556 PHYSIOLOGY. 



There are from 8 to 12 impulses for a slow movement and from 18 

 to 20 impulses per second for a rapid movement. Ten vibrations 

 per second may be taken as the average. 



Elasticity of the Muscle. Of all the properties of muscle, elas- 

 ticity is the one least well known, the one which is most difficult to 

 explain and understand. 



Physicists say that a body is perfectly elastic when, after having 

 been removed from its first position, it returns exactly to the original 

 position. Thus, an ivory ball is perfectly elastic; after it has been 

 flattened by an external force it returns exactly to its original shape. 



Fig. 201. Extensibility of Elastic Band and Muscle. (WALLER.) 



If a piece of rubber is stretched by adding successive weights it 

 is found that the series of elongations are nearly proportional to the 

 weights. When the weights are successively removed it will be found 

 that the elasticity of the rubber is nearly perfect. But if over- 

 weighted for a long time it does not return completely to its 

 original length, and the elasticity disappears gradually. If now you 

 take a frog's fresh muscle and successively load it, the extension of 

 the muscle for each weight is not proportional to the weight used, 

 but with each increase in weight the muscle stretches rather less, the 

 greater the previous extension. On removing the weights the muscle 

 shortens but it does not return to its original length. A contracted 

 muscle is more extensible than a resting one. This prevents a rup- 

 ture of the muscle in a sudden contraction. 



Muscular elasticity preserves the tension of the muscle under 



