THE MUSCLES. 557 



all usual conditions. The muscles attached to the bones are in a 

 state of elastic tension which is favorable to the action of the muscle, 

 diminishing the danger of rupturing its fibers. The elasticity of 

 muscle favors the economical expenditure of work by the muscle. A 

 muscle is always taut, never in a state of relaxation, and it is then 

 ready to efficiently exert mechanical force the moment it begins to 

 contract. Heating to a certain extent increases and cooling decreases 

 elasticity. The curve of muscle, when stretched by weights, is not 

 a hyperbola, but one peculiar to muscle. 



In rigor 



In tetanus 



Normal 



Fatigued 



Fig. 202. Extensibility of Muscle in Various States. (WALLER.) 

 Tested by 50 grammes applied for short periods. 



Muscular Work. While treating of elasticity and its modifica- 

 ion, tonicity, it might be well to give a brief discussion upon mus- 

 ilar work. The amount of mechanical work which a muscle per- 

 >rms equals the product of the weight lifted and the height to which 

 le weight is lifted. Thus, the work = height X the weight. 



When a muscle begins to contract, it is then that it lifts the 

 iatest load; as the contraction continues, the muscle is capable of 

 fting less and less. 



