40 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



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 



Fig. 17. To show the decrease in extent of contraction of the gastrocnemius muscle 

 of a frog with increase in load. In the first contraction, to the right, the load was 14.2 



EAt each successive contraction the load was increased by 5.3 gms. With a load of 

 ?ms. the lever gave only the slightest indication of a shortening, and this may have 

 due to some lateral movement. 



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 'con- 

 structing a curve of work by plotting off along the abscissa at equal 

 intervals 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 



