MUSCLE 549 



and a curve plotted with the weights for abscissae and the amounts 

 of elongation for ordinates would be a straight line. But this is not 

 a fundamental distinction between animal tissues, and the materials 

 of unorganized nature, as some writers seem to suppose. For when 

 the slow after-elongation which follows the first rapid increase in 

 length in the loaded, excised muscle is waited for, the curve of 

 extensibility comes out a straight line (Wundt), and within limits 

 this is also the case for human muscles in the intact body. And 

 although a steel rod much more quickly reaches its maximum elon- 

 gation for a given weight when loaded, and its original length when 

 the weight is removed, than does a muscle, time is required in both 

 cases, and the difference is one of degree rather than of kind. 



Dead muscle is less extensible and much less elastic than living. 

 In the state of contraction the extensibility is increased in excised 

 frog's muscle. When fatigue comes on after many excitations, the 

 after-elongation becomes more pronounced, but the return after un- 

 loading is very incomplete. Bonders and Van Mansveldt have found 

 that contraction causes little difference in the muscles of a living man, 

 although fatigue increases the extensibility. The great extensibility 

 and elasticity of muscle must play a considerable part in determining 

 the calibre of the vessels, and in lessening the shocks and strains 

 which the heart and the vascular system in general are called upon 

 to bear, and must contribute much to the smoothness with which the 

 movements of the skeleton are carried out, and immensely reduce 

 the risk of injury to the bones as well as to the muscles themselves, 

 the tendons and the other soft tissues. And not only is smoothness 

 gained, but economy also ; for a portion of the energy of a sudden 

 contraction, which, if the muscles were less extensible and elastic, 

 might be wasted as heat in the jarring of bone against bone at the 

 joints, is stored up in the stretched muscle and again given out in its 

 elastic recoil. The skeletal muscles, too, are even at rest kept 

 slightly on the stretch, braced up, as it were, and ready to act at a 

 moment's notice without taking in slack. This is shown by the fact 

 that a transverse wound in a muscle 'gapes,' the fibres being retracted, 

 in virtue of their elasticity, towards the fixed points of origin and 

 insertion. 



If a muscle is so overweighted that it cannot contract, it elongates 

 slightly on stimulation (Weber's paradox). This has by some been 

 held to indicate that the increase of extensibility associated with con- 

 traction still occurs in the excited state when actual contraction is 

 mechanically prevented. 



In the further study of muscle it is necessary first of all to consider 

 the means we have of calling forth a contraction in other words, the 

 various kinds of stimuli. 



Stimulation of Muscle. A muscle may be excited or 

 stimulated either directly or through its motor nerve ; and 

 the stimulus may be electrical, mechanical, chemical, or 

 thermal. Electrical stimuli are by far the most commonly 



