486 



Prof. C, S. Sherrington. 



[Jan. 31, 



It is easy to evoke reflex-extension of the hind limb by stimulation of the 

 skin of the opposite hind limb. With the spinal dog laid on its side (e.g., left) 

 and a thread attaching the knee of the slightly flexed right limb to a recording 

 lever, the delivery of a certain stimulus (e.g., 15 break-shock at 30 per 

 second) by unipolar faradisation through a gilt needle at a skin-point of the 

 left foot, evokes reflex-extension at right hip and knee. If this stimulus, at 

 moderate and unchanged intensity, be given at regular interval {e.g., once per 

 minute) a series of extension reflexes of regular height and duration are 

 obtained. If in the course of such a series the right limb is, during one of 

 the 60-second intervals, thrown into strong reflex flexion (e.g., by faradisation 

 of the skin of its own foot and the reflex flexion be maintained for some 

 time, e.g., 40 seconds) the next extension-reflex following on the intercurrent 

 flexion differs from those prior to it in being more ample and more prolonged 

 (figs. 5 and 6). Its after-discharge is greatly increased and its latency is 

 sometimes diminished. If the test stimulus for the extension-reflex be 

 adjusted at just subliminal value, the intercurrent flexion-reflex will make it 

 supraliminal. The exaltation of the extension-reflex may remain perceptible 

 for five minutes ; in the example furnished in fig. 5 it is quite recognisable 

 for four minutes. 



The mode of production of this exaltation seems the same as that traced 

 above in the knee-extensor, with the knee-jerk as test-sign, and in the 

 stepping reflex after interruption by tail inhibition. It has been shown that 

 central inhibition of the extensor of the knee is part and parcel of the 

 homonymous flexion-reflex of the leg. In the present case, therefore, 

 during the intercurrent flexion-reflex, the reflex arc of extension was under 

 inhibitory depression. After discontinuance of that inhibition the extensor 

 reflex is found exalted to a degree of activity beyond that which it showed 

 prior to the inhibition. 



A similar successive spinal induction is evident in the following. The 

 spinal animal (cat) being supine, the nerves of one hind limb are severed 

 save for the nerve to vasto-crureus. The limb is supported with femur 

 vertical and anticrus therefore fairly flexed at knee. The central end of the 

 hamstring nerve is farad ised ; this causes the usual reflex inhibition of 

 vasto-crureus, an inhibition which, if there be little or no tonus in that 

 muscle at the time, is difficult to detect by mere inspection, though easily 

 revealed by abolition of the knee-jerk. If the faradisation of the central 

 end of the hamstring nerve be continued for some seconds, e.g., three, on its 

 cessation there often ensues a marked reflex extension of the knee. This 

 is no mere return to previously existent slight tonus ; it is a fairly intense 

 contraction of the vasto-crureus, often sufficient to extend the knee fully 



