CH. XI.] 



EXTENSIBILITY OF MUSCLE. 



135 



it shortens more, when a greater weight it shortens less ; till 

 when it shortens least it lifts the greatest weight. 



This experiment illustrates the general truth that when a 

 muscle is contracted it is more extensible. At the point P the 

 energy tending to shorten the muscle (its contractile power) is 

 exactly equal to the energy tending to lengthen it against its 

 elastic force. Thus we have the apparent paradox at this point 

 that a muscle when contracted has exactly the same length as 

 when uncontnicted ; 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 



200 



250 



Fig. 157- 



the effort; they have not contracted in the narrow sense of 

 becoming shorter, but that is not the only change a muscle 

 undergoes when it contracts ; the other changes, electrical, ther- 

 mal, 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 1 5 7 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. 



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 con- 



