782 CHEMISTR Y OF RESPIRA TION. 



In pure oxygen the tissues absorb more oxygen, but do not discharge 

 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 ; 6 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 



(C 6 H 5 CH 2 OH), and the aldehyde of salicylic acid C 6 H 4 < 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 5 .C0 2 H), or of 



salicylic acid (C 6 H 4 ^ Q|T )> as the case may be, are produced. 



Ehiiich 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 oxyhaemoglobin 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 8147, 

 involuntary muscle 7 2 '4, 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 1'air avec les etres organises," par Senebier, Geneve, 1807, 

 tome i. p. 447 ; tome ii. pp. 44, 56. 



2 Paul Bert, loc. tit. 



3 Regnard, " Rech. expe"r. sur les combustions respiratoires," Paris, 1879, p. 23. See 

 also "Animal Heat," this Text-book, vol. i. p. 840. 



4 Hermann, "Untersuch. u. d. Stoffwechsel der Muskeln," Berlin, 1867, S. 37. 



5 Tissot, Arch, de physiol. norm, et path., Paris, 1894, tome xxvi. p. 838 ; 1895, tome 

 xxvii. 



6 Arb. a. d. physiol. Anst. zu Leipzig, 1868. 



7 Schmiedeberg, Arch. f. exper. Path. u. Pharmakol., Leipzig, 1876, Bd. vi. S. 233 ; 

 1881, Bd xiv. S. 288, 379. For further details and references, see Neumeister, " Lehrbuch 

 der physiol. Chemie," Jena, 1893, Th. 1, S. 8, et seq. 



8 "Der Sauerstotfbediirfniss des Organisnms," Berlin, 1885. 



9 Untersuch. a. d. physiol. Inst. d. Univ. Halle, 1888, Heft 1, S. 107. 



10 Journ. Physiol. , Cambridge and London, vol. vi. p. 93. See also Vierordt, Ztschr. f. 

 Bid., Miinchen, 1875, Bd. xi. S. 195 ; Denning, ibid., 1883, Bd. xix. S. 483. 



