438 



THE FUNCTIONS OF CROSS-STRIATED MUSCLES 



creases as the tension rises, whereas the primary lift caused by the white 

 muscle fibers, decreases as a rule with a rising tension (cf. page 436). 



3. The Absolute Power of a Muscle. The method of after-loading* has been 

 used for the purpose, among other things, of determining the so-called absolute 

 power of the muscle. A muscle is loaded only with a lever, and the lever is 

 supported mechanically so that the weights hung on it, which constitute the 



FIG. 176. Isotonic (upper) and isometric (lower) contraction curves under the same primary 

 tension, after Fick. To be read from left to right. The curves a, 6, c, d, represent the short- 

 ening of the muscle corresponding to the isometric contractions a, /3, /, 5. 



after-load, do not affect the muscle so long as it is resting. It is loaded by 

 the weights only when contraction begins, and lifts them only when its tension 

 overcomes the after-load. By adding weights one reaches finally a mass which 

 the muscle no longer has the power to lift. This weight is taken as the absolute 

 power of the muscle (E. F. Weber). 



It is evident that, other things being equal, the absolute power of a muscle 

 must be proportional to its cross section, or in other words, among muscles com- 

 posed of the same kind of fibers the thickest is the strongest. In tetanus the 

 absolute power is greater than in simple contractions, and for the voluntary 



