THE BLOOD. 91 



to cause the haemoglobin to give up its oxygen, it is believed that the 

 oxygen is but loosely combined with the substance. 



Haemoglobin. Haemoglobin is a crystallizable body which constitutes 

 by fur the largest portion of the colored corpuscles. It is intimately dis- 

 tributed throughout their stroma, and must be dissolved out of it before 

 it will undergo crystallization. Its percentage composition is C. 53*85; 

 11. 7-32; K". 16-17; 0. 21-84; S. -63; Fe. '42; and if the molecule be sup- 

 posed to contain one atom of iron the formula would be C 600 , H 960 , N 1M , 

 Fe S s , 179 . The most interesting of the properties of haemoglobin are its 

 powers of crystallizing and its attraction for oxygen and other gases. 



Crystals. The haemoglobin of the blood of various animals possesses 

 the power of crystallizing to very different extents (blood-crystals). In some 

 animals the formation of crystals is almost spontaneous, whereas in others 

 crystals are formed either with great difficulty or not at all. Among the 

 animals whose blood coloring-matter crystallizes most readily are the 

 guinea-pig, rat, squirrel, and dog; and in these cases to obtain crystals it 

 is generally sufficient to dilute a drop of recently -drawn blood with water 

 and expose it for a few minutes to the air. Light seems to favor the for- 

 mation of the crystals. In many instances oth ?r means must be adopted, 

 e.f./., the addition of alcohol, ether, or chloroform, rapid freezing, and 

 then thawing, an electric current, a temperature of 140 F. (60 C.), or 

 the addition of sodium sulphate. 



Human blood crystallizes with difficulty, as does also that of the ox, 

 the pig, the sheep, and the rabbit. 



FIG. 81. Crystals of oxy-hsemoglobin prismatic from human blood. 



The forms of haemoglobin crystals, as will be seen from the appended 

 figures, differ greatly. 



Haemogloblin crystals are soluble in water. Both the crystals them- 

 selves and also their solutions have the characteristic color of arterial 

 blood. 



