64 HUMAN PHYSIOLOGY 



Tone changes are noted in the heart as well as in the 

 organs in which smooth muscle is present. The heart, 

 like the stomach although within narrower limits, may 

 be a larger organ at one time than at another. If 

 we make use of any method to record the beating of the 

 heart we may find that the rhythmic beats do not rise 

 from a uniform but from a variable base-level. This is 

 to say that the degree of relaxation is sometimes more 

 profound than an average and sometimes less so. Tone 

 changes in skeletal muscle are not so readily apparent 

 as in the other kinds of contractile tissue but there is 

 no doubt that they occur. The consequences are, in 

 this case, somewhat different from those that have been 

 described. 



It has been shown that the value of tone changes such 

 as take place in smooth muscle consists largely in 

 adapting hollow viscera to their varying contents. 

 The skeletal muscles do not, as a rule, bend around 

 cavities; usually each one is arranged to cause motion 

 on the part of a bone. There can ordinarily be recog- 

 nized for each skeletal muscle another, or perhaps 

 more than one, adapted to counteract its tension and 

 to produce an opposite movement. So we speak of 

 antagonistic or opposing muscles in the skeletal system. 



Increase of tone in skeletal muscles has, as its main 

 result, the establishment of a greater rigidity and so 

 of a greater resistance to external forces applied to cause 

 motion at the joints. Where smooth muscle with 

 heightened tone tends to diminish the cavity of whose 

 wall it forms a part, skeletal muscle is known to be in 

 tonic contraction by the firmness that is noticed in the 

 affected region and by its own relative hardness. Good 

 posture depends on proper tone maintained by many 

 associated muscles. 



