THE UTILIZATION OF ENERGY. 495 
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consist largely in a breaking down or cleavage of some substance or 
substances contained in the muscle, resulting in a rapid increase in 
the excretion of carbon dioxide and the consumption of oxygen by 
the animal. In this process of breaking down or cleavage there is a 
corresponding transformation of energy, a portion of the potential 
energy of the metabolized material appearing finally as heat, while 
a part may take the form of mechanical energy. The inquiry 
naturally arises what proportion of the total energy liberated during 
the increased metabolism is recovered as mechanical work and what 
proportion takes the form of the (for this purpose) waste energy of 
heat. The question is not only one of great theoretical interest to 
the physiologist, but the efficiency of the working animal regarded 
as a machine for the conversion of the potential energy of feeding- 
stuffs into mechanical work is also of the highest practical im- 
portance. 
Erriciency oF SIncLE Muscie.—A large amount of experi- 
mental work has been devoted to the study of the single muscle as a 
machine. The subject is a complicated one, and unanimity of views 
upon it has by no means been attained, especially as to the mechan- 
ism of muscular contraction. As regards the efficiency of the muscle 
as a converter of energy, however, one fact is perfectly well estab- 
lished, viz., that it varies within quite wide limits. 
If the two ends of a muscle be attached to fixed points, so that 
it cannot shorten, a suitable stimulus will still cause it to contract 
in the technical sense of the word; that is, a state of tension will 
be set up in the muscle tending to pull the two supports nearer 
together (isometric contraction). In such a contraction there is 
an expenditure of potential energy and a corresponding increase 
of muscular metabolism. but no external work is done. In other 
words, all the potential energy finally takes the form of heat and 
the mechanical efficiency is zero. This is the case, for example, in 
the standing animal. A not inconsiderable muscular effort is 
required to maintain the members of the body in certain fixed 
positions, and a corresponding generation of heat takes place, but 
no mechanical work is done. 
But even when the muscle is free to shorten and thus do mechan- 
ical work, its efficiency is found to be variable, the chief determin- 
ing factors being the load and the degree of contraction. The 
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