238 THE VITAL FUNCTIONS. 



during its passage through the lungs; and we have only col- 

 lateral evidence to guide us in the inquiry.* 



The most obvious effect resulting from the action of the 

 air is a change of colour from the dark purple hue, which 

 the blood has when it is brought to the lungs, to the bright 

 vermilion colour, which it is found to assume in those or- 

 gans, and which accompanies its restoration to the qualities 

 of arterial blood. In what the chemical difference between 

 these two states consists may, in some measure, be collected 

 from the changes which the air itself, by producing them, 

 has experienced. 



The air of the atmosphere, which is taken into the lungs, 

 is known to consist of about twenty per cent, of oxygen gas, 

 seventy-nine of nitrogen gas, and one of carbonic acid gas. 

 When it has acted upon the blood, and is returned from the 

 lungs, it is found that a certain proportion of oxygen, which 

 it had contained, has disappeared, and that the place of this 

 oxygen is almost wholly supplied by an addition of carbonic 

 acid gas, together with a quantity of watery vapour. It ap- 

 pears also probable that a small portion of the nitrogen gas 

 is consumed during respiration. 



For our knowledge of the fact of the disappearance of ox- 

 ygen we are indebted to the labours of Dr. Priestley. It had, 

 indeed, been long before suspected by Mayow, that some 

 portion of the air inspired is absorbed by the blood; but the 

 merit of the discovery that it is the oxygenous part of the 

 air which is thus consumed is unquestionably due to Dr. 



* Some experiments very recently made by Messrs. Macaire and Marcet, 

 on the ultimate analysis of arterial and venous blood, taken from a rabbit, 

 and dried, have shown that the former contains a larger proportion of oxy- 

 gen than the latter; and that the latter contains a larger proportion of carbon 

 than the former: the proportions of nitrogen and hydrogen being the same 

 in both. The following are the exact numbers expressive of these propor- 

 tions: 



Carbon. Oxygen. Nitrogen. Hydrogen. 



Arterial blood 50.2 . . . 26.3 . . . 16.3 ... 6.6 



Venous blood 55.7 . . . 21.7 . . . 16.2 ... 6.4 



Memoires de la Socittt de Physique et d'Hist, Naturdk de Geneve. T. v. 

 p. 400. 



