CHEMICAL BASIS OF THE ANIMAL BODY. 1291 



When decomposed in the absence of oxygen (air), as for 

 instance by the action of organic acids, more dilute mineral 

 acids or best of all by caustic alkalis, it yields a proteid, of 

 which but little is known (see p. 1175), and a coloured sub- 

 stance called haemochromogen. The latter on exposure to air 

 absorbs oxygen and becomes ordinary haematin ; it is in fact the 

 substance usually spoken of as reduced haematin. (See below.) 



2. Oxy-haemoglobin. 1 When haemoglobin is exposed to 

 the air it rapidly unites, molecule for molecule, with oxygen, 

 thus becoming oxy-haemoglobin, the characteristic constituent 

 of the red-corpuscles to which the scarlet colour of arterial 

 blood is due. It may be readily set free from the corpuscles by 

 the addition to dehbrinated blood of such fluids as alcohol, 

 ether, chloroform, water and solutions of bile-salts or by re- 

 peatedly freezing and thawing the blood; when thus set free 

 it passes into solution in the adjacent serum. From this 

 solution it may be obtained as crystals with more or less readi- 

 ness, dependently upon the kind of animal whose blood is used 

 for its preparation (see § 275), the difference being due partly 

 at least to the varying solubility of the several haemoglobins. 



To obtain rapidly a microscopic preparation of oxy-haemo- 

 globin crystals it suffices to take a drop of the blood of some 

 animal whose haemoglobin crystallizes readily (rat, guinea-pig 

 or dog), to mix a drop of it on a slide with a minute drop of 

 water and allow the mixture to evaporate until a ring of dried 

 substance is formed at the periphery. If it be now covered 

 with a cover-slip, crystals usually form in a short time, espe- 

 cially if it be kept cooled. Crystals may be also readily 

 obtained even from human blood by admixture with serum in 

 a state of slight putrefactive decomposition. For laboratory 

 purposes large quantities of crystallized oxy-haemoglobin may 

 be very readily obtained from dog's blood as follows. The 

 blood is dehbrinated and strained through fine muslin: it is 

 then placed in a flask and ether is added with frequent shaking 

 until the blood is just 'laky,' i.e. transparent. The flask is 

 now surrounded by a freezing mixture of ice and salt and in a 

 short time its contents usually become almost pasty from the 

 mass of crystals which form in it. These are then centrifu- 

 galized off, dissolved in a minimal amount of water, filtered, 

 cooled to 0°, and after the addition of one quarter of its bulk 

 of cooled alcohol again immersed in a freezing mixture. The 

 second crop of crystals thus obtained may be again recrystallized 

 as already described. The crystals are finally washed with 



1 Haemoglobin is united to corpuscles in the blood of all vertebrates with two 

 exceptions, Amphioxus and Leptocephahts. In invertebrate blood it is usually- 

 found in solution in the plasma, but there are a few (eight) exceptions to this 

 rule. 



