870 THE SPINAL CORD. 



The lengthening which some muscles have been proved to exhibit after 

 separation from the motor root cells, may be taken as evidence that, in the state 

 which is called "rest," their fibres are not perfectly at rest in the sense of 

 devoid of contraction. Their state is then, moreover, due to indirect ex- 

 citation through the nervous system. The sensible changes in the environ- 

 ment are so numerous and frequent, that an unceasing reflex activity of the 

 nervous system is not surprising. It is, however, siirprising, on the A r iew 

 generally taken, to find that in regard to the skeletal musculature the reflex 

 effect takes the form of an equable, minute, continuous contraction. The 

 fundamental observation on this subject is Brondgeest's 1 experiment. 



In addition to its neural tonus, the muscle has of course, in common with 

 all living cell tissue, its vegetative tonus, on which Virchow has laid stress. 

 The doctrine of a spinal tonus expressed by muscles is older than Brondgeest's 

 experiment. J. Miiller wrote in his text-book (1837), "The muscles, even 

 in the condition of rest, are continually being acted on by their nerves." 

 Marshall Hall, two years later, pointed to the difference between the tension 

 of muscles, in the decapitated rabbit when the cord is destroyed, and 

 when undestroyed. In the " spinal " animal the tonus of the muscles, 

 especially of some muscles, is less than in the normal condition ; but tonus does 

 still exist. As a rule, it may be said that the " spinal " tonus of the flexors is 

 greater than those of the extensors ; in other words, a spinal transection de- 

 presses the tonus of the extensors more than that of the flexors, at least at first. 

 This is seen not only in the spinal mammal, but in the crustacean and the 

 insect. 2 It is typically illustrated by Brondgeest's fundamental experiment. 



Chloroform and ether abolish this neural influence ; so also does severance 

 of the afferent roots. The neural influence is exerted chiefly on the muscles 

 from which reflex contraction can in the spinal animal be evoked first and 

 with most facility. Great depression of the tonus of the flexors is pro- 

 duced by section of afferent spinal roots alone, as well as by section of the 

 efferent. 3 When the afferent spinal roots of a limb have been severed, the 

 muscles of the limb feel flaccid, more flaccid than the muscles of the spinal 

 animal. Some influence acts on the cord through the afferent roots causing 

 portions of the limb musculature to be to a certain degree steadily braced and 

 more tightly strung than when completely denervated. The influence does 

 not, however, originate in the cutaneous sense organs, 4 or at least there 

 only in minor degree. Its maintenance appears to be one of the functions 

 — a subconscious function — of the organs of "muscular sense." The severance 

 of the afferent roots of a limb, besides causing ataxy of its "intended move- 

 ments," renders certain of them impossible; 5 besides these obviously sensual 

 impairments of the local skeletal musculature, are others bearing more on sub- 

 conscious reflex influence on the muscles. The section of the afferent roots 

 delays the onset of rigor mortis 6 much as does section of their motor nerves. 

 It also acts upon the extensor muscles of the limbs, for, after transection in 

 front of the mesencephalon, decerebrate rigidity 7 does not occur in the limb 

 the afferent roots of which have been divided. 



The knee-jerk and allied phenomena. — The reflex neural tonus of 

 the skeletal musculature influences not only the resting length and 

 tension of the muscles, but also their direct excitability to a particular 

 form of mechanical stimulation. When the leg is held supported in a 



1 See p. 798, supra; "Acad. Preisschrift," Utrecht, 1860. 



2 Cf. J. Loeb and Bethe, op. cit., supra. 



3 Marshall Hall maintained (1S39) that tonus and reflex action are but modifications of 

 one and the same function of the cord. 



4 Mommsen, Virchow's Archiv, Bd. ci. S. 22. 



6 Exner and Pineles, Arch. f. d. gcs. Physiol., Bonn, Bd. xlviii. S. 17; Mott and 

 Sherrington, Proc. Roy. Soc. London, 1895, vol. lv. 



"Sherrington, ibid., 1893, vol. Hi. 



7 Sherrington, Journ. Physiol., Cambridge and London, 1897, vol. xxii. 



