On Reciprocal Innervation of Antagonistic Muscles. 553 
effect, because the nerve of that muscle had been severed above its place of 
stimulation; indeed, all the nerves of the limb had been severed except only 
that of the vasto-crureus (extensor muscle) itself. 
The result, therefore, if traceable to centripetal impulses, is traceable to 
some among them which have their origin in vasto-crureus itself. In accord 
with this one finds (fig. 1, Z.7.1) that on simply taking with the hand the 
desensitised limb below the knee, and gently but firmly flexing the tonically 
Fia. 1. 
extended knee, the same result is obtained as that just described when the knee 
is bent by the flexor muscle. The forced movement, as executed by manipu- 
lation, experiences resistance from the tonic contraction of the extensor (vasto- 
erureus), but as the flexion movement is executed this resistance somewhat 
abruptly lessens almost to vanishing ; on then discontinuing the forced flexion 
the limb is found to remain practically where the forced movement (figs. 24, 
4a, L.r.) had carried it. In short, the extensor muscle, in result of a forced 
stretch, assumes in permanence a new tonic length. This reaction of the 
extensor may, for brevity, be termed the “ lengthening reaction.” 
The tonus exhibited by the extensor in decerebrate rigidity has been shown 
to be reflex in nature,* and to depend on afferent nerve-fibres arising in the 
extensor muscle itself.t The lasting increase in the tonic length of the muscle 
brought about by stretching it appears to be likewise reflexly produced. 
Evidence indicating this is as follows:—(1) The lengthening reaction is not 
obtained after severance of those afferent spinal roots through which pass the 
afferent nerve-fibres proceeding from the muscle (vasto-crureus) itself. 
(2) Together with the “lengthening reaction ” of the extensor, itself stretched, 
* Sherrington, ‘Journ. of Physiology,’ vol. 22, 1898. 
+ Ibid. 
