282 Prof. C. 8. Sherrington. [Apr. 17, 
former, and vice versd. The skin nerves in question all excite reflex contrac- 
tion of the hamstring muscles. I have shown* experimentally that massage 
and other mechanical treatment of the exposed hamstring muscles them- 
selves discharge centripetal impulses up their afferent nerves, and that these 
centripetal impulses inhibit the tone of the antagonistic extensor muscle. 
Verwornt has demonstrated the same thing for the flexors of the ankle in 
respect to the extensor digitorum pedis communis, the antagonist to the flexor. 
There are experimental grounds for thinking that the contractive activity 
of the hamstring muscles themselves discharges centripetal impulses up 
their afferent nerves, and that these impulses will reinforce the inhibitory 
action of the skin-reflex upon the extensor muscles which is antagonistic 
to the flexors set in action. The reflex inhibitory action of the skin nerves 
in question will in that way be supplemented by the afferent nerves of 
the muscles that the stimuli to the skin excite reflexly to contraction, and 
all this will occur together as a spinal reflex. The reflex arcs arising in the 
skin, and the reflex arcs arising in the muscles which those cutaneous arcs 
excite, are, therefore, synergic; they are what I have termed “allied arcs,’t 
and their actions mutually reinforce. | 
IV. The knee-jerk, though there are objections to considering it a true 
reflex, is, nevertheless, dependent on the integrity of a reflex spinal are. 
It was shown that this reflex arc in the case of the vastus medialis and 
crureus—the chief muscles involved in the jerk—arises in and returns to 
end in that very muscle itself,§ and that all other reflex arcs are inessential 
to the phenomenon. Hence, regarding the knee-jerk as an index of the 
reflex spinal tonus of the muscle, it was argued|) that the reflex arc which 
maintains the tonus of the extensor muscle arises in that very muscle itself. 
But not all authorities allow that the knee-jerk is of itself a true index 
of the reflex tonus. Now in “ decerebrate rigidity’ the tonus of the extensor 
muscle is so great that it is of itself, without appeal to the phenomenon 
of the knee-jerk, easily observable by mere inspection. Using the vastus 
medialis aud crureus under decerebrate rigidity, I find that the tonus of 
those muscles—which for the present purpose can, without artificiality, be 
considered one muscle—persists practically unaltered after severance of all 
the afferent nerves of both hind limbs, excepting only the nerve of that 
muscle itself, either in one limb or in both. On then cocainising or severing 
the afferent fibres from that muscle itself, but leaving intact the efferent 
* ‘Roy. Soc. Proc.,’ vol. 52, p. 556, 1893. 
+ ‘Arch. f. Physiologie,’ Supplem. Band, 1900, p. 117. 
{ ‘Brit. Assoc. Reports,’ Cambridge, 1904, Address to Section I. 
§ ‘Roy. Soc. Proc.,’ vol. 52, p. 557, 1893. 
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