THE PHENOMENON OF CONTRACTION. 21 



of its elasticity. In our bodies the muscles stretched from bone 

 to bone are, in fact, in a state of elastic tension. If a muscle is 

 severed by an incision across its belly the ends retract. The 

 extensibility and elasticity of the muscles add to the effective- 

 ness of the muscular-skeletal machinery. A muscle that is in a 

 state of elastic tension contracts more promptly and more effec- 

 tively for a given stimulus than one which is entirely relaxed. 

 Moreover, in our joints the arrangement of antagonists — flexors 

 and extensors — is such that the contraction of one moves the 

 bone against the pull of the extensible and elastic antagonist. 

 It would seem that the movements of the skeleton must gam 

 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 



F^K. 4. — n. Curve of extension of a rubber band, to show the equal extensions forequal 

 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 ordinate? 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 e.xtension 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 hmits 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 



