126 EXTENSIBILITY, ELASTICITY, AND WORK OF MUSCLE [CH. XI. 



elastic, i.e., it opposes no great resistance to external force; it is, 

 however, perfectly elastic; that is, it returns to its original shape 

 very exactly after stretching. This is true in the case of living muscle 

 within the body, but after very great stretching even in the body, 

 and still more so after removal from the body, when it begins to 

 undergo degenerative changes culminating in death, its elasticity is 

 less perfect. 



The cohesion of muscular tissue is less than that of tendon. 

 E. Weber stated that a frog's muscle one centimetre square in 

 transverse section will support a weight of a kilogramme (over 

 2 Ibs.) without rupture, but this diminishes as the muscle gradually 

 dies. 



The extensibility of any material may be studied and recorded by 

 measuring the increase of length which occurs when that material is 

 loaded with different weights. In Helmholtz's myograph (fig. 140), 

 different weights may be placed in the scale-pan beneath the muscle, 

 and the increase of length recorded on a stationary blackened cylinder 

 by the downward movement of the writing point ; the cylinder may 

 then be moved on a short distance, more weight added, and the 

 additional increase of length similarly recorded, and so on for a 

 succession of weights. 



If this experiment is done with some non-living substance, like 

 a steel spring or a piece of india-rubber, instead of a living muscle, 

 it is found that the amount of stretching is proportional to the weight ; 

 a weight = 2 produces an extension twice as great as that produced 

 by a weight = 1 ; in this way one obtains a tracing like that seen on 

 the left hand of figure 154, and the dotted line drawn through the 

 lowest points of the extensions is a straight one. 



With muscle, however, this is different ; each successive addition 

 of the same weight produces smaller and smaller increments of ex- 

 tension, and the dotted line obtained is a curve. 



A continuous curve of extensibility may be obtained by placing 

 a gradually and steadily increasing force beneath the muscle instead 

 of a succession of weights added at intervals. The most convenient 

 way of doing this is to use a steel spring, which is gradually and 

 steadily extended; and the writing point connected to the muscle 

 inscribes its excursion on a slowly moving cylinder. If, then, after 

 the muscle has been stretched, the steel spring is gradually and 

 steadily relaxed, the muscle retracts and again writes a curve now in 

 the reverse direction, until it regains its original length.* But in 

 muscles removed from the body, unless they are very slightly loaded, 

 the return to the original length is never complete ; the muscle is 



* A mathematical examination of these curves shows that they are not rect- 

 angular hyperbolas as they were once considered. They are very variable in form, 

 ana cannot be identified with any known mathematical curve. 



