THE CIRCULATING LIQUIDS OF THE BODY 47 



of i per cent., and there is also a little sulphur, the amount of 

 which stands in a very simple relation to the quantity of iron (i atom 

 of iron to 3 of sulphur in dog's haemoglobin, and i atom of iron to 

 2 of sulphur in the haemoglobin of the horse, ox, and pig). Haemo- 

 globin appears to be made up of a proteid element which contains 

 all the sulphur, and a pigment which contains all the iron, the proteid 

 constituting by far the larger portion of the gigantic molecule, whose 

 weight has been estimated at more than 16,000 times that of a 

 molecule of hydrogen. Since its percentage composition is still 

 undetermined with absolute precision, it is impossible to give an 

 empirical formula that is more than approximately correct. For 

 dog's haemoglobin Jaquet gives C T5S H 120 3N 195 S 3 FeO 218 , which would 

 make the molecular weight 16,669. 



The most remarkable property of haemoglobin is its power 

 of combining loosely with oxygen when exposed to an 

 atmosphere containing 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. 231, 236), 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 haemoglobin is in the oxidized state in the state of 

 oxyhaemoglobin, as it is called. If the oxygen is removed 

 by means of reducing agents, such as ammonium sulphide, 

 or by exposure to the vacuum of an air-pump, the colour 

 darkens, the blood-pigment being now in the form of reduced 

 haemoglobin. In ordinary venous blood a large proportion 

 of the pigment is in this condition, but there is always 

 oxyhaemoglobin present as well. In asphyxia (p. 217), how- 

 ever, the whole of the oxyhaemoglobin may disappear. 



Crystallization of Hcemoglobin. In the circulating blood the 

 haemoglobin is related in such a way to the strorna of the corpuscles 

 that, although the latter are suspended in a liquid readily capable of 

 dissolving the pigment, it yet remains under ordinary circumstances 

 strictly within them. In a few invertebrates, however, it is normally 

 in solution in the circulating liquid. As a rare occurrence haemo- 

 globin may form crystals inside the corpuscles. When it is in any 

 way brought into solution outside the body, it shows in rrany animals* 



