PHYSIOLOGY. 77 



think it is too much, but if you consider how much it is con- 

 nected with the other parts of our subject, you will not think 

 so. I proceed to some of the laws and circumstances of mus- 

 cular action, though we cannot arrive at the fundamental 

 theory." 



CI. In living and healthy animals, the muscular fibres have 

 a constant tendency to contract ; and this tendency we call their 



TONE, Or TONIC POWER. ^ 



" The constant tendency of muscular fibres to contract in liv- 

 ing bodies, is well illustrated by the gaping of wounds in mus- 

 cles ; and many experiments show, that, whatever takes off the 

 stretching powers, gives occasion to the shortening of muscular 

 fibres. This constant tendency to contract we call their Tone, 

 or Tonic Power. The term tone has been employed in a loose 

 manner, but this is the strict and proper meaning to be affixed 

 to it." 



CII. The tonic power of muscular fibres necessarily supposes 

 their being constantly in a state of extension beyond their natu- 

 ral or most contracted state ; and in this state they are con- 

 stantly kept (1) by the action of antagonist muscles, (2) by the 

 weight of the parts they sustain, (3) by fluids distending the 

 cavities they surround, and (4) by their connexion with such 

 distended cavities, particularly the blood-vessels. 



" If the muscles show this constant tendency to contract 

 upon taking off the stretching powers, they must be constant- 

 ly in an extended state, which I can show to be beyond their 

 natural, and it is of some consequence to inquire by what means 

 they are kept in this state. I cannot enumerate all the causes, 

 but here certainly are the principal ones: (1)1 say that they are 

 constantly kept in this state by the action of antagonist muscles. 

 In whatever manner we account for the original extension of 

 muscles, which is probably from the growth of the bones, the 

 fact is so, that the muscles are constantly in the extended state 

 of which I speak, and that chiefly by their being provided with 

 antagonists which have a contrary tendency ; which appears 

 from this, that, if you cut through a muscle, its antagonist is 

 contracted. But these are manifestly not of equal strength ; 

 and, therefore, the one is sometimes more extended or contract- 

 ed than the other, according to the position that is most favour- 



