EFFECTS OF RESPIRATION ON THE BLOOD. 405 



The analysis of Marcet gives a more decided predominance of Carbon in 

 Venous blood and of Oxygen in Arterial; according to him, venous blood 

 contains 557 per cent, of carbon, and only 21-7 per cent, of oxygen; whilst 

 arterial blood contains only 50-2 per cent, of carbon, but as much as 26-3 per 

 cent, of oxygen. The discrepancy between these results is probably to be 

 accounted for by the fact to be presently noticed, regarding the facility with 

 which important changes are effected in the gaseous contents of the blood, by 

 a short exposure of it to the atmosphere. The analysis of Dr. Marcet proba- 

 bly over-states the difference between arterial and venous blood, as that of 

 Michaelis underrates it ; but from these and other data, the general fact of the 

 predominance of oxygen in the former, and of carbon in the latter, may be 

 confidently stated. Here, then, we have an important confirmation of the 

 doctrine, that there is an absolute removal of oxygen from the air, during the 

 process of respiration ; and not a mere conversion of this gas into carbonic acid. 

 541. In what precise form the variable amount of these bodies is contained 

 in the Blood, has not yet been clearly shown. That they must be partly com- 

 bined with its other ingredients, and not merely dissolved in the fluid, is clear, 

 from the changes which they produce in its aspect and properties; these 

 changes are the most evident in the red corpuscles ; but they are also con- 

 siderable in the fibrinous portion of the blood. Indeed, as the Invertebrata do 

 not possess red corpuscles, it can only be upon the Liquor Sanguinis that their 

 respiration operates. There seems good reason to believe, that the red Cor- 

 puscles are the chief carriers of oxygen from the lungs to the tissues, and of 

 carbonic acid from the tissues to the lungs; whether or not we hold with 

 Liebig, that they possess this power in virtue of the iron which enters into 

 their composition. The numerous experiments of Scudamore, Clanny,Bischoff 

 and others, have shown that a small quantity of these gases may be removed 

 from fresh-drawn blood by exposing it to a vacuum. But the amount thus 

 obtained is small in proportion to that which may be procured by treating it 

 with hydrogen or nitrogen : for these gases possess, according to the laws of 

 mutual diffusion already referred to, a much greater power of displacing the 

 carbonic acid and oxygen diffused through the blood, than is exerted by a 

 vacuum. Carbonic acid, however, may be obtained from venous blood in con- 

 siderable amount, by agitating it with atmospheric air, the oxygen and nitrogen 

 of which have a powerful displacing influence upon it; and it is probable that 

 a large quantity is thus removed, during the flow of blood from the vein in 

 ordinary bleeding, especially when the fluid does not spout forth in a full 

 stream, but trickles down the arm in a shallow current. Hence, in all experi- 

 ments upon the gaseous contents of the blood, it is essential that it should flow 

 direct from the orifice into the gas which is to operate upon it ;* and to the 

 neglect of this precaution may be traced much of the discrepancy that has 

 prevailed among the several results which have been made public. The 

 quantity of carbonic acid that may be obtained from venous blood by continued 

 agitation of it with atmospheric air, is stated by Miiller at half a cubic inch 

 from seven cubic inches of the fluid; but when it is agitated with hydrogen, 

 the quantity of carbonic acid obtained is sometimes as much as one-sixth of the 

 volume of the blood. Hence we understand the mode in which the respira- 

 tion of hydrogen is a powerful cause of the extrication of carbonic acid from 

 the lungs of those animals which can support life for some time without oxy- 

 gen. The most important and satisfactory experiments that have been hith- 

 erto made upon the gases of the blood, are those of Magnus. He has shown 

 that carbonic acid, oxygen, and nitrogen, may be extracted both from arterial 



* An apparatus contrived for this purpose by Dr. Stevens, is described by him m 

 the Phil. Trans., 1834. 



