496 PRINCIPLES OF ANIMAL NUTRITION. 
maximum efficiency of the muscle is reached when the load is such 
that the muscle can just raise it, while this maximum load dimin- 
ishes as the muscle contracts until when the latter reaches the limit 
of shortening it of course becomes zero. Conversely, if the muscle 
be stretched beyond what may be called its normal length, as is 
the case in the living body, the weight which it can lift, and conse- 
quently its efficiency, is increased. In these respects the muscle 
behaves like an elastic cord, and some authorities, notably Chau- 
veau,* regard the essence of muscular contraction as consisting of 
a direct conversion of the potential energy of the “contractile 
material” of the muscle into muscular elasticity. 
EFrricIENcy oF THE Living AnrMaL.—According to the above 
principles ‘the maximum efficiency of a muscle would be obtained 
when it was loaded to its maximum at each point in the contraction ; 
that is, when the load diminished uniformly from the maximum 
corresponding to the initial length of the muscle to zero at the point 
of greatest contraction. Such conditions, however, rarely if ever 
obtain in the animal. Of its many muscles some serve largely 
or wholly to maintain the relative positions of the different parts of 
the body, and consequently have an efficiency approaching zero. 
Others contract to a varying extent and under loads less than the 
maximum. Some muscles, owing to their anatomical relations, 
work at a less mechanical advantage than others, while the extent 
to which a given group of muscles is called into action will vary 
with the nature of the work. 
If, then, the efficiency of the single muscle is variable, that of 
the body as a whole would seem likely to be even more so, thus 
rendering it difficult to draw any trustworthy direct conclusions 
as to the efficiency of the bodily machine from studies of the effi- 
ciency of the single muscle. Moreover, the performance of labor 
by an animal sets up various secondary activities, notably of the 
circulatory and respiratory organs, which consume their share of 
potential energy and yet do not contribute directly to the per- 
formance of the work, and the extent of these secondary activities 
varies with the nature and the severity of the work. When, there- 
fore, as is here the case, we consider the whole animal in the light 
of a machine for converting the potential energy of the food into 
* Le Travail Musculaire. Paris, 1891. 
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