336 THE BLOOD. 



Nitric Oxide Haemoglobin. This compound is more stable even 

 than carbon monoxide haemoglobin, and, like this, may be obtained 

 in crystalline form. Its spectrum is similar to that of carbon mon- 

 oxide haemoglobin. The bands, however, are less sharply defined 

 and paler than those of that compound, and, like these, do not dis- 

 appear on the addition of a reducing agent. The substance is met 

 with in poisoning with the gas in question. 



Methaemoglobin. Methaemoglobin is a pigment which normal Iv 

 does not occur in the blood, but is found after the ingestion of large 

 amounts of potassium chlorate, antifebrin, potassium permanganate, 

 turpentine, kairin, thallin, following the inhalation of nitrite of 

 amyl, ether, etc. It is encountered also in hemorrhagic transudates 

 and cystic fluids, and may occur in the urine when methaemoglobin- 

 semia exists. 



The elementary composition of methaemoglobin is the same as 

 that of oxyhaemoglobin, but its molecular structure is manifestly 

 different, as in a vacuum it does not give up its oxygen. On 

 treating with reducing agents or on exposure to putrefactive organ- 

 isms, in the absence of oxygen, it is converted into haemoglobin. 

 When oxy haemoglobin is decomposed with dilute acids or alkalies, 

 methaemoglobin is formed at some stage of the process, and precedes 

 the formation of haematin. During the preservation of oxy haemo- 

 globin in the dry state, moreover, a partial transformation into 

 methaemoglobin is very likely to occur. The substance is crystal- 

 lizable, and may be obtained in this form by treating a concentrated 

 solution of oxyhaemoglobin with a saturated solution of potassium 

 ferricyanide until the color has changed to a port-brown. The 

 mixture is cooled to C., and treated with one-quarter of its 

 volume of cooled alcohol. When kept in the refrigerator crystal- 

 lization takes place in the course of a few days. The crystals are 

 of a brown color, and occur as needles, prisms, or hexagonal plate- 

 lets. They may be purified by recrystallization from water in the 

 presence of alcohol. An aqueous solution of the substance is 

 brown, while its alkaline solutions are beautifully red. On exposure 

 to sunlight its neutral and dilute solutions gradually assume a 

 dark-red color, which is thought to be referable to a transformation 

 of methaemoglobin into photomethcemoglobin. On spectroscopic 

 examination such solutions show one broad band of absorption in 

 the green portion of the spectrum no matter whether the reaction is 

 alkaline, neutral, or acid. Methaemoglobin proper under the same 

 conditions gives a fairly broad band of absorption between C and D, 

 nearer C, which is characteristic, and disappears on the addition of 

 sodium hydrate solutioli. In addition, tw r o different bands may at 

 times be seen between D and E, which are thought to be referable, 

 however, to a contamination of the substance with haemoglobin. 

 Some observers further speak of an additional band near F, but 

 this is not characteristic. On reduction of the alkalini/ed solution 

 with ammonium sulphide the spectrum of haemochromogen results* 



