456 COLOUR OF BLOOD. [BOOK 11. 



oxygen present is insufficient to satisfy wholly the haemoglobin 

 of the red corpuscles ; the haemoglobin is, to a large extent, 

 reduced, hence the purple colour of venous blood. When ordi- 

 nary venous blood, diluted without access of oxygen, is brought 

 before the spectroscope, the two bands of oxyhaemoglobin are 

 seen. This is explained by the fact that in partly reduced 

 haemoglobin, which we may conveniently regard as a mixture 

 of oxyhsemoglobin and (reduced) haemoglobin, the two sharp 

 bands of the former are always much more readily seen than 

 the much fainter band of the latter. Now in ordinary venous 

 blood there is always some loose oxygen, removable by dimin- 

 ished pressure or otherwise ; the haemoglobin is only partly 

 reduced, there is always some, indeed a considerable quantity, 

 of oxyhaemoglobin as well as (reduced) haemoglobin. It is only 

 under special circumstances, as for instance after death by what 

 we shall presently speak of as asphyxia, that all the loose oxy- 

 gen of the blood disappears ; and then the two bands of the 

 oxyhaemoglobin vanish too. If even only a small quantity of 

 oxygen be present so distinct are the two bands that a solution 

 of completely reduced haemoglobin may be used as a test for 

 the presence of oxygen ; if oxygen be present in any fluid to 

 which the reduced haemoglobin is added, the single band imme- 

 diately gives way to the two bands of oxyhaemoglobin. 



As the venous blood passes through the capillaries of the 

 lungs, this reduced haemoglobin takes from the pulmonary air 

 its complement of oxygen, all or nearly all the haemoglobin of 

 the red corpuscles becomes oxyhaemoglobin, and the purple 

 colour forthwith shifts into scarlet. For careful observations 

 shew that the haemoglobin of arterial blood is saturated or 

 nearly saturated with oxygen , it probably falls short of com- 

 plete saturation by about 1 vol. of oxygen in 100 vols. of blood. 

 By increasing the pressure of the oxygen, an additional quan- 

 tity may be driven into the blood, but this, after the haemoglobin 

 has become completely saturated, is effected by simple absorp- 

 tion. The quantity so added is extremely small compared with 

 the total quantity combined with the haemoglobin. 



Passing from the left ventricle to the capillaries of the tis- 

 sues the oxyhaemoglobin gives up some of its oxygen to the 

 tissues, becoming, in part, reduced haemoglobin, and the blood 

 in consequence becomes once more venous, with a purple hue. 

 Thus the red corpuscles by virtue of their haemoglobin are em 

 phatically oxygen-carriers. Undergoing no intrinsic change in 

 itself, the haemoglobin combines in the lungs with oxygen, which 

 it carries to the tissues; these, more greedy of oxygen than 

 itself, rob it of its charge, and the reduced haemoglobin hurries 

 back to the lungs in the venous blood for another portion. The 

 change from venous to arterial blood is then in part (for as we 

 shall see there are other events as well) a peculiar combination 



