INFLUENCE OF MUSCULAR EXERTION UPON METABOLISM: 193 
than during rest. Apparently the stimulation of the respiratory 
center is brought about by the other products of muscular metab- 
olism, whatever they may be, which find their way into the blood. 
Under the influence of this stimulus the respiratory movements 
increase in frequency or depth or both, thus making possible a 
more active gaseous exchange between the blood and the air in the 
lungs. This action is usually so efficient that the expired air dur- 
ing work contains a smaller proportion of carbon dioxide than it 
does during rest, notwithstanding the fact that the total quantity 
eliminated is much greater. 
Since respiration, like circulation, is maintained by muscular 
action, it is true in the former case as in the latter that a greater 
activity of the function necessitates a greater metabolism for 
that purpose. Zuntz & Hagemann * have recently investigated the 
work of respiration in the horse, the augmented respiratory activ- 
ity being brought about by an admixture of carbon dioxide to the 
inspired air, this resulting in a marked increase in the depth of the 
respiratory movements. With the animal upon which most of the 
experiments were made they found an increment of from 2.02 c.c. to 
5.23 c.c. of oxygen consumed for each increment of one liter in the 
volume of air respired. In general, although with some exceptions, 
the work of respiration as thus measured increased with the in- 
creased depth of the respiratory movements. The results upon 
other horses were somewhat variable. It was observed, however, 
that in the performance of ordinary work by the horse the effect 
was chiefly upon the frequency of respiration rather than its depth. 
The former effect the authors believe to involve less work than the 
latter and moreover an amount largely independent of the total 
volume of air respired. 
§ 2. Effects upon Metabolism. 
It is obvious from the foregoing paragraphs that the production 
of external work is a complex phenomenon. As regards its effects 
upon the total metabolism, the main features involved seem to be: 
1. An explosive decomposition of some unknown “contractile 
substance” in the muscles. 
* Landw. Jahrb., 27, Supp. III, 361. 
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