1908. ] Innervation of Antagonistic Muscles. 555 
reaction of vasto-crureus, as it annuls likewise other inhibitory relaxations of 
that muscle known to be reflex in nature.* 
It appears, therefore, that the “lengthening reaction” of the extensor of 
the knee is a reaction excited by forced stretch of the muscle acting on the 
reflex tonus through receptive nerves arising in the muscle itself. 
II. There is similarly observable in the decerebrate extensor muscle 
another reaction practically the converse of the foregoing. Suppose that in 
the limb of the decerebrate animal, prepared as above, the knee, in conse- 
quence of the “lengthening reaction” just described, is remaining flexed. 
Suppose, then; some skin-surface or afferent nerve appropriate for evoking 
reflex extension of the knee is stimulated either mechanically or by faradisa- 
-tion. The vasto-crureus, isolated as above, contracts, and in consequence the 
knee is extended. After cessation of the application (figs. 2 and 3, a) of the 
stimulus, the knee, instead of falling back into flexion, remains extended, and 
may remain so for many minutes. Light is again thrown on the nature of 
this persistence of the extensor effect by comparing with it the effect of 
passive manipulation of the limb. Suppose that, as before, the limb starts 
with its knee flexed. Then if the observer simply take the desensitised limb 
below the knee, and move it with his hand into a posture of extension at 
knee, on his releasing the limb the knee does not drop back into the flexed 
posture ; it retains almost to the full the new extended posture given it by 
the manipulation (fig. 1, S.7v.). Passive approximation of the ends of insertion 
and origin of the vasto-crureus has caused that muscle to assume a new tonic 
length, shorter than it exhibited before. 
This same lasting change in the tonic length of the muscle results 
whether the initiatory approximation of the ends of the muscle be made by 
passive manipulation or by active contraction of the muscle itself. The 
maintenance of the extended knee-posture which follows a reflex contraction 
of vasto-crureus presents the same characters as the maintenance of posture 
following a simple passive lift of the knee. If during the application of the 
exciting stimulus, and for a second or two longer, the knee which the reflex 
would extend be mechanically prevented from extending, it does not on 
release assume the extended posture. Prevention of the shortening of the 
muscle in the first stage of the reflex prevents the occurrence of the long- 
maintained extension after-effect of the reflex, although that usually endures 
for minutes. 
The conclusion indicated is, therefore, that the shortening of the muscle, 
produced either actively or passively, induces it to assume in relative 
* Sherrington, ‘Roy. Soc. Proc.,’ B, vol. 76, 1905. 
