THE FLUIDS OF THE BODY 67 



if its prodigious molecular weight is borne in mind. In a sense, 

 the reason for the great size of its molecule is not far to seek. 

 The atomic weight of iron (Fe = 56) is much greater than that 

 of either of the other elements contained in haemoglobin. The 

 molecule needs to be very great to float an atom of iron. As 

 it is, the corpuscles are heavier than the plasma which sur- 

 rounds them, in the proportion of about 13 to 12. Although 

 haemoglobin is a crystallizable substance, its immense molecule 

 is absolutely non-diffusible. It cannot pass through a mem- 

 brane. This is of no consequence as regards the relation of 

 haemoglobin to the walls of the capillary bloodvessels, since it is 

 contained in corpuscles ; but it is of great importance as 

 regards its relation to the discs which carry it. A very small 

 quantity of enveloping substance suffices to prevent it from 

 diffusing into the plasma of the blood. The great molecules 

 are held together and isolated from the fluid in which they 

 float by a minimal amount of insoluble globin. 



The iron needed for the making of haemoglobin is obtained 

 both from meat and vegetables. The constituents of an 

 ordinary diet provide from 2 to 3 centigrammes of iron a day. 

 The whole of the blood contains about 4-5 grammes. When 

 corpuscles are being destroyed in the spleen, the iron which 

 their pigment contains is largely reabsorbed and rendered 

 available for further use. The iron in a mixed diet is more 

 than sufficient to counterbalance any loss. Milk contains 

 extremely little iron. Before birth the liver and spleen accu- 

 mulate a store of iron which lasts until the end of the nursing 

 period, unless this be unduly prolonged. If it be prolonged, the 

 child is apt to become anaemic. Iron has been administered in 

 the treatment of anaemia ever since its presence in the red clot 

 of blood was recognized a hundred and fifty years ago. Physi- 

 cians are agreed that in the anaemia of young people it is of 

 value ; but observations made with a view to obtaining definite 

 data as to the increase in number of blood-corpuscles which 

 results from the administration of iron, without any other 

 alteration in the diet or the habits of the patient, have not given 

 accordant results. Some observers have obtained an increase 

 with organic compounds of iron, others with inorganic com- 

 pounds ; some are in favour of small doses, others of very large 

 ones. As in the treatment by drugs of other abnormal condi^ 



62 



