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



of the skin and with the action of association paths in the lumbar cord, are 

 sufficient to produce the phenomenon in the absence of contact with the 

 ground. 



The difficulty in explaining this phenomenon has been emphasised by 

 Sherrington.* He points out that, in the intact animal (cat, dog), severance 

 of all the sensory nerve trunks directly distributed to all four feet up to and 

 above the wrists and ankles scarcely impairs the act of progression. He also 

 notes that reflex stepping is not annulled, or even obviously impaired, by 

 severance of all the various cutaneous nerves of the limb. And stretching 

 of the prominent fold of skin which runs along the outer edge of the groin 

 cannot be of essential importance in the act, because cocainisation of this 

 region does not interfere with reflex stepping. The extensor thrust may 

 also be abolished — by that division of the sensory nerves of the foot 

 •described above — without noticeably changing the acts of the walk and 

 trot. He therefore concludes that the intrinsic stimuli for reflex stepping 

 of the limb are not referable to any part of the skin of the limb. 



In continuation of his work on proprioceptive reflexes,f Sherrington finds 

 in the sensory end-organs of the muscles themselves the seat of the intrinsic 

 stimuli for reflex stepping.:}: He considers that the mode of process in 

 reflex walking is as follows : The spinal step is a rhythmic reflex which may 

 be excited by continuous stimuli applied either to the cross-section of the 

 divided spinal cord or to various peripheral points outside the limb itself. 

 The generating stimulus is continuous, but the movement of the limb is 

 determined by the alternate action of two antagonistic reflexes. The 

 primary stimulus sets one of these in action. This act generates in that 

 limb a proprioceptive reflex antagonistic to itself. The proprioceptive reflex 

 interrupts the primary reflex, and in this interruption abolishes the stimulus 

 which engendered itself. The primary reflex is then reconstituted and 

 again calls forth the interrupting reflex, and so on. The secondary reflex is 

 determined by the combination of three main factors — centripetal impulses 

 from the deep structures moved passively by the primary reflex (joints, etc.) ; 

 centripetal impulses from the muscles which move actively in the primary 

 reflex ; and the central change underlying " rebound." The phenomenon of 

 " reflex reversal " and the " extensor thrust " may also play a part. 



Of particular significance is this factor of " central rebound." Sherrington§ 



* ' Journ. Physiol.,' 1910, vol. 40, p. 28. 



t ' Boy. Soc. .Proc.,' B, 1908, vol. 80, p. 552 ; ' Q. Journ. Exp. Physiol.,' 1909, vol. 2, 

 p. 109. 



% 1 Journ. Physiol.,' 1910, vol. 40, p. 28. 



§ 'Boy. Soc. Proc.,' B, 1905, vol. 76, p. 160 ; 'Boy. Soc. Proc.,' B, 1906, vol. 77, p. 478. 



