330 The Outline of Science 



contain. It becomes part and parcel of the blood, and will now, 

 after passing through the liver, be pumped round the body for the 

 various organs to select from it the nourishment they need. 



The blood is not the simple matter which it seemed to our 

 ancestors ; indeed, it has proved of late years to be full of subtle 

 interest. If you prick your finger with a needle, and put a small 

 drop of blood under the microscope, you see myriads of little 

 disks, many of them in rows like columns of pennies, in a watery 

 or yellowish fluid. The fluid the serum or plasma is the liquid 

 food of the body and the medium for conveying away the soluble 

 waste matter. The red disks or "corpuscles" are the bodies that 

 convey oxygen from the lungs to the tissues. There are about 

 five millions of them in every cubic millimetre of the blood of a 

 healthy man a woman's allowance is half a million less and 

 it is these which give the blood its red colour. We have about 

 twenty-five thousand trillions of them in all. 



Thus the microscope discovered a new and unexpected com- 

 plexity in the blood, and further research has shown that it is 

 the iron-containing pigment in these red corpuscles which is 

 chiefly concerned in the carriage of oxygen. There is very little 

 iron in the blood, and it is absurd to think that we increase its 

 quantity (once it is normal) by eating things "with iron in 

 them," as people say. But what iron there is may be called a 

 precious metal in the human body, and the red corpuscles have 

 it in a form that still more or less baffles the chemist. There are 

 believed to be something like two thousand atoms to the molecule 

 in the red pigment (haemoglobin) of the corpuscle! These disks, 

 or corpuscles, are formed, mostly, in the red marrow of the boneSj 

 and, after serving for a few weeks, they are broken up in the 

 liver or the spleen. 



Warfare in the Body 



But this is only the beginning of the interest of the blood. 

 In the drop which we observe under the microscope there are, as 



