736 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES 



less and less extension. If, for instance, the sartorious or semi-mem- 

 branosus of a frog be connected with a lever writing on a blackened 

 surface, and weights increasing by equal amounts be successively 

 attached to it, the recording surface being allowed to move the same 

 distance after the addition of each weight, a series of vertical lines, 

 representing the amount of each elongation, will be traced. When the 

 lower ends of all the vertical lines are joined, a smooth curve with the 

 concavity upwards is obtained (Fig. 243). This is a property common 

 to living and dead muscle and to other animal structures, such as 

 arteries. Marey's method, in which the weight is continuously in- 

 creased from zero and then continuously decreased to zero again by 

 the flow of mercury into and out of a vessel attached to the muscle, 

 gives directly the curve of extensibility. 



The elongation of a steel rod or other 

 inorganic solid is proportional within 

 limits to the extending force per unit of 

 cross-section ; 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 dis- 

 tinction 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, 



243 Curves of Extensi- excised muscle is waited for, the curve of 

 bility. M, of muscle; S, of an extensibility comes out a straight line 

 ordinary inorganic solid. (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 

 elongation 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. When muscle 

 (striated or smooth) is not stretched beyond the limit of physiological 

 relaxation, the amount of stretching is proportional to the weight, and 

 the same is true of all the simple tissues of the body (Haycraft). 



Dead muscle is less extensible than living, and its limits of elasticity 

 are much narrower. 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 unloading is very incomplete. Donders and Van Mans- 

 veldt 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 con- 

 siderable 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 ol a sudden contraction, which, if the muscles were less ex- 

 tensible 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 



