112 EXTENSIBILITY, ELASTICITY, AND WORK OF MUSCLB [CH. X. 



contracted has exactly the same length as when uncontracted ; but 

 this is a matter of everyday experience ; if one tries to lift a weight 

 beyond one's strength, one fails to raise it, but nevertheless one's 

 muscles have been contracting in the effort ; they have not contracted 

 in the restricted sense of becoming shorter, but that is not the only 

 change a muscle undergoes when it contracts; the other changes, 

 electrical, thermal, chemical, etc., have taken place, as evidenced in 

 one's own person by the fact that the individual has got warm in his 

 efforts, or may even feel fatigue afterwards. 



But the paradox does not end here, for if diagram 130 is again 

 looked at, it will be seen that beyond the point P the two curves 

 cross ; in other words, the muscle may even elongate, due to increase 

 of extensibility, when it contracts. This is known after its discoverer 

 as Weber's paradox. The increase of extensibility of muscle during 



100 



150 



200 



250 



Contracted 

 Uncontracted --I- 



FIG. 130. 



contraction is protective and tends to prevent rupture in efforts to 

 raise heavy weights. 



Influence of Temperature on Extensibility. If a piece of iced 

 india-rubber is taken and stretched by a weight, its retractility when 

 the weight is removed is very small. If, now, when the weight is on 

 it, it is warmed at one point, as by placing the hand on it, its 

 retractility is increased and it contracts, raising the weight. Some 

 physiologists have considered that muscular contraction can be 

 explained in this way ; they have supposed that the heat formed in 

 muscular contraction acts like warmth -as applied to india-rubber. 

 This view is, however, incorrect. It is much more probable that 

 there is no causal relationship between the temperature-change and 

 the extensibility-change which occur when muscle contracts; both 

 are simultaneously produced by the stimulus. 



Moreover, the influence of heat on muscle is by no means the 

 same as that on india-rubber. This influence is not invariable, and 



