924 TONE OF SKELETAL MUSCLES. [BOOK m. 



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 

 * 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 fundamental 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 diminution or loss of tone, 

 in other cases to exaggeration of tone, manifested often as con- 

 spicuous 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 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 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 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 



