1910, vol. 40, p. 86; 'Brain,' 1910, vol.33, p. 13; also Eoaf and 

 Sherrington, ' Quart. Journ. Exper. Physiol.,' vol. 3, p. 210 ; and G. Brown and Sherring- 

 ton, ibid., vol. 4, p. 202. 



