THE BLOOD 91 



-of sodium chloride in addition. Fresh blood contains sufficient 

 sodium chloride in itself. 



The action of the acetic acid is (1) to split the haemoglobin into 

 haematin and globin ; and (2) to evolve hydi-ochloric acid from the 

 sodium chloride. The haematin unites with the hydrochloric acid, 

 and thus haemin is formed. The formula for haemin is C35H35N4PeC104 

 (Morner). 



Haematoporphyrin is iron-free haematin ; it may be prepared by 

 mixing blood with strong sul- 

 phuric acid : the iron is taken >tr^' "^ ^j^ "^^ 

 out as ferrous sulphate. This j^^^^tt -^ x 

 substance is also found sometimes i " -^ ^ j >*.^^^ 

 in nature: it occurs in certain ^ ^i, "^ Ht^ ^V ^>^ 

 invertebrate pigments, and may ^ ^^^ | X " _t ^ 

 also be found in certain forms of . -I " ^ . A 

 pathological urine. It shows well- r" r "^x *^i^ 

 marked spectroscopic bands, and *!{• ^J "^ ^^ ^.^^ +^ 



so is not identical with thii iron- '^ ■^ ^ 7 • ^ 



free derivative of haemoglobin -^ *%"/ -^u^* 



called haematoidin which is formed Fig. 31.— Haemin crystals magnified. (Preyer.) 



in extravasations of blood in the 



body (see p. 70). The two substances are possibly isomeric. 



COMPOUNDS OF HiEMOGLOBIN WITH GASES 



Haemoglobin forms at least four compounds with gases : — 



With oxygen jl. Oxyhaemoglobin. 



(2. Methaemoglobm. 

 With carbonic oxide. 3. Carbonic oxide haemoglobin. 

 With nitric oxide. 4. Nitric oxide haemoglobin. 



These compounds have similar crystalline forms : each probably 

 consists of a molecule of haemoglobin combined with one of the 

 gas in question. They part with the cflmbined gas somewhat 

 readily, and are arranged in order of stabiUty in the above list, the 

 least stable first. 



Oxyhaemoglobin is the compound that exists in arterial blood. 

 Many of its properties have been already mentioned. The oxygen 

 linked to the h£emoglobin, which is removed by the tissues through 

 which the blood circulates, may be called the respiratory oxygen of 

 hcemoglohin. The processes that occur in the lungs and tissues, 

 resulting in the oxygenation and deoxygenation respectively of the 

 haemoglobin, may be imitated outside the body, using either blood or 



