812 THE CENTRAL NERVOUS SYSTEM 



action of the contralateral foot occurs, and the rate of the rhythm, is 

 not affected by variation in the frequency of the stimulus. 



In all the above cases the refractory period may be held responsible 

 for the rhythmic nature of the contraction. In other reflexes it exists 

 for another purpose. In the case of the extensor thrust, which it will 

 be remembered is elicited by pressure applied to the pads of the plantar 

 aspect of the foot, the momentary extension of the leg lasts only for a 

 little less than two-tenths of a second, but is followed by a refractory 



Fig. 211. Tracing from the hind limb of a spinal dog during the scratching movements pro- 

 duced by applying stimuli at two skin points (A and B), the application of the stimuli beJng in- 

 dicated by the signals. Not only were the stimuli applied at different points, but at B they 

 were of much greater frequency than at A. Although there is a slight change in "local sign," it 

 will be observed that there is no alteration in rhythm, indicating that this property must be a 

 function of the final common path. (From Sherrington.) 



period lasting nearly a whole second, during which a second stimulus 

 elicits no response. The object of this long refractory period is no doubt 

 that opportunity may be given for the flexor muscles to perform the 

 contraction that would naturally ensue during the normal occurrence 

 of the extensor thrust, as in the act of walking. When the animal 

 places his foot on the ground, the sudden pressure exerted on the pad 

 of the foot immediately calls forth the extensor thrust, by means of 



