1896.] Relative Efficiency of Animals as Machines. 787 
ratios, and attention should be directed to the erroneous inter- 
_ pretations of organic processes that are made in the application 
of this false theory in connection with a fancied analogy of 
the animal machine to a steam engine. 
We are told that “ when coal is burned in the furnace a 
part of its potential energy is transformed into the mechanical 
power which the engine uses forits work. The rest is changed 
to heat which the engine does not utilize, and which, there- 
fore, is wasted. The potential energy of the food is trans- 
formed in the body into heat and mechanical power. The heat 
is used to keep the body warm. The mechanical power is employed 
for muscular work.” 
As an outcome of this false analogy the term “ fuel value of 
foods ” has been introduced to serve as an index of their ca- 
pacity “to keep the body warm,” and provide for muscular 
work. The absurdity of these crude and superficial views of 
energy as a factor in nutrition will be readily recognized by 
physiologists and we need only notice some obvious inaccura- 
cies of statement. 
In the first place, there are no processes of combustion in 
physiological activities, and fuel as such, can have no value 
in animal nutrition. The assumption that the potential en- 
- ergy of foods not expended in muscular work is“ used to keep 
the body warm,” is in direct conflict with familiar physiolog- 
ical activities. There are large expenditures of energy in 
transforming to the form of vapor the water exhaled by the 
lungs and thrown off as perspiration by the skin and it isa 
well known fact in physiology that the body is cooled by the 
evaporation of water from the surface that is constantly tak- 
ing place. 
The laboring man, perspiring freely in hot weather, ex- 
pends a considerable part of the potential energy of his food 
in the cooling process of vaporizing the water discharged by 
the skin as a result of his exertions. The law of the conserva- 
tion of energy is strictly observed and there isno demand for fuel 
to burn to “keep the body warm.” The heat liberated in the de- 
structive metabolism of the tissues, or what we speak of as the 
wear and tear of the sytem, is disposed of in various ways, 
