PHYSIOLOGICAL PROPERTIES OF THE MUSCLES. 459 



elasticity is brought into play in opposing muscles or sets of 

 muscles ; one set acting to move a part and extend the 

 antagonistic muscles, which, by virtue of their elasticity, 

 retract when the extending force is removed. Their tonicity 

 is an insensible, and more or less constant contraction, by 

 which the action of opposing muscles is balanced when both 

 are in the condition of what we call repose. Their sensibil- 

 ity is peculiar, and is expressed chiefly in the sense of fatigue, 

 and in the appreciation of weight and resistance to contrac- 

 tion. Their contractility, or irritability is the property which 

 enables them to contract and exert a certain amount of 

 mechanical force under the proper stimulus. All of these 

 general properties strictly belong to physiology, as do some 

 special acts that are not necessarily involved in the study 

 of ordinary descriptive anatomy. 



Elasticity of Muscles. The true muscular substance 

 contained in the sarcolemma is eminently contractile ; and 

 though it may possess a certain degree of elasticity, this 

 property is most strongly marked in the accessory anatomi- 

 cal elements. The interstitial fibrous tissue is loose and pos- 

 sesses a certain number of elastic fibres, and, as we have 

 seen, the sarcolemma is very elastic. It is probably the sar- 

 colemma that gives to the muscles their retractile power after 

 simple extension. 



It is unnecessary to follow out in detail all of the numer- 

 ous experiments that have been made upon the elasticity 

 of muscles. There is a certain limit, of course, to their 

 perfect elasticity (understanding by this the degree of ex- 

 tension that is followed by complete retraction), and this 

 cannot be exceeded in the human subject without dislocation 

 of parts. In some late experiments by Marey, it was found 

 that the gaetrocnemius muscle of a frog, detached from the 

 body, could be extended about one-fiftieth of an inch by a 

 weight of a little more than three hundred grains. This 

 weight, however, did not extend the muscle beyond the 



