716 KNEE-JERK. [Book hi. 



nerve, and this result may be taken as refuting the argument, 

 quoted above, against the existence of tone, which is based on 

 the statement that a muscle cannot be observed to lengthen 

 after section of its nerve. It may be here remarked that if the 

 brainless frog, whose hind limbs are more or less pendent when 

 the body is suspended, be placed on its belly the hind limbs are 

 brought into a flexed position under the body by means of 

 obvious muscular contraction; and from this it might be inferred 

 that the maintenance of the position of the pendent limb was 

 also the result of a feeble contraction. But no obvious contrac- 

 tions can be observed in the latter case, as in the former; and 

 when in the former the limb has once been brought into the 

 flexed position, that position, like the pendent position, is main- 

 tained without obvious contractions. As we said above ' tone ' 

 may pass into something which appears to be identical with a 

 contraction, but where no obvious contractions are observed it 

 seems preferable to speak of the state of the muscle as one 

 of tone. 



In the dog, after division of the cord in the thoracic region, 

 the hind limbs during the period of shock are limp and tone- 

 less. In the warm-blooded animal, as we have said, the effects 

 of shock are much more lasting than in the cold-blooded ani- 

 mal ; and in the dog the tone of the skeletal muscle returns 

 much more slowly than in the frog. Indeed when the division 

 of the cord has taken place low down the skeletal tone returns 

 very slowly, and may be manifested very feebly, or even be 

 absent altogether. But under favourable circumstances, when 

 a sufficient length of cord has been left, a fairly normal tone is 

 reestablished. In man, in accordance with the facts previously 

 mentioned (§ 464) skeletal tone, which has been lost through 

 the continuity of the cord being broken by disease or accident, 

 appears rarely if ever to return fully in the regions below the 

 lesion. 



We may therefore on the whole of the evidence conclude 

 that the maintenance of skeletal tone is one of the functions 

 of the cord ; but we may here repeat that the condition of the 

 cord, on which depends the issue from the cord along efferent 

 nerves of the influences, whatever their nature, which produce 

 tone in the muscle, may be, and indeed is, in its turn dependent 

 on afferent impulses. In the case of the frog quoted above 

 the tone of the pendent limbs disappears or is greatly lessened 

 when the posterior roots of the sciatic nerves are divided, 

 though the anterior roots be left intact. In the absence of 

 the usual stream of afferent impulses passing into it, the cord 

 ceases to send forth the influences which maintain the tone. 

 Hence the maintenance of tone presents many analogies with a 

 reflex action especially when we remember that, as stated above, 

 tone passes insensibly into contraction ; and it may seem a mere 



