ADVANCED EXPERIMENTAL PHYSIOLOGY 289 



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. 



PIG. 172. Height of contractions of gastrociiemius with increasing load. The 

 number above each contraction is its observed height in mm. Magnification, 5. The 

 number below each contraction is the weight in grm. hung at the axis of the lever ; 

 the actual load on the muscle was half of this number. (A.P.B.) 



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

 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 arrangement 

 depends upon the increased extensibility of contracting muscle; for, 

 in this way liability to rupture is reduced ; further, there is a saving 

 of energy in pulling at a dead weight through an elastic spring, 

 instead of through an inelastic cord, since some of the energy expended 

 would be lost in a sudden jerk, but, in the case of the spring, is 

 stored up in it and given out again as its elastic recoil. Thus smooth- 

 ness is imparted to even the most sudden movements. We have 

 also seen that as a muscle shortens its absolute contractile force 

 decreases ; therefore, it is clear that the after-load should be thrown 

 on to the muscle at the instant of contraction, when the contractile 



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