MUSCLE 631 



efficient of elasticity is the quotient of the deforming force acting on 

 unit area of the given body by the deformation produced (within the 



TJ 1 T Tf 



limits of elasticity). In the above example it is -^-^,that is, -=-' 



the reciprocal of the extensibility e. For steel the coefficient of 

 elasticity is very large, for muscle small. Or, as we may otherwise 

 express it, living muscle within its limits of elasticity is very ex- 

 tensible ; a small force per unit area of cross-section of a prism of it 

 will produce a comparatively great elongation. The extensibility, 

 however, diminishes continually with the elongation, so that equal 

 increments of stretching force produce always less and less extension. 

 If, for instance, the sartorius or semi-membranosus 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. 219). 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 increased 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 FlG - 2I 9-~ ; 

 amounts of elongation for ordinates 



would be a straight line. But this is M, of muscle ; S, of an ordinary 

 not a fundamental distinction between inorganic solid, 

 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. When 

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

 rJhysiological 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 Mansveldt have found that contraction causes 



