9 1 2 METABOLISM. 
living animal, and by Ludwig and Schmidt l upon separated and perfused 
muscles of the dog. Most of these experiments showed an increase of CO., 
during contraction, but in some there was no increase. Minot 2 (also with 
Ludwig), using serum for perfusion instead of blood, could find no relation 
between the C0 2 output and the contraction of the muscles. He came to the 
conclusion that CO., is not one of the disintegration products formed during 
contraction. Frey and Gruber, 3 using somewhat improved methods, have, 
however, obtained more distinct evidence of an increase of C0 2 during con- 
traction ; and a similar result was got by Chauveau and Kaufmann, 4 who 
investigated the amount of CO., in the blood passing to and from the levator 
labii inferioris of the horse when at rest, and when in natural activity during 
mastication. It must be stated, however, that the results obtained by per- 
fusion of separated mammalian muscles are not altogether free from the 
objection raised by Fletcher regarding the excised surviving muscles of the 
frog, that prolonged excitation may tend to hasten the approach of rigor. 
It is therefore not absolutely certain whether the C0 2 , which is ultimately 
produced as a result of muscular activity, actually leaves the muscle as such, 
or in some other form, such as lactic acid, which is destined to be further 
oxidised elsewhere. 5 
The most interesting question in connection with the special meta- 
bolism of the muscles which remains to be considered, is the effect 
which their exercise produces upon the proteid metabolism of the body. 
It was the opinion of Liebig that the energy of muscular contraction 
was produced by the oxidation of muscular substance, and it would 
follow from this that the exercise of the muscles must tend, ceteris 
paribus, to increase the amount of nitrogen excreted in the urine. 
This doctrine of Liebig was accepted for many years by physio- 
logists, but was, for a time at least, completely overthrown by the 
results of the famous experiment of Fick and Wislicenus, 6 known as 
the experiment of the ascent of the Faulhorn. It was shown by 
these observers that at least three times as much work was done 
during the ascent as could be accounted for by the oxidation of proteid, 
as estimated by the amount of nitrogen eliminated by them during and 
after the work. 
The work, therefore, could only have been caused by the oxidation 
of non-proteid matter. Similar results were obtained by Parkes 7 and 
others in man, and by C. Voit in dogs. 8 This, combined with the fact 
that the C0 2 output of the body is increased in proportion to the 
amount of exercise, led to the view being widely adopted that the 
energy of the body is mainly, if not entirely, obtained by oxidation of 
non-proteid materials, and that the splitting and oxidation of proteid 
1 Arb. a. d. physiol. Anst. zu Leipzig, 1868. 
■Ibid., 1877. 
3 Arch. f. Physiol, Leipzig, 1885, S. 519. 
^Coinjif. rend. Acad. d. sc, Paris, 1887. 
5 For other observations and statistics on the subject, see article "Chemistry of 
Respiration." 
6 Vrtljschr. d. naturf. Gesellsch. in Zurich, 1865, Bd. x. S. 317. 
7 Proc. Roy. Soc. London, 1872, vol. xx. p. 402. 
8 See Hermann's "Handbuch," Bd. vi. S. 187. The experiments of Parkes, Voit, and 
North (as well as those by Pavy, Austin Flint, and others, which cannot here be re- 
ferred to in detail), were made upon men and dogs taking walking exercise. It has been 
determined by Zuntz and Katzenstein (in man and horse) that each kilogrammetre of ascent 
work is accompanied by a consumption of oxygen thirteen times that consumed in each 
metre of walking exercise (Arch. f. Physiol., Leipzig, 1890, S. 367, Verhandl. d. physiol. 
Gesellsch . zni Berlin). 
