CH. XI.] 



EXTENSIBILITY OF MUSCLE. 



133 



If the muscle is slowly loaded and slowly unloaded, the curva- 

 ture 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 com- 

 plicated 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 de- 

 scribes a straight line ; at 

 the apex of the tracing 

 unloading began and went gl 

 on steadily till the spring 

 once more regained its 

 initial length. The up- 

 stroke, like the downstroke, 

 is a straight line. In B 

 and C muscles were used ; 

 it will be noticed that the 

 muscle docs not regain its 

 original length after unload- 

 ing, and that after unload- 

 ing the upward tendency 

 of the tracing 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 of the period of unloading shows 

 more curvature. 



This introduces us to what is 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 



C 



Fig. 155. Curves of extensibility. (Brodie.) 



