ADVANCED EXPERIMENTAL PHYSIOLOGY 101 



contraction recorded on a stationary drum. The drum is rotated a 

 short distance by hand ; an additional load of 50 grins, is hung 

 from the lever, and another contraction recorded. The process is 

 repeated until the muscle is no longer able to raise the load off the 

 after-loading screw. Fig. 40 gives the result of such an experiment ; 

 in it the magnification was 5, and the actual load on the muscle half 

 of the weight hung near the axis of the lever. The following table 

 gives in grm. mm. the work done by the muscle with the various 

 loads : 



From the last column in this table we see that, although the height 

 of the contractions diminishes continuously, the actual work done 

 by the muscle increases at first rapidly and then more slowly, until it 

 reaches its maximum with a load of 200 grms. After that point 

 the work done begins to decrease slowly, and then more rapidly 

 until at 700 grms. a load is reached which the muscle is unable to 

 lift. This weight represents the " absolute contractile force " of this 

 muscle, that is, the load which, brought to bear on the muscle at the 

 instant of contraction, is just able to prevent it from shortening. 

 Although the muscle is unable to lift this load, and therefore, when 

 stimulated, does no visible mechanical work, it nevertheless liberates 

 energy chiefly as heat. 



We are now in a position to recapitulate, so far as load is con- 

 cerned, the conditions necessary to obtain an optimal contraction of 

 a muscle and to see how far they exist in the living body. Initial 

 tension, we have seen, decreases the latent period and increases the 

 power of the muscle to do work. In the body the muscles are 

 constantly loaded to a slight extent, and are thus kept stretched 

 and free from " slack." In this way movements with a short latent 

 period, and with an absence of jerkiness, are obtained ; and the 

 muscles by being stretched are kept irritable, awake and fit for 

 sudden work. On the other hand, we see that a muscle, when purely 

 after-loaded, is at a disadvantage for doing work ; yet in the body 

 the main load is thrown on as an after-load. The advantage of this 



