CHAP, i.] THE SPINAL COED. 715 



ments. This resistance, which is quite independent of, that is 

 to say, which may be clearly recognized in the absence of all 

 distinct muscular contractions of volitional or other origin, is 

 an expression of muscular tone, of the effort of the various 

 muscles to maintain their ' natural ' length. In many cases of 

 disease this resistance is felt to be obviously less than normal; 

 the limb is spoken of as "limp" or "flabby"; or as having 4 a 

 want of tone.' In other cases of disease, on the other hand, 

 this resistance is markedly increased; the limb is felt to be stiff 

 or rigid; more or less force is needed to change it from a flexed 

 to an extended, or from an extended to a flexed condition; and, 

 in the range of disease, we may meet with very varying amounts 

 of increased resistance, from a condition which is only slightly 

 above the normal to one of extreme rigidity. In some cases 

 the condition of the muscle is such as at 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 shew the funda- 

 mental identity of the skeletal tone with the more obvious 

 arterial tone. 



Clinical experience then shews 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 diminu- 

 tion or loss of tone, in other cases to exaggeration of tone, mani- 

 fested 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 shew the exist- 

 ence 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 influ- 

 ence proceeding from the spinal cord is shewn by dividing the 

 sciatic nerve on one side; the hind limb on that side now hangs 

 quite helpless. This more pendent position shews that some of 

 the flexors have lengthened in consequence of the section of the 



