THE MECHANICAL CHANGES OF MUSCLE 221 



muscle with 40 grams 1 millimetre from the axis than with 1 gram 40 millimetres 

 from the axis, though the tension put on the muscle will be the same in both cases. 

 In the first case the energy of the moving mass will be proportional to 



= 800, and it is this energy which 



40 x (I) 2 1 x 



; = 20, and in the second to 



determines the overshooting of the lever and the deformation of the curve. 

 Since throughout the contraction the lever follows the muscle in its movement, 





, , TO DRUM 3SCM -. 



u 



Q 



D 



FIG. 55A. 



Myograph for optical registration of muscular 

 contraction. (K. LUCAS.) 



the tension on the muscle remains the same throughout, and the method is 

 therefore known as the isotonic method. 



In many cases it is of importance to be able to record the development of 

 the energy (i.e. the tension) of the active muscle apart from any changes in 

 its length. For this purpose the muscle is allowed to contract against a strong 

 spring, the movements of which are magnified by means of a very long lever. 

 Thus the shortening of the muscle is almost entirely prevented, but the in- 

 crease in its tension causes a minute but proportionate movement of the spring, 

 which is recorded by means of the lever. Since in this case the length or 

 measurement of the muscle remains approximately constant, while the tension 

 is continually varying throughout the contraction, it is known as the isometric 

 method. The great magnification necessary in this method introduces serious 

 sources of error ; but it seems that, if all due precautions be taken to avoid 

 these errors, the isometric curve differs very little in form from the isotonic, 

 displaying only a somewhat quicker development of energy at the beginning of 

 contraction. It would probably be better to eliminate the lever altogether and 

 magnify the minute movements of the spring by attaching to it a small hinged 

 mirror by which a ray of light is reflected through a slit on to a travelling 

 photographic plate. Since the ray of light has no inertia, magnification of the 

 movements may be carried to any extent without increasing the instrumental 

 deformation of the curve (Fig. 55 A). 



