THE BLOOD. 35 



that the mere presence of iron is not in itself sufficient to 

 account for the colour of the blood ; this depends most pro- 

 bably upon the state of combination of the iron in the blood. 

 Liebig states, as will be shown immediately, that the iron 

 in the blood exists in the varying conditions of peroxide, 

 protoxide, and carbonate of the protoxide of iron. 



USES OF THE RED CORPUSCLES. 



In connexion ivith Respiration. Observation has taught 

 us the fact that the colour of the blood changes considerably 

 according as it is exposed to the influence of oxygen and car- 

 bonic acid gases, it becoming bright red under the influence 

 of the former, and dark red, almost black, under that of the 

 latter gas. 



Now the microscope has revealed to us the additional fact 

 that the colouring matter of the blood resides within the red 

 corpuscles ; and hence we are led to infer that the changes of 

 colour alluded to are accompanied by alterations in the con- 

 dition of the colouring matter contained in those corpuscles. 



Further, the alterations of colour which have been men- 

 tioned take place not only in blood withdrawn from the 

 system, but also in that which still circulates in the living 

 body, the vital fluid being exposed in the lungs to the in- 

 fluence of the oxygen contained in the atmosphere, and to 

 carbonic acid in the capillary system of vessels. 



But it is not merely a change of colour which the blood 

 undergoes, or rather the coloured blood corpuscles undergo, 

 on exposure to either of the gases particularised, but they 

 also experience at the same time, as might easily be inferred, 

 a positive change of condition, a portion of one or other of 

 the gases to which the blood corpuscles are exposed being 

 imbibed by them. 



That it is really the red corpuscles which absorb the oxygen, 

 or the carbonic acid, as the case may be, admits of demon- 

 stration, and is proved by the fact that these gases lose but 

 little volume when placed in contact with the liquor sa.nguims, 

 or serum of the blood. 



b 2 



