160 
On Reciprocal Innervation of Antagonistic Muscles.— 
Seventh Note. 
By C. 8. SHERRINGTON, F.R.S. 
Physiology Laboratory, University of Liverpool. 
(Received February 28,—Read April 6, 1905.) 
In a previous note* on this subject, instances were pointed out in which 
the passive posture already obtaining in a limb influences the character 
of the spinal reflex elicitable from the limb. It was there shown how 
passive flexion, for instance, favours reflex extension. On resorting to postures 
assumed and maintained not passively but actively a like influence is evident. 
A reflex described in the previous note gives good illustration of this. Light 
pressure applied to the planta of the spinal dog evokes a brisk extension of 
the limb at hip, knee, and ankle. The pressure applied js such as resembles 
that which the weight of the dog would, in its own step, apply to the planta 
on contact with the ground. This reflex—which may be termed the 
“extensor thrust ’—employs the antagonist muscles to those employed by the 
well-known flexion reflex provocable by noxious stimuli apphed to the planta, 
eg., by a prick. This latter reflex, the flexion reflex, although it throws into 
action a group of muscles different from that thrown into action by the 
extensor thrust, yet exerts a marked influence over that reaction. If the 
extensor thrust be compared as elicited before and after a prolonged flexion 
reflex, the “thrust” reflex is found more facile and vigorous just after the 
flexion than it was before it. 
Again, if the crossed extension reflex of the limb be examined before and 
after a prolonged flexion reflex a similar alteration is evident init. Whena 
carefully adjusted electrical stimulus is at regular intervals applied to the 
afferent path of one limb and the resultant extensor reflex of the crossed 
limb is noted, it is found that if in one of the intervals a flexion reflex of the 
latter limb is induced and maintained for 15 seconds or more, the extensor 
reflex becomes altered in consequence. For a period immediately following 
the flexion reflex the extension reflex is increased. The intensity of the reflex 
is heightened its duration is prolonged, and its latent time is reduced. If the 
testing stimulus be subliminal the threshold value of the stimulus required 
by the reflex is found lowered. In short the activity of the flexion arcs 
* © Roy. Soc. Proc.,’ vol. 66, p. 67, 1900. 
