THE PHENOMENON OF CONTRACTION. 



39 



The average results of such experiments show an efficiency of 

 about 21 per cent., which compares favorably with the figures ob- 

 tained for mechanical engines, such as the steam engine (15 to 25 

 per cent.) . It must not be assumed, however, from this comparison 

 of the muscle with a steam engine that in the former, as in the 

 latter, there is a conversion of part of the heat to mechanical work. 

 This view was held formerly in physiology, but considerations, 

 which are referred to below (p. 73), have convinced most physiol- 

 ogists that the muscle is not a heat engine. The energy that is 

 expressed as work is derived in some other as yet undetermined 

 way. Nevertheless, heat is evolved by chemical changes and 

 material is consumed in muscular contraction as an apparently 

 necessary part of the process, and the work performed bears to 

 this energy liberated as heat the proportion indicated. 



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: 



Load in Grams. 



5 



15 



25 



35 



Lift in Millimeters. 



27.6 



25.1 



11.45 



6.3 



Work Done in Gram-millimeters 

 Load X Lift. 

 138.0 

 376.5 

 286.25 

 220.5 



