406 ON RESPIRATION. 



and venous blood, but in varying proportion. The amount of oxygen in arte- 

 rial blood equals at least one-third, and frequently almost one-half that of the 

 carbonic acid ; whilst in venous blood the oxygen is scarcely ever more than 

 one-fourth, and often less than one-fifth, of the carbonic acid. The proportion 

 of nitrogen seems to be continually varying, without any fixed law ; it is some- 

 times as little as one-twelfth of the whole quantity of gas extracted from the 

 blood; and sometimes nearly a fourth. 



542. That the change of the colour of Venous blood to that of Arterial, is 

 principally due to the replacement of its carbonic acid by oxygen, is very 

 easily shown. The simple removal of the carbonic acid by hydrogen will not 

 produce the alteration ; it has been observed by Magnus, however, that a slight 

 change of colour takes place in blood under the vacuum of an air-pump, 

 although it does not nearly acquire the arterial tint. This falls in with what 

 is known of the influence of carbonic acid on the blood : in common with 

 other acids it has a blackening effect upon it, so that arterial blood when ex- 

 posed to it becomes venous, and venous blood is rendered still darker ; but the 

 simple removal of it is not sufficient to restore the original hue. This restora- 

 tion may be effected in two ways, -either by the addition of saline matter to 

 the blood, or by exposing the fluid to oxygen. The presence of a certain 

 amount of saline matter appears, from the experiments of Dr. Stevens, to be a 

 condition necessary for the due influence of oxygen upon the colouring matter 

 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, without any extrication of car- 

 bonic acid or absorption of oxygen. Hence it appears that the presence of 

 saline matter in the blood is an essential condition for the due effect of the 

 process of oxygenation ; and that the change of colour may be regarded as 

 resulting from the conjoint operation of the removal of carbonic acid, and the 

 absorption of oxygen.* 



* [Some experiments by Schererf both confirm the opinion of Nasse, that the change 

 from the arterial to the venous colour of the blood depends in great measure on the form, 

 of the blood-corpuscles, and explain most of the observations of Dr. Sievens on the effects 

 of distilled water and salts upon the blood. Their general conclusions are: 1. That 

 when fresh-stirred and bright-red ox-blood is mixed with distilled water, it acquires a 

 dark-red colour, and its corpuscles, by imbibing water, become spherical, and at last 

 vanish. But, 2. That if, after the change has begun, and not gone far, a concentrated 

 solution of a neutral salt be added, the blood-corpuscles again acquire their natural form, 

 and the bright-red colour is restored. 3. That when oxygen is passed through blood 

 darkened by the addition of distilled water, it is not changed in colour, and the blood- 

 corpuscles do not reappear; but that the same kind of blood, mixed with a small quan- 

 tity of milk, or oil, or finely-powdered chalk, or gypsum, soon regains its bright red 

 colour. 4. Again, by the long-continued contact of concentrated saline solutions with 

 the blood-corpuscles, they become jagged and decomposed, and the blood becomes black; 

 and those which have been reddened by the action of salts, become black again on being 

 expanded by the imbibition of water. 5. By adding carbonic acid to bright-red blood, its 

 corpuscles change their biconcave for a biconvex form, and at the same time its colour 

 changes from red to black. So that there are always changes in the shape of the blood- 

 corpuscles, coincident with the changes in the colour of the mass of blood ; whenever 

 they are dilated, as by distilled water or carbonic acid, the dark colour is produced; 

 whenever they are contracted into the biconcave form, the bright-red colour is restored. 



Mulder, also,t espouses the opinion of the changes of colour in the blood being im- 

 mediately due to physical rather than to chemical changes of the corpuscles, and has 

 added many facis to those just quoted in disproof of the opinion of Liebig.that the changes 

 are due to the alternate production of the carbonate of the protoxyde, and of the peroxyde, 



f [ Henle and Pfeufler, Zeitschrift, &c., and Oesterr. Medic. Wochenschrift, Nov. 4, 

 1843. B. & F. Med. Rev., vol. xix, p. 253.] 



* [ Verslag van de Vertiende Vergadering van het Nederlandsche Instituut in ' Het In- 

 stituut,' 1844, No. iv, and Physiologische Scheikunde, pp. 361-77. Lond. Med. Gaz., 

 No. 13, Dec. 1844.] 



