EFFECTS OF RESPIRATION ON THE BLOOD. 587 



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. 

 Collard de Martigny repeated this experiment in nitrogen, with the same re- 

 sults. 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 ex- 

 periments have been since repeated by Miiller and Bergemann, who took the 

 additional precaution of removing, by means of the air-pump, all the atmo- 

 spheric 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 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 

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

 the absorption of oxygen is necessary to enable carbon to be set free from the 

 tissues. It is impossible, however, for an adult Bird or Mammal 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 capillary action necessary 

 for the passage of the blood through the lungs will not take place without it. 

 But Dr. Edwards has shown, that young Mammalia can sustain life in an 

 atmosphere of hydrogen or nitrogen, fora 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 them, as in Reptiles and Invertebrata. 



770. That the changes which Venous Blood undergoes in the lungs, are to 

 be explained upon principles of a purely chemical and physical nature, is evi- 

 dent from the fact, that the same changes will take place when it is exposed 

 to the air out of the body, even through the medium of a thick membrane, such 

 as a bladder. Such changes, however, only affect the surface of the fluid; but 

 this is exactly what we should expect, since the air has no access to the part 

 beneath. The Blood, whilst circulating through the capillaries of the Lungs, 

 is divided into an innumerable multitude of minute streamlets, each so small 

 as to admit but a single layer of its corpuscles ; and in these, therefore, the 

 surface which is placed in contact with the air is so enormously extended, as 

 to be almost beyond calculation. Hence, then, we can at once understand 

 how such a change may be instantaneously effected in it, as would occupy 

 several hours, when the blood is less advantageously exposed to the influence 

 of oxygen. In studying the nature of these alterations, it is very necessary 

 to ascertain whether Oxygen and Carbonic Acid exist in a free state in the 

 Blood; and to what extent their proportions differ in Venous and Arterial 

 blood. The late researches of Professor Magnus have shown that Blood 

 possesses a very remarkable absorbing power for these gases, especially for 

 Carbonic acid. By freely exposing it to the latter gas, it was found that it 

 could take up as much as 1* times its bulk; and that after all its Oxygen and 

 Nitrogen had been thus displaced, it could still absorb as much as 16 per 

 cent, of its volume of Oxygen, and 6'3 of Nitrogen, on being exposed to those 

 gases respectively. The usual quantity of Oxygen present in arterial blood 



* Miiller's Physiology, p. 341. 



