BLOOD 489 



Derivatives of Haemoglobin. — The following pigments are 

 derived from haemoglobin : — 



(1) Methsemoglobin. — Hemoglobin forms another com- 

 pound with oxygen — methsemoglobin ; a substance which 

 must be acted on by strong reducing agents before it 

 will part with its oxygen. When, therefore, this pigment is 

 formed in the body, the tissues die from want of oxygen. 

 It may be produced by the action of various substances on 

 oxyhemoglobin. Among these are ferricyanides, nitrites, 

 and permanganates. It crystallises in the same form as 

 oxyhasmoglobin, but it has a chocolate brown colour. Its 

 spectrum is also different from hemoglobin and oxyhemo- 

 globin, showing a narrow sharp band in the red part of the 

 spectrum, with two or more bands in other parts according 

 to the reaction of the solution in which it is dissolved (fig. 

 199). It is of importance, since it occurs in the urine 

 in such pathological conditions as 'paroxysmal methaimo- 

 glohinuria. 



(2) Carboxyhsemoglobin. — Hemoglobin also combines with 

 some other gases. Among these is Carbon monoxide, CO. 

 Hemoglobin has a greater affinity for this gas than it has 

 for oxygen, so that, when carbon monoxide hemoglobin is 

 once formed in the body, the blood has little power of taking 

 up oxygen, and the animal dies. Carbon monoxide is 

 evolved freely in the fumes from burning charcoal, is present 

 in coal seas, and is found in the air of coal mines after 

 explosions. Carbon monoxide hemoglobin forms crystals 

 like oxyhemoglobin, and has a bright 2^inkis]i red colour, 

 without the yellow tinge of oxyhemoglobin. Since, after 

 death it does not give up its carbon monoxide and become 

 changed to purple hemoglobin, the bodies of those poisoned 

 with the gas maintain the florid colour of life. Its spectrum 

 is very like that of oxyhemoglobin, the bands being slightly 

 more to the blue end of the spectrum (fig. 199). It maybe 

 at once distinguished by the fact that when gently warmed 

 with ammonium sulphide it does not yield reduced hemo- 

 globin (Chemical Physiology). 



(3) Nitric oxide, NO, has even a greater affinity for Hb 

 than has CO. The compound is very similar in all its 



