912 METABOLISM. 



living animal, and by Ludwig and Schmidt 1 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 C0 2 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 clone 

 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. 



2 Ibid., 1877. 



3 Arch. f. Physiol., Leipzig, 1885, S. 519. 



4 Compt. 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. Ray. 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 kilogram metre 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. zu Berlin). 



