1906.] 



On Innervation of Antagonistic Muscles. 



487 



and passing off again in three or four seconds' time. It sets in, in my 

 experience, not at the very moment of withdrawal of the inhibitory stimulus, 

 hut in the course usually of the first three seconds following that with- 

 drawal. A tap given to the patellar tendon appears sometimes to elicit it 

 when otherwise it would not ensue spontaneously. 



Eelated to this phenomenon seems the following. When a flexion reflex 

 of the hind limb is by appropriate stimulation continued for a long time in 

 the spinal dog, the flexion tends to be broken through* from time to time 

 by short-lasting explosive extensions of the limb, much resembling the 

 " extensor-thrust." In all these cases the extensor arc during the flexion- 

 reflex has been under prolonged inhibition, and the superactivity which 

 it shows under a test stimulus, the " spontaneous " discharge which it 

 exhibits on relief from the inhibition, and the explosive outbreak which 

 it gives when the inhibitory reflex is getting fatigued, all seem to be 

 evidences of " successive spinal induction " supervening as a rebound after 

 inhibition. 



The effect upon the direct flexion-reflex of an intercurrent extension 

 reflex is, in my experience, much less marked than the converse just 

 described. This may be due in part to my having used the crossed 

 extension-reflex and not a direct extension-reflex as the intercurrent reflex. 

 The crossed reflex is less potent and powerful than the uncrossed reflex.f 

 But the only homonymous extension-reflex of the limb available is the 

 "extensor-thrust," and that is so unmanageable, and especially is so little 

 capable of prolongation, that it was unsuited to this purpose. However, 

 an influence can be traced, and in other ways than by intercurrent extension 

 in a series of reflex flexions. Thus, with the spinal animal vertical, the hind 

 limbs are taken and kept fully extended at hip, knee, and ankle ; then usually, 

 in a short time, a strong flexion-reflex at hip and knee supervenes. Again, 

 if similarly one hind limb be strongly passively flexed at hip but strongly 

 extended at knee and kept in that posture for a short time, it is usual for any 

 attempt to passively extend the hip to elicit at once strong reflex contraction 

 of the flexors of the hip, preventing passive extension. 



Nevertheless, the greater inductive effect of flexion upon extension than 

 of extension upon flexion as examined at the knee-joint, seems, in my 

 experience, marked. In regard to it one remembers that though electrical 

 stimulation of the afferent nerve-fibres from the flexor muscles has been 

 shown to inhibit the reflex contraction of the extensor,! it has not been 



* ' Journ. of Physiol.,' vol. 34, p. 34, fig. 21, phase 3. 

 + Sherrington, ' Brit. Assoc. Eeports,' ibid. 

 I ' Roy. Soc. Proc.,' vol. 52, p. 556. 



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