CHAP. L] THE SPINAL CORD. 925 



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 

 contractions 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 toneless. In 

 the warm blooded animal, as we have said, the effects of shock are 

 much more lasting than in the cold blooded animal; 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 mani- 

 fested 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 ( 591) 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 matter of words whether we speak of the maintenance 

 of tone as an automatic or as a reflex action of the cord. We may, 

 however, distinguish the part played by the afferent impulses in 

 assisting the cord to a condition in which it is capable of 

 maintaining tone from the part played by an afferent impulse in 

 causing a reflex action ; in the former the action of the afferent 



