318 Mr. Graham Brown. Intrinsic Factors in the [July 21, 



the other centre by the first decreases in efficiency. The point then arrives 

 when the second centre is no longer inhibited efficiently. But there is then 

 following upon the inhibitory depression an exaltation of its activity — 

 " rebound." The balance is therefore not only regained but overshot. The 

 exhibition of activity by the second centre then determines the contraction 

 of the antagonistic group of muscles, and at the same time there is an 

 inhibition of the activity of the first centre. The " fatigue " which 

 accompanies this activity of the second centre then sets in. The balance is 

 restored, but again overshot. And so the act proceeds. 



The phenomenon of rebound, which Sherrington* has shewn to be of 

 central locus, may play a very important part in the swinging of balance 

 between the spinal centres. And the phenomena which underlie the phasic 

 act of progression may be likened to the beating of a pendulum. The 

 activity exhibited may remain for a time flexion, may then swing back to 

 the neutral point of spinal balance, but may overshoot this and become 

 extension activity, may then swing back past the neutral point into flexion 

 activity — and so on. 



There remains the question of the part played by the proprioceptive 

 mechanism in the act. 



There can be no question of its importance nor of its suitability to 

 augment the central mechanism. It cannot, however, be regarded as 

 determining the refractory phases in the act. Its part must be regulative, 

 not causative. 



A purely central mechanism of progression ungraded by proprioceptive 

 stimuli would clearly be inefficient in determining the passage of an animal 

 through an uneven environment. Across a plain of perfect evenness the 

 central mechanism of itself might drive an animal with precision. Or it 

 might be efficient, for instance, in the case of an elephant charging over 

 ground of moderate unevenness. But it alone would make impossible the 

 fine stalking of a cat over rough ground. In such a case each step may be 

 somewhat different to all others, and each must be graded to its conditions 

 if the whole progression of the animal is to be efficient. The hind limb, 

 which at one time is somewhat more extended in its posture as it is in 

 contact with the ground, in another step may be more flexed. But the 

 forward thrust it gives as its contribution to the passage of the animal must 

 be of a comparatively uniform degree in each consecutive step. It may only 

 be so if it is graded by the posture of the limb when in contact with the 

 ground, and by the duration of its contact with the ground. This grading 

 can only be brought about by peripheral stimuli. Of these we must regard 

 * 'Q. Journ. Exp. Physiol.,' 1909, vol. 3, p. 109. 



