7 8 2 CHEMISTR Y OF RESPIRA TION. 
In pure oxygen the tissues absorb more oxygen, but do not discbarge 
a much greater quantity of carbon dioxide than they do in air ; even 
in nitrogen or hydrogen the tissues continue to give off carbon dioxide. 1 
The excised tissues of warm-blooded animals have a larger respiratory 
exchange than the corresponding tissues of cold-blooded animals, and 
differences are also observed in tissues from animals of different species. 2 
The respiratory exchange of isolated muscle rises and falls, within certain 
limits, with the external temperature. 3 
Experiments made upon excised tissues are liable to several sources 
of error. Putrefaction may begin, and cause an absorption of oxygen 
and a discharge of carbon dioxide ; 4 this danger, however, is small in 
tissues removed directly after the death of the animal, and kept at a low 
temperature, and free from septic contamination. 5 Another source of 
error is the loss of vitality in the tissues, and the accumulation of 
carbon dioxide and other waste products in the interior of the tissues. 
A much better method for the study of the respiratory changes in 
isolated tissues and organs is that introduced by Ludwig ; G an artificial 
circulation of blood is maintained, and the changes in the blood are 
determined. By these and similar experiments it can be shown that 
the tissues have the power of taking up oxygen, and also of oxidising 
various substances. This power is possessed in a different degree by 
the various tissues. 7 Schmiedeberg has shown that benzyl alcohol 
(CgHjCHoOH), and the aldehyde of salicylic acid (C 6 H 4 / q^ ) undergo 
no appreciable oxidation when placed in blood, but if blood containing 
one of these substances is made to circulate through a freshly excised 
kidney then considerable quantities of benzoic acid (C 6 H 6 .C0 2 H), or of 
salicylic acid (CyE^ q|t ), as the case may be, are produced. 
Ehrlich 8 found that most tissues could reduce and decolorise 
alizarine-blue and other pigments, but that the colour returned when 
the tissues were exposed to the air. Tissues placed in normal saline 
solution containing oxyhemoglobin quickly reduce that substance, and 
in this respect muscle is the most effective. Bernstein 9 found the 
following values for the rate of reduction: Muscle 100, liver 81*47, 
involuntary muscle 724, and the mucous membrane of the stomach 
57"05 ; lung tissue, on the other hand, had a very feeble power of 
reduction. This relative power of reduction holds good for tissues 
taken from frogs and from mammals. Somewhat similar experiments 
had been previously made by Yeo ; 10 he supplied a frog's heart with 
1 Spallanzani. " Rapports de l'air avec les etres organises," par Senebier, Geneve, 1807, 
tome i. p. 447 ; tome ii. pp. 44, 56. 
2 Paul Bert, loc. cit. 
■ 3 Regnard, "Reeh. exper. sur les combustions respiratoires," Paris, 1879, p. 23. See 
also "Animal Heat," tbis Text-book, vol. i. p. 840. 
4 Hermann, "Untersucb. u. d. Stoffwechsel der Muskelu," Berlin, 1867, S. 37. 
5 Tissot, Arch, de physiol. norm, et path., Paris, 1S94, tome xxvi. p. 838 ; 1895, tome 
xxvii. 
6 Arb. a. d. physiol. Anst. zu Leipzig, 1868. 
7 Schmiedeberg, Arch. f. exper. Path. u. Pharmacol., Leipzig, 1876, Bd. vi. S. 233 ; 
1881, Bd xiv. S. 288, 379. " For further details and references, see Xeumeister, " Lehrbuch 
der physiol. Chemie," Jena, 1893, Th. 1, S. 8, et seq. 
8 "Der Sauerstoffbedurfniss des Organismus," Berlin, 1885. 
9 Untersuch. a. d. physiol. Inst. d. Univ. Halle, 1888, Heft 1, S. 107. 
10 Jo urn. Physiol., Cambridge and London, vol. vi. p. 93. See also Vierordt, Ztschr. f. 
Biol., Mimchen, 1875, Bd. xi. S. .195 ; Denning, ibid., 1883, Bd. xix. S. 4S3. 
