EFFECTS OF RESPIRATION ON THE BLOOD. 403 



secondary causes. It appears that, in general, even in herbivorous animals, 

 the Exhalation of Nitrogen predominates over the absorption ; but nrrni 

 inquiries have shown, that their food ordinarily contains a supply of azotized 

 matter amply sufficient for their wants. It is yet a matter of doubt, however, 

 whether the Absorption would not predominate, when there is a deficiency of 

 azotized matter in the aliment. Such we may imagine to be the case in Insects 

 which feed upon the saccharine juices of plants ; the waste of their muscular 

 tissue being, from the activity of their movements, excessively rapid. 



III. Effects of Respiration on the Blood. 



538. That an important change is effected in the character of the Blood, by 

 exposure to atmospheric air in the lungs, has been known, from the time 

 when it was first ascertained that it is regularly transmitted to those organs. 

 The most obvious part of this change is the alteration in its colour, from the 

 dark purple of the venous fluid, to the rich crimson of the arterial. But this 

 alteration is only the index of changes far more important which occur in its 

 chemical constitution. Respecting the nature of these changes, there has 

 been, as formerly stated, much difference of opinion ; some maintaining that 

 the carbonic acid exhaled is formed in the lungs ; and others, that it is con- 

 tained in the venous blood, and is truly excreted from it. The latter opinion, 

 which was long since brought forwards by La Grange and Hassenfratz, has 

 recently obtained such full confirmation from the experiments of Spallanzani, 

 Edwards, Miiller, Bischoff, Magnus, and others, as to have a full claim for 

 adoption as a physiological truth. These experiments are of two kinds ; first, 

 those which show that an exhalation of carbonic acid may continue for a long 

 time, when the animal is breathing an atmosphere in which no oxygen exists ; 

 and, secondly, those which prove that much more carbonic acid exists in an 

 uncombined state in venous blood than in arterial, whilst more oxygen exists 

 in a similar condition in arterial blood than in venous. The results of these 

 will now be briefly stated. 



539. It was stated, Jong since, by Spallanzani, that Snails might be kept 

 for a long period in Hydrogen, without apparent injury to them ; and that 

 during this period they disengaged a considerable amount of Carbonic acid. 

 Dr. Edwards 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, if 

 correct, may be accounted for, by the superior displacing power, which (on 

 the laws of the diffusion of gases), hydrogen possesses for carbonic acid. Col- 

 lard de Martigny 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 experiments 

 have been since repeated by Miiller and Bergemann, 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 experi- 

 ments, that the quantity of carbonic 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, but 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 



* Miiller's Physiology, p. 338. 



