932 PHYSIOLOGY 



methsemoglobin molecule is derived from the oxidising agent added, 

 so that ferricyanide of potash, for instance, is converted into ferro- 

 cyanide.* Since the whole of the oxygen in the oxyhsemoglobin is 

 given off on the addition of potassium ferricyanide, we may use this 

 fact in order to determine the total amount of oxygen in combination 

 in the blood (v. p. 969). 



DERIVATIVES OF HAEMOGLOBIN. Haemoglobin is a compound 

 of an iron-containing coloured group (the prosthetic group) with 

 a protein, which probably varies somewhat in different animals. 

 The prosthetic group is identical in every case where it has been 

 examined. A separation of the prosthetic group from the protein 

 moiety can be effected with extreme ease, and occurs whenever 

 the haemoglobin is treated with weak acids, with alkalies, or is 

 heated above 70 C. The protein group is known as globin. In 

 order to separate this, oxyhaemoglobin crystals are dissolved in 

 water and treated with small quantities of very dilute hydrochloric 

 acid. A precipitate of pigment forms, which, if the haemoglobin 

 used be free from inorganic salt, rapidly dissolves in excess of the acid. 

 Alcohol and ether are then added in such relative quantities that the 

 ether separates rapidly from the aqueous solution. The colouring- 

 matter (haematin) dissolves in the ether, whilst the protein (globin) 

 remains in solution in the water. The solutions are separated by a 

 separating funnel and ammonia added carefully to the aqueous solu- 

 tion. This throws down a precipitate of the protein, which is soluble 

 in acids and alkalies and coagulable on heating ; the coagulum, how- 

 ever, is soluble in acids. It is precipitated by ammonia in the presence 

 of ammonium chloride. It contains as much as 16-89 per cent, 

 nitrogen, and yields a considerable amount of the basic derivatives 

 on hydrolysis. It is therefore classified with the histones. Haemo- 

 globin yields about 94 per cent, of globin and about 4*5 per cent, of 

 the chromogenic group, haematin. 



In order to obtain hcematin in a pure condition it is usual to start 

 with the crystalline derivative of haemoglobin known as hcemin. 

 When some dried blood is heated with a crystal of common salt and 

 placed in acetic acid on a slide, a residue is obtained in which a number 

 of reddish-brown needles are embedded known as Teichmann's crystals 

 or haemin crystals (Fig. 358). The preparation of these crystals is often 

 used as a convenient test for the identification of blood. 



In order to obtain them in large quantities the following method, devised 

 by Chalfejew, is employed. One volume of defibrinated blood is added to four 

 volumes of glacial acetic acid previously heated to 80 C. As soon as the 



* When the change is effected by reducing agents we must assume that the 

 oxygen of the water or air is the source of that required for the oxidation of the 

 reduced haemoglobin to methsemoglobin. 



