MUSCULAR WORK. 571 



longer capable of raising while in its natural passive form, without being 

 stretched by the weight at the moment of stimulation. 



In order to obtain a standard for the comparison of the absolute muscular 

 energy in different muscles and also in different animals, an estimation is made 

 of the absolute muscular force for one square centimeter of cross-section. The 

 mean cross-section of a muscle is determined by dividing its volume by its length. 

 The volume is equal to the absolute weight of the muscle in question divided 

 by the specific gravity of muscle-substance (1058). Thus, the absolute muscular 

 energy for one square centimeter of a frog's muscle is from 2.8 to 3 kilos; for 

 one square centimeter of human muscle from 7 to 8 or even from 9 to 10 kilos. 

 Analogous figures for crustaceans are from 1.8 to 3.2; for beetles from 3.4 to 6.9; 

 for mussels from 4.5 to 12.4 kilos. The transverse section of the muscles tested 

 in man is estimated from cadavers having the same constitution and muscular 

 development as the person under observation. 



In conformity with proposition 3 it is evident that a muscle during contraction 

 will develop the greater absolute muscular energy the more it is extended before 

 contraction. 



5. If a muscle in a state of tetanic contraction maintains a weight 

 in an elevated position, it performs no work during the time, but only in 

 the act of elevation. Nevertheless, the muscle in the state of tetanus 

 requires continued stimuli, and it exhibits metabolic changes and fatigue. 

 The transformation of its potential energy is applied to the generation 

 of heat. 



When the maximal stimulus is applied, a muscle is not capable of lifting as 

 heavy a weight at one contraction as when tetanic stimulation is applied. During 

 tetanic stimulation, moreover, the muscle develops the greater energy (even as 

 much as twice the ordinary) the more frequent the stimulation, as has been ob- 

 served with increasing frequency up to 100 stimuli in a second. 



If only moderate stimuli that do not excite the maximal contraction 

 are applied to the muscle two possibilities present themselves. If the 

 feeble stimulus remains constant, while the weight changes, the amount 

 of work performed follows the same law that is operative during maxi- 

 mal stimulation. If the weight remains the same, while the strength 

 of the stimulus varies, then, according to Pick, the height to which the 

 weight is raised varies in direct proportion to the strength of the 

 stimulus. 



The stimulus that sets a muscle into activity must, naturally, attain a certain 

 strength before it becomes effective liminal intensity of the stimulus. This is 

 independent of the weight attached to the muscle. With a minimal stimulus a 

 small weight is raised to a higher level than a large one; but as the stimulus is 

 increased, the contractions increase in greater proportion with a heavy weight. 



A contracting muscle is capable of performing considerably more 

 work if the weight to be lifted is attached to an inert mass that acts 

 like a fly-wheel, or if the weight is swung to a considerable height. 

 Starke was able almost to quadruple the work corresponding to a maxi- 

 mal contraction by a proper selection of materials for this purpose. 

 Also the production of heat is increased under such conditions, although 

 in much less degree, and it is much more quickly diminished on fatigue. 



If the resistance applied to prevent the movement of a limb whose muscles, 

 strained to the utmost degree, be suddenly removed, the limb will, with the greatest 

 energy and rapidity, assume the position brought about by the muscles. Such 

 springing movements are observed especially in grasshoppers, leaping beetles, and 

 cheese-mites. 



Under special conditions a muscle may perform considerable work through 

 its increase in thickness. 



In the intact body the vessels of a muscle dilate during muscular contraction, 



