38 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



a wheel, a smaller yield (25 per cent.) was obtained. It appears 

 from these figures that the muscular machine is an especially 

 efficient one as regards the amount of external work that can be 

 obtained from the oxidation of a given amount of material. Steam 

 engines are said to be capable of yielding only 10 to 15 per cent, 

 of the heat energy of the fuel in the form of mechanical or useful 

 work. 



The Curve of Work and the Absolute Power of a Muscle. 

 The statements in the preceding paragraph prove that the muscle, 

 judged from the standpoint of a machine to do work, compares most 

 favorably in its efficiency with machinery of human construction. 

 But it should be borne in mind that in this as in other respects the 

 properties of cross-striated muscular tissues vary greatly. In some 

 animals or individuals it is a much more efficient machine than in 

 others. This fact is indicated by our general experience regarding 

 variations in muscular strength in different individuals, and is proved 

 more precisely by direct experiments on single muscles. A frog's 

 muscle may be isolated and the extent of its contractions and the 

 work done may be estimated directly. Under such conditions it 

 will be found that, while the height of the successive contractions 

 diminishes as the load increases (see Fig. 17), the work done that 

 is, the product of the load into the lift first increases and then 

 decreases. For example : 



Work Done in Gram-millimeters. 

 Load in Grams. Lift in Millimeters. Load X Lift. 



5 27.6 138.0 



15 25.1 376.5 



25 11.45 286.25 



35 6.3 220.5 



A series of experiments of this kind furnishes data for construct- 

 ing a curve of work by plotting off along the abscissa at equal inter- 

 vals the equal increments in load and erecting over each load an 

 ordinate showing the proportional amount of work done. The 

 curve has the general form indicated in Fig. 18. Three facts are 

 expressed by this curve: First, that if the muscle lifts no weight 

 no work will be done; this follows theoretically from the formula 

 W = L H, in which W represents the work done, L the load, and 

 H the lift. If either L or H is equal to zero the product, of course, 

 is zero; that is, no external work is done; the chemical energy 

 liberated in the contraction takes the form of heat. Under such 

 circumstances the amount of heat given off from the muscle should 

 be greater than when a load is lifted. In accordance with this 

 fact it is found that a muscle lifting a light load gives off more heat 

 during the contraction than when lifting a heavier load. Second, 

 There is an optimum load for each muscle with which the greatest 



