THE PHENOMENON OF CONTRACTION. 21 



It would seem that the movements of the skeleton must gain 

 much in smoothness and delicacy by this arrangement. The 

 physical advantages of the extensibility and elasticity of mus- 

 cular tissue are evident not only in the contractions of our volun- 

 tary muscles, but, as we shall see, in a striking way also in the 

 circulation, in which the force of the heart beat is stored and 

 economically distributed by the elastic tension of the distended 

 arteries. The extensibility of muscular tissue has been studied 

 in comparison with the extensibility of dead elastic bodies. With 



Fif?. 4. a. Curve of extension of a rubber band, to show the equal extensions for equal 

 increments of weight. The band had an initial load of 17 gms., and this was increased 

 by increments of 3 gms. in each of the nine extensions, the final load being 44 gms. The 

 line joining the ends of the ordinates is a straight line. 6, Curve of extension of a frog's 

 muscle (gastrocnemius). The initial load and the increment of weight were the same as with 

 the rubber. The curve shows a decreasing extension for equal increments. The line join- 

 ing the ends of the ordinates is curved. 



regard to the latter it is known that the strain that the body 

 undergoes is proportional, within the limits of elasticity, to the 

 stress put upon it. If, for instance, weights are attached to a 

 rubber band suspended at one end, the amount of extension of 

 the band will be directly proportional to the weights used. If 

 the extensions are measured the relationship may be represented 

 as shown in Fig. 4, the equal increments in weight being indicated 

 by laying off equal distances on the abscissa, and the resulting 

 extensions by the height of the ordinates dropped from each 

 point. If the ends of the ordinates are joined, the result is a 

 straight line. When a similar experiment is made with a living 

 muscle it is found that the extension is not proportional to the 

 weight used. The amount of extension is greatest in the begin- 

 ning and decreases proportionately with new increments of 

 weight. If the results of such an experiment are plotted, as 

 above, representing the equal increments of weight by equal 

 distances along the abscissa and the resulting extensions by ordi- 

 nates dropped from these points, then upon joining the ends of 



