THE AUTOMATIC ACTIONS OF THE SPINAL CORD. 581 



first sight seems much more comparable to a permanent ordinary contraction 

 than to a mere exaggeration of normal tone ; but all intermediate stages are 

 met with, and indeed these extreme cases may be taken as indicating that 

 the molecular processes which maintain what we are now calling tone are at 

 bottom of the same nature as those which carry out a contraction ; they 

 serve to show the fundamental identity of the skeletal tone with the more 

 obvious arterial tone. 



Clinical experience then shows that the central nervous system does 

 exert on the skeletal muscles such an influence as to give rise to what we may 

 speak of as skeletal tone changes in the central nervous system, leading in 

 some cases to diminution or loss of tone, in other cases to exaggeration of 

 tone, manifested often as conspicuous rigidity. The question why the 

 changes take one direction in one case and another in another is one of great 

 difficulty (the occurrence of extreme rigidity being especially obscure), and 

 cannot be discussed here. We have called attention to the facts simply 

 because they show the existence of skeletal tone and its dependence on the 

 central nervous system. This conclusion is confirmed by experiments on 

 animals, and these also afford proof that in animals the spinal cord can by 

 itself, apart from the brain, maintain the existence of such a tone. In a 

 frog, after division of the cord below the brain, the limbs during the period 

 of shock are flabby and toneless ; but after a while, as the shock passes off, 

 tone returns to the muscles, and the limbs offer when handled a resistance 

 like that of the limbs of an entire frog. When the animal is suspended the 

 hind limbs do not hang perfectly limp and helpless, but assume a definite 

 position ; and that this position is due to some influence proceeding from the 

 spinal cord is shown by dividing the sciatic nerve on one side ; the hind limb 

 on that side now hangs quite helpless. This more pendent position shows 

 that some of the flexors have lengthened in consequence of the section of 

 the 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 mus- 

 cular 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 contrac- 

 tion. 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 maintained without 

 obvious contractions. As we said above, "tone" may pass into something 

 which appears to be identical with a contraction, but where no obvious con- 

 tractions 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 divis- 

 ion 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 favorable 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 ( 504), skeletal tone, which has been lost through 

 the continuity of the cord beino; broken by disease or accident, appears 

 rarely if ever to return fully in the regions below the lesion. 



