CHAPTEE X 



EXTENSIBILITY, ELASTICITY, AND WORK OF MUSCLE 



MUSCLE is both extensible and elastic. It is stretched by a weight, 

 that is, it possesses extensibility ; when the weight is taken off, it 

 returns to its original length, that is, it possesses elasticity. The two 

 properties do not necessarily go together ; thus a piece of putty is 

 very extensible, but it is not elastic ; a piece of steel or a ball of 

 ivory are only slightly extensible, but after the stretching force has 

 been removed they return to their original size and shape very 

 perfectly. 



A substance is said to be strongly elastic, when it offers a great 

 resistance to external forces ; steel and ivory are strongly elastic. 



A substance is said to be perfectly elastic, when its return to its 

 original shape is absolute ; again steel and ivory may be quoted as 

 examples. 



Muscle is very extensible, i.e., it is easily stretched ; it is feebly 

 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 b} 

 measuring the increase of length which occurs when that material is 

 loaded with different weights. In Helmholtz's myograph (fig. 113), 

 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 



108 



