66 THE BODY AT- WORK 



these organs. From the liver they disappear early. In most 

 mammals they disappear from the spleen about the time of 

 birth ; but in some the hedgehog, for example they are 

 found in the spleen throughout the whole of life. 



Haemoglobin is a substance which has the property of uniting 

 with oxygen to form oxyhaemoglobin a compound from which 

 the oxygen is, again, very readily withdrawn. It is extremely 

 soluble, but may be made to crystallize by adding alcohol to 

 blood, after setting the haemoglobin free from the corpuscles 

 by freezing and thawing. From the blood of Man and most 

 other animals it crystallizes in the form of rhombic prisms, 

 whether in the oxidized (oxyhaemoglobin) or non-oxidized 

 condition. The addition of oxygen does not affect its crystal- 

 line form ; although crystalline, it is absolutely non-diffusible. 

 This is due to the great size of its molecule, which is probably 

 larger than that of any other substance which is capable of 

 crystallizing. 



The percentage composition of haemoglobin conforms closely 

 with that of albumin and other proteins, with this most im- 

 portant difference : it contains a definite proportion of iron 

 0-336 per cent. That the percentage of carbon, hydrogen, 

 nitrogen, sulphur, and oxygen should agree with that com- 

 monly found in proteins is inevitable, since it may be split 

 into a part which contains all the iron, haematin, and a proteid 

 part resembling albumin ; and the latter constitutes 96 per 

 cent, of its weight. 



There is no doubt but that its value as a vehicle of oxygen 

 depends upon the presence of iron. In the matter of taking up 

 and dropping oxygen, haematin behaves somewhat in the same 

 manner as haemoglobin ; whereas if iron be removed from 

 haematin the " iron-free haematin " loses its respiratory value. 

 It is almost certain that a molecule of haemoglobin contains a 

 single atom of iron. On this supposition its molecular formula 

 may be calculated. It is not quite the same for all animals, 

 although the variations are slight. For the blood of the horse 

 it is as follows : 



C 712 H 113 oN 214 S 2 Fe0 245 . 



This means a molecular weight of 16708. We give the figures, 

 because the properties of haemoglobin will be better understood 



