5 2 THE CIRCULATING LIQUIDS OF THE BODY 



taining it, and of again giving it up in the presence of oxidizable 

 substances or in an atmosphere in which the partial pressure of 

 oxygen (pp. 250-253) has been reduced below a certain limit. It 

 is this property that enables haemoglobin to perform the part of an 

 oxygen-carrier to the tissues, a function of the first importance, 

 which will be more minutely considered when we come to deal with 

 respiration. 



The bright red colour of blood drawn from an artery or of venous 

 blood after free exposure to air is due to the fact that the haemo- 

 globin is in the oxidized state 

 in the state of oxyhaemo- 

 globin, as it is called. If the 

 oxygen is removed by means 

 of reducing agents, such as 

 ammonium sulphide, or by ex- 

 posure to the vacuum of an 

 air-pump, the colour darkens, 

 the blood-pigment being now 



f*H^1 *** in the form of reduced haemo- 



raflH globin. In ordinary venous 



\ I. ^A blood a large proportion of the 



pigment is in this condition, 

 but there is always oxyhaemo- 

 globin present as well. In 

 asphyxia (p. 276), however, 

 nearly the whole of the oxy- 

 haemoglobin may disappear. 



Crystallization of Hemoglobin. 

 In the circulating blood the 

 haemoglobin is related in such a 

 way to the stroma of the cor- 

 puscles that, although the latter 

 are suspended in a liquid readily 

 capable of dissolving the pig- 

 ment, it yet remains under 

 ordinary circumstances strictly 

 within them. In a few inver- 

 tebrates, however, it is nor- 

 mally in solution in the cir- 

 culating liquid. As a rare occurrence haemoglobin may form crystals 

 inside the corpuscles (p. 71). When it is in any way brought into solu- 

 tion outside the body, it shows in many animals, but not in the same 

 degree in all, a tendency to crystallization ; and the ease with which 

 crystallization can be induced is in inverse proportion to the solubility 

 of the haemoglobin. Thus, it is far more difficult to obtain crystals of 

 haemoglobin from human blood than from the blood of the rat, guinea- 

 pig, or dog, whose blood-pigment is less soluble than that of man, and 

 for a like reason the oxyhaemo globin of the bird, the rabbit, or the frog 

 crystallizes still less readily than that of human blood. 



As to the form of the crystals, in the vast majority of animals they 



Fig. 14. Oxyheemoglobin Crystals (Frey). 

 a, b, from man ; c, from cat ; d, from guinea- 

 pig; e, from hamster;/, from squirrel. 



