20 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



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 effectiveness of the muscular-skeletal 

 machinery. A muscle that is in a state of elastic tension con- 

 tracts more promptly and more effectively for a given stimur- 

 lus 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 gain much in smoothness and 

 delicacy by this arrangement. The physical advantages of the 

 extensibility and elasticity of muscular tissue are evident not 



Fig. 4. n, Curve of extension of a rubber band, to show the equal extension 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. 



only in the contractions of our voluntary 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, as it were, by 

 the elastic tension of the distended arteries. The extensibility of 

 muscular tissue has been studied in comparison with the extensi- 

 bility of dead elastic bodies. With 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 relation- 

 ship may be represented as shown in the accompanying figure, 

 the equal increments in weight being indicated by laying off equal 



