192 PRINCIPLES OF ANIMAL NUTRITION. 
whole is thus greatly augmented, while at the same time a larger 
percentage of the total blood passes through the muscles. For 
example, in the experiments of Chauveau & Kaufmann, cited 
above, the ratio between the circulation in the resting as compared 
with the active muscle varied from 1:3.35 to 1:6.60. Zuntz & 
Hagemann,* in their investigations upon the work of the heart, 
found the average amount of blood passing through the heart of a 
horse per minute to be during rest 29.16 liters and during work 
53.03 liters. By this increase in the rate of circulation through 
the muscles the carbon dioxide and other injurious products of 
muscular metabolism are rapidly removed and an abundant supply 
of oxygen is ensured. In fact it is usually true that during work 
which is not excessive the venous blood contains less carbon diox- 
ide and more oxygen than during rest. 
Since the heart is a muscular organ, it is obvious that this in- 
crease in the circulatory activity must add materially to its metab- 
olism. In the performance of work, therefore, there is an expend- 
iture of matter and energy, not only for the work of the skeletal 
muscles but likewise for the additional work of the heart. Zuntz 
& Hagemann in their experiments upon the horse just mentioned 
compute that during moderate work the metabolism due to the 
work of the heart amounts to 3.8 per cent. of the total metabolism 
of the body. 
RespiraTion.—The greater activity of the circulation conse- 
quent upon muscular exertion would be futile were not provision 
made for more efficient aeration of the blood in the lungs through an 
increased activity of the respiration. The latter appears to be 
brought about, like the increase in the circulatory activity, by the 
effect of the greater amount of metabolic products in the blood, 
acting in this case upon the respiratory center. It has been shown 
that an accumulation of carbon dioxide in the blood does not have 
this effect, but that a lack of oxygen, such as occurs, for example, 
in asphyxiation, provokes powerful movements of the respiratory 
organs. In ordinary work, however, whatever may be the case in 
excessive muscular exertion, the effect is not caused by a lack of 
oxygen, for the blood, as already noted, is usually more arterialized 
* Landw. Jabrb., 27, Supp. III, 405. 
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