482 



Prof. C. S. Sherrington. 



[Jan. 31, 



excitatory of the reflex remaining in action all the time, though unable to 

 produce the reflex owing to the intervening inhibition. 



The after-increase which ensues, in the second form of arrest, but not in 

 the first, might be explicable in either of two ways. It might be due to the 

 continuance of the exciting stimulus, E, during the period of arrest. That 

 stimulus might, though unable to evoke discharge of the motor neurone 

 during the inhibition, yet be charging a relay apparatus in the reflex arc, 

 and so lead to increased discharge after the inhibition was past. Or the 

 after-increase might proceed as a direct result from the inhibition itself, 

 the depressed activity of inhibition being followed by a rebound to super- 

 activity, and altogether apart from the continuance of any excitatory 

 stimulus during the inhibitory period. 



To decide between these possibilities the effect of strongly stimulating the 

 tail when at the same time both hind limbs were supported from below was 

 tried. The stimulus for the stepping reflex was thus held in abeyance at the 

 time of and during the whole period of the intercurrent inhibition. The 

 result was found to be an after-increase of the stepping reflex not less 

 marked than in the previous cases. 



It is not at first obvious what relation a stimulus to the tail bears to the 

 reflex of the limb. But it is often noticeable that in the "mark-time " reflex 

 the tail itself is alternately deflected to right and left, keeping time with the 

 stepping reflex. When the right limb begins to draw up in' flexion, and the 

 left limb to straighten out in extension, the tail begins to move from the 

 right to the left. The tail does really therefore participate in the locomotor 

 reflex, of which the stepping movement is also a part. Nocuous stimuli to 

 the side of the tail, e.g., by unipolar faradisation, evoke reflex abduction of 

 the tail from the side stimulated, and the organ is then usually kept abducted 

 for a time, just as the hind paw is drawn up and kept so for a time when 

 excited by similar stimulation. Moreover, such stimulation of the tail 

 excites reflex movement not only of the tail but of the hind limb, and the 

 limb's movement is usually extension at hip and knee. The result of this is, 

 that the tail stimulus can inhibit a flexion-reflex of the hind limb. If the 

 flexion-reflex be induced by inserting a hedgehog spine into the planta, and 

 if while that prolonged reflex is in progress and the limb is remaining 

 thoroughly flexed at hip and knee, a caudal skin-point is faradised, the limb 

 at once drops into the extended attitude under gravity. This occurs when 

 the nerves to the extensors to the hip and knee have been severed. The 

 tail stimulus therefore inhibits the flexors of knee and hip. 



The particular mode in which the tail-stimulus comes to inhibit the 

 stepping reflex seems to be that it inhibits the rhythmic flexion of the hip, 



