CH. X.] 



CURVES OF EXTENSIBILITY 



109 



then be moved on a short distance, more weight added, and the 

 additional increase of length similarly recorded, and so on for a 

 succession of weights. 



If this experiment is done with some non-living substance, like 

 a steel spring or a piece of india-rubber, instead of a living muscle, 

 it is found that the amount of stretching is proportional to the weight ; 

 a weight = 2 produces an extension twice as great as that produced 

 by a weight = 1 ; in this way one obtains a tracing like that seen on 

 the left hand of figure 127, and the dotted line drawn through the 

 lowest points of the extensions is a straight one. 



Elastic Band 



Muscle 



FIG. 127. (After Waller.) 



With muscle, however, this is different ; each successive addition 

 of the same weight produces smaller and smaller increments of ex- 

 tension, and the dotted line obtained is a curve. 



A continuous curve of extensibility may be obtained by placing 

 a gradually and steadily increasing force beneath the muscle instead 

 of a succession of weights added at intervals. The most convenient 

 way of doing this is to use a steel spring, which is gradually and 

 steadily extended; and the writing-point connected to the muscle 

 inscribes its excursion on a slowly moving cylinder. If, then, after 

 the muscle has been stretched, the steel spring is gradually and 

 steadily relaxed, the muscle retracts and again writes a curve now in 

 the reverse direction, until it regains its original length.* But in 

 muscles removed from the body, unless they are very slightly loaded, 

 the return to the original length is never complete; the muscle is 



* A mathematical examination of these curves shows that they are not rect- 

 angular hyperbolas as they were once considered. They are very variable in form, 

 and cannot be identified with any known mathematical curve. 



