CHARACTER OF THE MUSCULAR RESPONSE. 831 



and flexors of the knee. Each successive phase of the reflex move- 

 ment is therefore co-ordinate in this sense. 



The completion of the main movement executed in a spinal reflex is, 

 in the mammal, very frequently succeeded by a brief " twitch " of con- 

 traction. This after contraction is sometimes of considerable amplitude, 

 although " twitch "-like in brevity of duration; it is seen well at the 

 ankle-joint of the spinal rabbit after reflexes induced by skin stimuli 

 to the foot. This remaining after effect of a reflex tetanus excited from 

 the cord is often quite prolonged, 1 persisting even more than a minute. 



It has been often noticed that a reflex excited in the spinal animal 

 rapidly dwindles and ceases to be elicited if the stimuli applied at 

 a locus be repeated rather frequently. This kind of after effect has been 

 spoken of as fatigue, and in studying reflex actions by Tiirck's method 

 it is necessary to allow pauses of many minutes between the reapplica- 

 tions of the stimulus. Owing to the rule of spatial uniformity, it is 

 easy to bring the same motor apparatus into play from each of several 

 afferent channels. When, after the stimulus applied to one of these 

 channels has had its efficiency for producing the motor reaction greatly 

 depressed or set aside altogether by frequent repetition, if the incidence 

 of the stimulus be shifted to one of the other afferent channels playing 

 on the same motor apparatus, the latter may still perfectly respond. 2 

 This indicates that in the so-called " fatigue " of the reflex the block 

 occurs in the afferent channel. It occurs probably at the intraspinal 

 end of that channel, for the effect is seen when afferent nerve trunks are 

 stimulated, as well as when sensory surfaces are used ; and that nerve 

 fibres themselves are practically unfatigable has been established 

 (Bernstein, Bowditch, Waller, Wedenskii, and others). 



But when more natural stimuli are applied, the reflex movement 

 may persist much longer without becoming exhausted ; for instance, 

 the chelate appendage of an Astacus reduced to the spinal condition 

 has been observed to prosecute continuously for many hours its reflex 

 attempts to give the morsel it holds into the grasp of the maxillipedes. 3 



The contraction of the muscles in spinal reflexes is never, in my 

 experience, very forcible. No very great opposition is required to restrain 

 the movements, and the postures assumed by the limbs never attain the full 

 extremes of flexion possible under vigorous willed action, or under excitation 

 of the peripheral nerves. The average character of the contraction reflexly 

 elicited from different muscles is obviously different for different muscles, 

 e.g. that for the gastrocnemius and extensors of the hip is short and sharp as 

 compared with that for the flexors of the ankle and hip, both in the frog and dog. 



When a motor nerve is excited by induction shocks of not too high 

 frequency (e.g. 42 per sec), the contracting muscle vibrates at a similar rate of 

 frequence. 4 If the transection of the cord be excited instead of the motor 

 nerve, the muscle vibrates, but with a frequency independent of that of the 

 serial stimulus and far slower, e.g. about 10 per sec. 5 A similar rate of vibra- 

 tion had been seen in spinal movements under strychnin. 6 



1 de Boeck, Arch. f. Physiol., Leipzig, 1889. 



2 Sherrington, Phil. Trans., London, 1896. 



3 Bethe, Arch. f. d. ges. Physiol., Bonn, 1897, Bd. lxviii. 



4 Helmholtz, Monatsbl. Perl. Akad., 1864. 



5 Schafer and Horsley, Journ. Physiol., Cambridge and London, 1886, vol. vii. p. 100 ; 

 cf. v. Kries, Arch. f. Physiol., Leipzig, 1886 ; Kronecker and Stanley Hall, Arch. f. 

 Physiol., Leipzig, 1879, Snppl. S. 12, had previously obtained a much higher result. 



Loven, Nord. mcd. Ark., Stockholm, 1879 ; Centralbl. f. d. Tried. Wissensch., Berlin, 

 1881. 



