110 EXTENSIBILITY, ELASTICITY, AND WORK OF MUSCLE [CH. X. 



permanently longer to a slight extent, which varies with the amount 

 of the previous loading. 



If the muscle is slowly loaded and slowly unloaded, the curvature 

 of its tracing is much more marked than if the experiment is done 

 rapidly. 



The following three tracings are reproduced from some obtained 

 by Dr Brodie. In the method used, the records are not complicated 

 by the curve of a lever, but the movement was simply magnified by 

 a beam of light falling on a mirror attached to the end of the muscle, 

 and reflected on to a travelling photographic plate. Each tracing is 

 to be read from right to left ; the first one (A) shows the result of 

 stretching a steel spring by a steadily increasing force ; the end of 



the spring gets lower and lower, 

 and describes a straight line; at 



^^^^^ ^^^^^ ^^^^^^ the apex of the tracing unloading 

 B9B&^^H7^^^^I began and went on steadily till 



the spring once more regained its 

 initial length. The upstroke, like 

 the downstroke, is a straight line. 

 In B and C muscles were used ; 

 it will be noticed that the muscle 

 does not regain its original length 

 after unloading is completed, and 

 the upward tendency of the tracing 



jj&^ISr^S ^ beyond this point represents after- 



retraction. In B, the extension 

 was applied rapidly, the tracing 

 is almost a straight line; in C, 

 the extension was brought about 

 more slowly, and the tracing is a 



__^^_ ^^^^^_^^^__ curve; in both cases the tracing 

 - BSfESBfcfe^V^^^^^I ^ the period of unloading shows 



more curvature. 



This introduces us to what is 



Fio. 123. Curves of extensibility. (Brodie.) ,, , ,, 



called after-extension and after- 

 retraction. That is to say, after a muscle is weighted there is an 

 immediate elongation, followed by a gradual elongation which 

 continues for some time ; or if a muscle has been weighted and is 

 then unloaded there is an immediate slackening, followed by a 

 gradual after-retraction. 



This may be shown by looking at the graphic records shown in 

 the next diagram. It will be noticed that the extension is greatest 

 when the muscle is in a contracted condition, and smallest when it is 

 dead (in rigor). In fatigue the after-extension is very marked, and 

 the return after unloading very imperfect. 



