CIRCULATION.] PHYSIOLOGY, . 51 



Blood, then, is a very wonderful fluid : wonderful for 

 being made up of coloured corpuscles and colourless 

 fluid, wonderful for its fibrin and power of clotting, 

 wonderful for the many substances, for the proteids, 

 for the ashes or minerals, for the rest of the things 

 which are locked up in the corpuscles and in the 

 serum. 



But you will not wonder at it when you come to 

 see that the blood is the great circulating market of 

 the body, in which all the things that are wanted by 

 all parts, by the muscles, by the brain, by the skin, by 

 the lungs, liver, and kidney, are bought and sold. 

 What the muscle wants, it, as we have seen, buys from 

 the blood ; what it has done with it sells back to the 

 blood ; and so with every other organ and part. As 

 long as life lasts this buying and selling is for ever 

 going on, and this is why the blood is for ever on the 

 move, sweeping restlessly from place to place, bringing 

 to each part the things it wants, and carrying away 

 those with which it has done. When the blood ceases 

 to move, the market is blocked, the bupng and selling 

 cease, and all the organs die, starved for the lack of 

 the things which they want, choked by the abundance 

 of things for which they have no longer any need. 



We have now to learn how the blood is thus kept 

 continually on the move. 



HOW THE BLOOD MOVES. § V. 



26. You have already learnt to recognize the 

 blood-vessels of the rabbit, and to distinguish two 

 kinds of blood-vessels — the arteries, which in a dead 

 animal generally contain little or no blood, and have 



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