198 HUMAN PHYSIOLOGY 



It may be at once distinguished by the fact that when 

 gently warmed with sulphide of ammonia it does not yield 

 reduced haemoglobin. (Chemical Physiology, p. 14.) 



Decomposition of Haemoglobin. Haemoglobin is a some- 

 what unstable body, and, in the presence of acids and alkalies, 

 splits up into about 96 per cent, of a colourless proteid sub- 

 stance belonging to the globulin group, and about 4 per cent, 

 of a substance of a brownish colour called haematin. The 

 spectrum and properties of this substance are different in 

 acid and alkaline media. In acid media it has a spectrum 

 closely resembling methaemoglobin, but it can at once be 

 distinguished by the fact that it is not changed by reducing 

 agents. In medicine it is sometimes important to distinguish 

 between these pigments. Haematin in alkaline solution 

 can take up and give off oxygen in the same way as 

 haemoglobin does. Reduced alkaline haematin or hsemo- 

 chromogen has a very definite spectrum (Fig. 101), and its 

 preparation affords a ready means of detecting old blood 

 stains. Haematin contains the iron of the haemoglobin, and 

 it is this pigmented iron-containing part of the molecule 

 which has the affinity for oxygen. Apparently it is the pre- 

 sence of iron which gives it this property, because, if the iron 

 be removed by means of sulphuric acid, a purple-coloured 

 substance, iron-free hcematin, haematoporphyrin, is formed, 

 which has no affinity for oxygen. This pigment occurs in 

 the urine in some pathological conditions. (Chemical Physi- 

 ology, p. 15.) 



In the liver haemoglobin is broken down to form bilirubin 

 and the other bile pigments. These are iron-free, and, like 

 iron-free haematin, do not take up and give off' oxygen. But 

 not only is this iron-free pigment formed from haemoglobin 

 in the liver, but the cells of any part of the body have the 

 faculty of changing haemoglobin in blood extravasations into 

 a pigment known as haematoidin, which is really the same as 

 bilirubin. 



Haemin the hydrochloride of haematin is formed when 

 blood is heated with chloride of sodium and glacial acetic 

 acid. It crystallises in small steel black rhombic crystals, 

 and its formation is sometimes used as a test of blood stains. 

 (Chemical Physiology, p. 16.) 



