BED CORPUSCLES OF BLOOD. 139 



The solution, even when diluted, has a dark colour ; and possesses all 

 the properties of the colouring matter of venous blood. The iron may 

 be separated from the haematine by strong reagents which combine with 

 the former, and the latter still possesses its characteristic colour. This 

 hue cannot be dependent, therefore, on the presence of iron in the stat* 

 of peroxide ; as some have supposed. On the other hand, the iron is 

 most certainly united firmly with the ingredients of the haematine, as 

 contained in the red corpuscles ; for this may be digested in dilute sul- 

 phuric or muriatic acid for many days, without the least diminution in 

 the quantity of iron, the usual amount of which may be afterwards ob- 

 tained by combustion from the haematine that has been subjected to 

 this treatment. This experiment seems further to prove, that the iron 

 cannot be united with the haematine in the state of either protoxide or 

 peroxide, as maintained by Liebig ; since weak acids would then dissolve 

 it out. Regarding the nature of this compound, and the changes which 

 it undergoes in respiration, there is still much to be learned ; and until 

 these points have been more fully elucidated, the precise uses of the 

 Red Corpuscles in the animal economy cannot be understood. There 

 is evidence, however, that the production of Haematine is (like the pro- 

 duction of the red colouring matter of the Protococcus nivalis, 26), a 

 result of chemical action taldng place in the cells themselves ; for no 

 substance resembling Haematine can be found in the liquid in which 

 these cells float, and scarcely a trace of iron can be detected in it; 

 whilst, on the other hand, the fluid portion of the chyle holds a large 

 quantity of iron in solution, which seems to be drawn into the red cor- 

 puscles, and united with the other constituents of haematine, as soon as 

 ever it is delivered into the circulating current. 



222. It has been usually supposed, until recently, that the difference 

 in colour between Arterial and Venous blood is due to different states 

 of combination of the Haematine they respectively contain, with Oxygen 

 and Carbonic acid. For in its passage through the capillaries of the 

 system, the arterial blood loses its bright florid hue, and assumes the 

 dark purple tint which distinguishes ordinary venous blood ; and the 

 converse change takes' place in the capillaries of the lungs, the original 

 florid hue being recovered. Now it is certain that the blood, in its 

 change from the arterial to the venous condition, loses oxygen, and 

 becomes charged with an increased amount of carbonic acid, although 

 its precise mode of combination is not known ; on the other hand, in its 

 return from the venous to the arterial state, the 1 blood gives off this 

 additional charge of carbonic acid, and imbibes oxygen. The change 

 of colour, under similar conditions, takes place out of the body, as well 

 as in it. Thus if venous blood be exposed for a short time to the air, 

 its surface becomes florid ; and the non-extension of this change to the 

 interior of the mass is evidently due to the impossibility of bringing air 

 into relation with every particle of the blood, in the manner which the 

 lungs are so admirably contrived to effect. If venous blood be exposed 

 to pure oxygen, the change of colour will take place still more speedily ; 

 and it is not prevented by the interposition of a thick animal membrane, 

 such as a bladder, between the blood and the gas. On the other hand, 

 if arterial blood be exposed to carbonic acid, it loses its brilliant hue, 



