CHAPTER XII 



HOW A " HEAD OF PRESSURE " IS MAINTAINED IN 

 THE ARTERIES 



We have seen that the muscular engines of the body- 

 are permeated from end to end with a network of fine 

 capillary vessels. From this network countless cylinders 

 of the muscles are fed with fuel and oxygen. We have 

 also seen that all the levers of the body are traversed by 

 a similar system of fine blood-vessels from which the 

 bone-builders draw material for their sustenance and work. 

 Indeed every organ of the body — the brain, liver, stomach, 

 skin, and the heart itself — is really a spongework of capil- 

 laries with the living units or corpuscles built within the 

 exceeding fine interstices of the meshwork. If we could 

 wash out all the living units from a man's body and yet 

 preserve the capillary network — as the rain sometimes 

 does to withered leaves — we should have before us the 

 complete outline of a man made up of a network of 

 vessels so minute that we should require a microscope to 

 detect the meshes. We should see in such a sponge-like 

 spectre of a man the arteries branch and rebranch until 

 they ultimately ended in the capillary field of vessels ; 

 we should see the veins begin on the edges of the field 

 as exceedingly small vessels, which unite and reunite 

 until the large channels leading towards the heart become 

 formed. We are now to see how the heart maintains a 

 constant flow of blood throughout this vast capillary field. 

 We cannot prick the living body anywhere without draw- 

 ing blood, showing that the flow is everywhere, and that 

 it never ceases ; no matter which artery we wound, the 



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