CH. xxvi.] HAEMOGLOBIN. 427 



is found in the blood after asphyxia. It also occurs in all venous 

 blood that is, blood which is returning to the heart after it has 

 supplied the tissues with oxygen. Venous blood, however, always 

 contains a considerable quantity of oxy haemoglobin also. Haemo- 

 globin is the oxygen-carrier of the body, and it may be called a 

 respiratory pigment.* 



Crystals of oxyhaemoglobin t may be obtained with readiness 

 from the blood of such animals as the rat, guinea-pig, or dog ; 

 with difficulty from other 

 animals such as man, ape, 

 and most of the common 

 mammals. The following 

 methods are the best : 



1. Mix a drop of de- 

 fibrinated blood of the rat 

 on a slide with a drop of 

 water ; put on a cover- 

 glass ; in a few minutes 

 the corpuscles are rendered 

 colourless, and then the 

 oxyhscmoglobin crystal- 

 lises out from the solu- 

 tion so formed. 



2. Microscopical sped- ^ j^-c^tals of oxytamortobin -prismatic, 

 incus may also be made from human blood. 



by Stein's method, which 



consists in using Canada balsam instead of water in the foregoing 



experiment. 



3. On a larger scale, crystals may be obtained by mixing the 

 blood with one-sixteenth of its volume of ether ; the corpuscles 

 dissolve and the blood assiimes a laky appearance. After a period 

 varying from a few minutes to days, abundant crystals are 

 deposited. 



In nearly all animals the crystals are rhombic prisms (fig. 359); 

 but in the guinea-pig they are rhombic tetrahedra, or four-sided 

 pyramids (fig. 360) ; in the squirrel and hamster, hexagonal 

 plates (fig. 361). 



The crystals contain a varying amount of water of crystalli- 



* In the blood of invertebrate animals haemoglobin is sometimes found, 

 but usually in the plasma, not in special corpuscles. Sometimes it is replaced 

 by other respiratory pigments, such as the green one, chlorocruorin. found 

 in certain worms, and the blue one, haemocyanin, found in many molluscs 

 and Crustacea. Chlorocruorin contains iron ; haemocyanin contains copper. 



t ( 'rystals of haemoglobin can also be obtained by carrying out the crystal- 

 lisation in an atmosphere free from oxygen. 



