INFLUENCE OF MUSCULAR EXERTION UPON METABOLISM. 189 
several hours, giving out considerable amounts of carbon dioxide, 
and Bunge * has made similar observations upon the movements 
of certain intestinal worms (Ascaris) in one per cent. salt solution 
made as nearly oxygen-free as possible. 
Weinland t has shown that in the latter case the energy is 
derived chiefly from the cleavage of glycogen with the production 
of carbonic and valerianic acids. 
Summary.—The three classes of facts just adduced justify the 
conclusion that the chemical changes by which energy is liberated 
in a muscular contraction are not simply oxidations, but are of the 
nature of a cleavage of some complex substance or substances with 
evolution of carbon dioxide. There is, in other words, a sudden 
“explosive” decomposition of substances elaborated in the muscle 
during rest. Of the nature of the material thus broken down we 
have little definite knowledge. We can say, however, that if it is 
nitrogenous matter its nitrogen is ordinarily retained in the muscle 
in some form and that in effect the metabolized material is non- 
nitrogenous. The increase in the consumption of oxygen during 
work appears to be to a certain extent a secondary process, accom- 
plishing the further oxidation of the primary products of metabolism. 
At the same time, the fact that the amount of oxygen consumed 
responds very promptly to work and also to rts cessation shows that 
these primary products, whatever they may be, are very speedily 
oxidized, either in the muscle or elsewhere in the organism. 
Thermal Changes during Contraction.—A considerable por- 
tion of the energy set free during muscular exertion always takes 
ultimately the form of heat. When the muscle acts without shorten- 
ing, as when supporting a weight (isometric contraction) — that is, 
when no external work is done—all the metabolized energy takes 
the form of heat. If, on the other hand, the weight be lifted (iso- 
tonic contraction) —if external work is done—a portion of the 
energy takes the form of motion. The interesting question of the 
relation between the external work performed and the total amount 
of energy metabolized will be considered later. For the present it 
is sufficient to state that muscular action always produces heat 
and that a very considerable share of the metabolized energy 
ultimately takes this form. 
* Zeit physiol. Chem. 8, 48. f Zeit. f. Biol., 42, 55. 
