22 



THE PHYSIOLOGY OF MUSCLE AND NERVE. 



the ordinates we obtain a curve concave to the abscissa. At 

 first the muscle shows a relatively large extension, 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 increased 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 

 shown in the accompanying 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 incre- 

 ments of weight. It is only after pass- 

 ing this limit that the law stated above 

 holds good. It should be added also 

 that the amount of deformation ex- 

 hibited 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 vis- 

 cous 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 of time. It has been shown 

 that the extensibility of a muscle is greater in the contracted 

 than in the resting state. 



The curve of extension described above for skeletal muscle 

 holds also for so-called plain muscle. This latter tissue forms a 

 portion of the walls of the various viscera, the stomach, bladder, 

 uterus, blood-vessels, etc., and the facts shown by the above curve 

 enter frequently into the explanation of the physical phenomena 

 exhibited by the viscera. For instance, it follows from this curve 

 that the force of the heart beat will cause less expansion in an 

 artery already distended by a high blood-pressure than in one in 

 which the blood-pressure is lower. 



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

 fHaycraft, "Journal of Physiology," 31, 392, 1904. 



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; at a the limit 

 of elasticity is passed and the 

 muscle lengthens by increasing 

 extensions for equal increments; 

 at x rupture (750 gms. for frog's 

 gastrocnemius) . 



