OXYHJ3MOGLOBIN. 233 



The combinations which haemoglobin enters into are numerous, 

 and throw much light upon the function of the corpuscles. 



As already stated, the coloring matter, when exposed to the 

 air, combines with oxygen to form a loose chemical compound 

 called oxyhsemoglobin. This is the condition in which the color- 

 ing matter of the blood is commonly met with. Although so 

 prone to combine with oxygen, the oxyhaemoglobin very readily 

 parts with some of it. In the circulation it is always united with 

 oxygen, normally leaving the lungs in a state of saturation. On 

 its way through the capillaries of the tissues it parts with some of 

 its oxygen, becoming more or less reduced (haemoglobin), but even 

 the most venous blood always contains some oxyhaemoglobiu. 



The oxygen can be removed by reducing the pressure under an 

 air-pump, or by exposing the solution to a mixture of nitrogen 

 and hydrogen. Various reducing agents rob the oxyhsemoglobin 

 of its oxygen ; and if blood or a solution of oxyhaemoglobin be 

 sealed in a glass tube so as to exclude the air, the loose oxygen 

 is taken up by some of the other constituents of the blood, and 

 the oxyhsemoglobin becomes gradually reduced to haemoglobin. 

 This depends on the putrefactive changes in the proteids, and 

 may be prevented by careful aseptic precautions. If the reduced 

 haemoglobin be shaken for a few moments with air, the bright 

 color characteristic of oxyhaemoglobin soon reappears, and if the 

 reducing agent be not injurious to the blood, the reduction and 

 reoxidation may be repeated several times, the haemoglobin going 

 through the changes which take place in it during normal respi- 

 ration. 



The union of oxygen with haemoglobin solutions is not mere 

 absorption of the oxygen by the liquid, but a definite chemical 

 combination. This is seen from the following facts : (1.) When 

 the pressure is removed the oxygen does not come away from the 

 liquid in accordance with the law which governs the escape of 

 absorbed gas (vide p. 239). (2.) The two substances give a dif- 

 ferent result when examined with the spectroscope. The reduced 

 haemoglobin gives one wide diffuse band, which lies between the 

 D and E lines of the solar spectrum, and much of the violet end 



