ELASTICITY OF MUSCLES. 27 



in the vice (7, and is connected witli the lever E E, the 

 point of which touches the plate of smoked glass. The 

 weight of the lever is held in equipoise by the balance 

 II. When weights are placed in the scale-pan at F, the 

 lever moves upward, and its point marks a straight line 

 which affords opportunity for measuring the amount of 

 the extension. 



But in whatever way examined, muscle, in common 

 with all other soft bodies, exhibits another variation 

 from the bearing of rigid bodies. We have seen that 

 in steel or similar bodies the extension is exactly pro- 

 portionate to the weight applied ; that is to say, if a 

 given steel wire is extended one millimetre by one 

 kilogramme, then the amount of extension caused by 

 two kilogrammes is two millimetres, that by three kilo- 

 grammes is three millimetres, and so on. It is not so 

 in the case of muscle and other soft bodies. They are 

 comparatively more extensible by light than by heavy 

 bodies. For instance, if the extension of a muscle 

 when carrying ten grammes is five millimetres, when 

 carrying a weight of twenty grammes it is, not ten 

 millimetres, but perhaps only eight; when carrying 

 thirty grammes it is only ten millimetres, and so on. 

 The extension, therefore, becomes continually less as 

 the weight increases, and finally becomes unnoticeable 

 by the time that the point at which the muscle is torn 

 by the applied weights is reached. This behaviour is 

 of importance, because the conditions of elasticity play 

 an important part in muscular operations. The muscle 

 on contracting is capable of lifting aweigh-t. The same 

 weight, however, extends the muscle, and the co-opera- 

 tion of the two forces — the contractile tendency and 

 the elastic extension — produces, as we shall find, the 

 final operation on which labour depends. 



