456 COLOUR OF BLOOD. [Book ir. 



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 oxyhemoglobin are 

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

 haemoglobin, which we may conveniently regard as a mixture 

 of oxyhemoglobin 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 oxyhemoglobin as well as (reduced) hemoglobin. 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 

 oxyluemoglobin vanish too. If even only a small quantity of 

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

 of completely reduced hemoglobin may be used as a test for 

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

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

 diately gives way to the two bands of oxyhemoglobin. 



As the venous blood passes through the capillaries of the 

 lungs, this reduced hemoglobin takes from the pulmonary air 

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

 the red corpuscles becomes oxyhemoglobin, and the purple 

 colour forthwith shifts into scarlet. For careful observations 

 shew that the hemoglobin 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 hemoglobin 

 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 hemoglobin. 



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

 sues the oxyhemoglobin gives up some of its oxygen to the 

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

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

 Thus the red corpuscles by virtue of their hemoglobin 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 hemoglobin 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 



