342 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



Solutions of carbon-monoxide haemoglobin also give a spectrum with two 

 absorption bands closely resembling in position and appearance those of oxy- 

 haemoglobin (see PI. I. spectrum 7). They are distinguished from the oxy- 

 haemoglobin bands by being slightly nearer the blue end of the spectrum, as 

 may be demonstrated by observing the wave-lengths or, more conveniently, 

 by superimposing the two spectra. Moreover, solutions of carbon-monoxide 

 haemoglobin are not reduced to haemoglobin by adding Stokes's liquid, two 

 bands being still seen after such treatment. A solution of carbon-monoxide 

 haemoglobin suitable for spectroscopic examination may be prepared easily by 

 passing ordinary coal-gas through a dilute oxyhaemoglobin solution for a few 

 minutes and then filtering. 



Derivative Compounds of Haemoglobin. A number of compounds 

 directly related to haemoglobin have been described, some of them being 

 found normally in the body. Brief mention is made of the best known of 

 these substances, but for the details of their preparation and chemical proper- 

 ties reference must be made to the section on " The Chemistry of the Body." 



* Methcemoglobin is a compound obtained by the action of oxidizing agents 

 on haemoglobin ; it is frequently found, therefore, in blood stains or patho- 

 logical liquids containing blood which have been exposed to the air for some 

 time. It is now supposed to be identical in composition with oxyhaemoglobin, 

 with the exception that the oxygen is held in more stable combination. 

 Methaemoglobin crystallizes in the same form as oxyhaemoglobin, and has a 

 characteristic spectrum (PI. I. spectrum 8). 



Hcemochromogen (C 34 H 36 N 4 FeO 5 ) is the substance obtained when haemo- 

 globin is decomposed by acids or by alkalies in the absence of oxygen. It 

 crystallizes and has a characteristic spectrum. 



Hcematin (C 32 H 32 N 4 FeO 4 ) is obtained when oxyhaemoglobin is decomposed 

 by acids or by alkalies in the presence of oxygen. It is amorphous and has a 

 characteristic spectrum (PI. I. spectra 9 and 10). 



Hcemin (C 32 H 30 N 4 FeO 3 HCl) is a compound of haematin and HC1, and is 

 readily obtained in crystalline form. It is much used in the detection of 

 blood in medico-legal cases, as the crystals are very characteristic and are easily 

 obtained from blood-clots or blood-stains, no matter how old these may be. 



Hcematoporphyrin (C 32 H 36 N 4 O 6 ) is a compound characterized by the absence 

 of iron. It is frequently spoken of as " iron-free haematin." It is obtained 

 by the action of strong sulphuric acid on haematin. 



Hcematoidin (C 16 H 18 N 2 O 3 ) is the name given to a crystalline substance 

 found in old blood-clots, and formed undoubtedly from the haemoglobin of 

 the clotted blood. It has been shown to be identical with one of the bile- 

 pigments, bilirubin. Its occurrence is interesting in that it demonstrates the 

 relationship between haemoglobin and the bile-pigments. 



Histohcematins are a group of pigments said to be present in many of the 

 tissues for example, the muscles. They are supposed to be respiratory pig- 

 ments, and are related physiologically, and possibly chemically, to haemoglobin. 

 They have not been isolated, but their spectra have been described. 



