CHAPTER XI 



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 



Elastic Band 



Muscle 



FIG. 154. (After Waller.) 



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 



125 



