THE PHENOMENON OF CONTRACTION. 



21 



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 beginning and decreases propor- 

 tionately with new increments of weight. If the results of such 

 an experiment are plotted, as above, representing the equal incre- 

 ments of weight by equal ^distances 

 along the abscissa and the resulting 

 extensions by ordinates dropped from 

 these points, then upon joining the 

 ends of the ordinates we obtain a curve 

 concave to the abscissa. At first the 

 muscle shows a relatively large exten- 

 sion, but the effect becomes less and 

 less with each new increment of weight, 

 the curve at the end approaching slowly 

 to a horizontal. If the weight is in- 

 creased until it is sufficient to overcome 

 the elasticity of the muscle the curve 

 is altered it becomes convex to the 

 abscissa, or, in other words, the amount 

 of extension increases with increasing 

 increments of weight up to the point 

 of rupture, as is shown in the accom- 

 panying curve* (Fig. 5). Haycraftf 

 calls attention to the fact that under 

 normal conditions the physiological 

 extension of the frog's muscles in the 

 body is equal to that produced by a 

 weight of 10 to 15 gms., and that 

 when the excised muscle is extended 

 by weights below this limit it follows 

 the law of dead elastic bodies, giving 

 equal extensions for equal increments 

 of weight. It is only after passing this limit that the law stated 

 above holds good. It should be added also that the amount of 

 extension exhibited by a muscle or other living tissue placed 

 under a stress varies with the time that the stress is allowed 

 to act. The muscle is composed of viscous material, and yields 

 slowly to the force acting upon it. In experiments of this kind, 

 therefore, the weights should be allowed to act for equal intervals 



* See Marey, "Du mouvement dans les fonctions de la vie," 1868, p. 284. 

 t Haycraft, "Journal of Physiology," 31, 392, 1904. 



JC 



Fig. 5. Curve given by 

 Marey to show the effect upon 

 the extension of muscle caused 

 by increasing the load regularly 

 to the point of rupture : From 

 o to a the extension of the 

 muscle decreases as the weight 

 increases, giving a curve concave 

 to the abscissa, ox; at a the 

 limit of elasticity is passed and 

 the muscle lengthens by in- 

 creasing extensions for equal 

 increments ; at x rupture (,750 

 gms. for frog's gastrocnemius). 



