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Rh matters, as Liebig supposed; but, as I have said, an increase in the non-nitrogenous waste matter, carbonic acid. So long as carbo-hydrates or fats are freely supplied, no increase in nitrogenous waste matters follows prolonged muscular exertion; but if they are withheld, or if the muscular exertion is excessive, then there is an increase in the waste nitrogenous substances. This shows that when the little machine cannot give its output of energy at the expense of carbo-hydrate or of fat, it sacrifices a part of its own framework. The muscle engineer, when he finds himself short of the ordinary fuel, seizes hold of combustible portions of his own engine, as if he were determined at all costs to do the work required of him. This is only a somewhat fanciful analogy, but it gives us an insight into what probably occurs in a muscle.

An engineer is desirous, chiefly for the sake of economy, to get as much effective work as possible out of the engine he constructs. The engine is intended to do work by liberating mechanical energy; but part of the energy appears as heat, and the heat is of no use to the engineer. The engineer knows the amount