EFFECTS OF RESPIRATION ON THE AIR. 397 



blood, and that the inhalation of oxygen is therefore immediately necessary 

 for its production, yet it is now quite certain that this carbonic acid exists 

 preformed in venous blood, and that the oxygen introduced is carried into 

 the arterial circulation, instead of being at once returned to the air in a state 

 of Carbonic acid ($ 104). Hence an exhalation of Carbonic acid may con- 

 tinue for a considerable period (in cold-blooded animals especially) whilst 

 the animal is breathing an atmosphere in which no oxygen exists. Thus it 

 was shown by Spallauzani, 1 that snails might be kept for a long time in 

 Hydrogen, without apparent injury to them; and that during this period 

 they disengaged a considerable amount of Carbonic acid. Dr. Edwards 2 

 subsequently ascertained that, when Frogs were kept in hydrogen for several 

 hours, the quantity of Carbonic acid exhaled was fully as great as it would 

 have been in atmospheric air, or even greater; this latter fact, which how- 

 ever is very doubtful, might be accounted for by the superior displacing 

 power which (on the laws of the diffusion of gases) hydrogen possesses for 

 Carbonic acid. Collard de Martigny 3 repeated this experiment in Nitrogen, 

 with the same results. In both sets of -experiments, the precaution was used 

 of compressing the flanks of the animal, previously to immersing it in the 

 gas, so as to expel from the lungs whatever mixture of oxygen they might 

 contain. These expsriments have been since repeated by Miiller and Berg- 

 mann, who took the additional precaution of removing, by means of the air- 

 pump, all the atmospheric air that the lungs of the frog might previously 

 contain, together with the Carbonic acid that might exist in the alimentary 

 canal. They found, in one of their experiments, that the quantity of Car- 

 bonic acid exhaled in hydrogen was nearly a cubic inch in 6^ hours ; and 

 in another, that nearly the same amount was given off in nitrogen, though 

 this required rather a longer period. It appears from the table of their 

 results,* that the amount was not ordinarily greater in the experiments, 

 which were prolonged for twelve or fourteen hours, than in those which were 

 terminated in half the time ; hence it may be inferred, that the quantity 

 which the blood is itself capable of disengaging is limited, .and that the 

 absorption of oxygen is necessary to enable Carbonic acid to be continuously 

 set free from the body. It is impossible, however, for an adult Bird or Mam- 

 mal to sustain life for any considerable time in an atmosphere deprived of 

 oxygen ; since the greatly increased rapidity and energy of all their vital 

 operations necessitate a much more constant supply of this vivifying agent, 

 than is needed by the inferior tribes; and, as we shall presently see, the cap- 

 illary action requisite for the passage of the blood through the lungs will 

 not take place without it ( 321). But Dr. Edwards has shown that young 

 Mammalia can sustain life in an atmosphere of hydrogen or nitrogen, for a 

 sufficient length of time to exhale a sensible amount of Carbonic acid ; so 

 that the character of the process is clearly proved to be the same in warm- 

 blooded animals as in Reptiles and Invertebrata. 



316. The seat of those changes which result in the formation of Carbonic 

 acid has been the subject of much research and discussion. It may either 

 be conceived that the oxygen taken up by the blood-corpuscles, and feebly 

 held in combination by their hoemoglobin, escapes by diffusion through the 

 bloodvessels, and directly enters into combination with the constituents of 

 the tissues, the Carbonic acid formed being given up in exchange for fresh 

 oxygen ; or we may suppose that the products of the disintegration of the 



1 Memoires sur la Kespiration, traduit par Senebier, Geneve, 1804. 



2 De 1 'Influence des Agens Physiques sur la Vie, Paris, 1824. 



3 Ivei-herches Experimentales, etc., in Magendie'a Journal de Physiologic, torn. x. 



4 Huller's Elements of Physiology, translated by Baly, vol. i, p. 338. 



