70 On Reciprocal Innervation of Antagonistic Muscles. 
words, in this reflex there is developed in the extensor neurones a slight 
refractory phase, slight because though effectively blocking a weak inhibitory 
stimulus, it fails before the insistence of a strong one. Thus analogy is 
at once revealed between this reflex and one superficially very different 
from it, namely, the “ scratch-reflex.” In the latter a feature of co-ordina- 
tion is the occurrence of marked refractory phase. During the progress 
of the reflex the external stimulus which excites the contraction of the 
flexor muscles fails to excite them at rhythmically recurring periods owing 
to supervention of a refractory phase in the spinal centre. And the analogy 
the extensors of the limb as 
well as the flexors contract rhythmically,* and presumably their rhythmic 
33 
is the closer since in the “scratch-reflex 
contractions alternate with those of their antagonists. The recurrent 
refractory phase would therefore not only block the flexor excitatory 
stimulus, but at the same time block the extensor inhibitory stimulus. 
This is what it seems to be doing in the “step-reflex.” The difference 
between the two reflexes becomes ~one of quantity and period rather than 
any essentially qualitative one. 
The facts indicate that the reflex movement of stepping, with its two 
opposite phases of flexion and extension, can be excited by one single form of 
stimulus intermittently or even merely unequally applied, that stimulus 
being the one which directly excites flexion. This suggests an explanation 
for the striking inequality with which flexion and extension respectively 
are represented in the receptive field, both deep and superficial, of the limb 
itself. The direct stimulation, electrical or mechanical (2.e., ligation), of any 
afferent limb-nerve, whether cutaneous or deep, excites as its immediate 
result flexion of the limb itself, not extension.t Somewhat similarly, in the 
motor cortex (especially of the hind lmb) the primary representation of 
flexion greatly preponderates over that of extension. The explanation of 
* By appropriate isolation the following muscles in the cat are seen to exhibit the 
characteristic rhythmic contraction constituting the scratch-reflex—tensor fascie femoris, 
psoas magnus, gluteus minimus, front portion of gluteus maximus, pectineus, sartorius, semt- 
tendinosus, tibialis anticus, posterior portion of biceps femoris, flexor longus digitorum, 
adductor parvus, vasto-crureus, semimembranosus and anterior part of biceps femoris. 
Although most of these are flexor muscles, some, eg., the three last mentioned, are 
extensors. These three can be seen to contract simultaneously together in the rhythm of 
the reflex. For the rhythmic abduction which accompanies the rhythmic flexion of thigh 
in the reflex gluteus minimus seems chiefly responsible, since that rhythmic movement 
persists when all muscles of limb except gl. min. have been paralysed by severance of 
their nerves. This list probably does not exhaust the whole set of muscles exhibiting the 
rhythmic contraction of the reflex.—C. 8. S. 
+ ‘Integrative Action of the Nervous System,’ p. 291, 1906; ‘Roy. Soc. Proc.,’ B, 
vol. 76, p. 293, and B, vol. 79, p. 337. 
