588 OF RESPIRATION. 



is, according to the experiments of Magnus, about 10 per cent.; but while 

 passing through the systemic capillaries, this is diminished about one-half, so 

 that Venous blood does not contain more than 5 per cent, of its volume of 

 Oxygen. On the other hand, the Carbonic acid of Arterial blood is about 20 

 per cent, of its volume ; and this proportion is increased in Venous blood to 

 nearly 25 per cent. The amount of Nitrogen varies considerably, being some- 

 times as little as 1'7 per cent, of the volume of the blood, and sometimes 

 nearly double that proportion ; it does not appear to differ, according to any 

 constant law, in arterial and venous blood.* 



771. There can be little doubt, then, that the changes which the function 

 of Respiration effects in the Blood have reference in great part to the relative 

 proportions of the different gases, which it holds in solution. And although 

 it might appear that the change of colour, which the Red Corpuscles undergo, 

 is a proof of a change of composition in the Haematine which they contain, 

 yet such a supposition is not borne out by experiment ; for no difference of 

 composition has been detected, between the Hsematine of Venous and that of 

 Arterial blood ; and it appears from the researches of Peligot on the action of 

 the protoxide of nitrogen upon solutions of the salts of the protoxide of iron, 

 that liquids may have their colour changed by the absorption of gases, which 

 form no chemical union with them. There seems reason to conclude, how- 

 ever, from the statements formerly quoted ( 115) in regard to the difference 

 between the Fibrine of Venous and that of Arterial blood, that Oxygen derived 

 from the inspired air enters into actual combination with this element ; and 

 the same may very probably be true of other constituents of the blood: so 

 that we are to regard the influence of Respiration as partly exerted in modify- 

 ing the proportions of the gases dissolved in the blood, substituting Oxygen 

 for a portion of its Carbonic Acid; and partly in enabling the ingredients of 

 the liquid to enter into new combinations with the Oxygen of the air. For 

 the reasons formerly stated ( 150) it appears probable that, whether or not 

 their Haematine be chemically affected by the change, the Red Corpuscles are 

 the chief carriers of the two gases to be interchanged, between the pulmonary 

 and systemic capillaries. 



772. Although the alteration in the relative proportions of Oxygen and Car- 

 bonic acid which it contains, is doubtless the essential change effected in the 

 Blood by the Respiratory process, the alteration in its colour is the most ob- 

 vious; and this is, under ordinary circumstances, an indication that the other 

 change has taken place. Thus, if Arterial blood be exposed, out of the body, 

 to carbonic acid, it will acquire the dark hue of venous blood; and Venous 

 blood exposed to it becomes darker still. On the other hand, if Venous 

 blood be exposed to Oxygen, it acquires the Arterial hue. The presence of a 

 certain amount of saline matter appears, from the experiments of Dr. Stevens 

 and others, to be a condition necessary for the due influence of oxygen upon 

 the colour of the blood; since, if it be deficient, the contact of oxygen will not 

 produce its usual effect. On the other hand, the addition of saline matter 

 (especially nitre) will occasion a decided change of hue in venous blood, with- 

 out any extrication of carbonic acid or absorption of oxygen. 



* For the latest researches of Prof. Magnus, which have had their origin in the objections 

 of M. Gay Lussac to those previously published by him, see the Annalen der Physik und 

 Chemie, vol. Ixvi., p. 177, and an Abstract in the Philosophical Magazine, Dec. 1845, Suppl. 

 In these researches, far greater success was obtained in removing the gases from the blood, 

 than in any previous experiments; and the account of their proportions, therefore, is more 

 satisfactory. It is extremely diflicuk to avoid all sources of error, in such researches; but 

 the constancy of the results obtained by Magnus indicates that we may place much confidence 

 in them. 



