OF THE VESSELS, (fec. 91 



CHAP. X. 



OF THE VESSELS OF ANIMAL BODIES. 



The circulation of the hlood, through the bodies of men 

 and quadrupeds, and the apparatus by which it is carried 

 on. compose a system, and testify a contrivance, perhaps 

 the best understood of any part of the animal frame. The 

 lymphatic vessels, or the nervous system, may be more sub- 

 tile and intricate ; nay, it is possible that in their structure 

 they may be even more artificial than the sanguiferous ; 

 but we do not know so much about them. 



The utility of the circulation of the blood, I assume as 

 an acknowledged point. One grand purpose is plainly 

 answered by it ; the distributing to every part, every ex- 

 tremity, every nook and corner of the body, the nourish- 

 ment which is received into it by one aperture. What en- 

 ters at the mouth finds its way to the fingers' end. A more 

 difficult mechanical problem could hardly I think be pro- 

 posed, than to discover a method of constantly repairing 

 the waste, and of supplying an accession of substance to 

 every part of a complicated machine at the same time. 



This system presents itself under two views : first, the 

 disposition of the blood-vessels, i. e. the laying of the pipes ; 

 and, secondly, the construction of the engine at the centre, 

 viz. the heart, for driving the blood through them. 



I. The disposition of the blood-vessels, as far as regards 

 the supply of the body, is like that of the water pipes in a 

 city, viz. large and main trunks branching off by smaller 

 pipes (and these again by still narrower tubes,) in every 

 direction, and towards every part, in which the fluid, which 

 they convey, can be wanted. So far the water pipes, 

 which serve a town, may represent the vessels, which carry 

 the blood from the heart. But there is another thing 

 necessary to the blood, which is not wanted for the water; 

 and that is, the carrying of it back again to its source. 

 For this ofiice a reversed system of vessels is prepared, 

 which uniting at their extremities with the extremities of 

 the first system, collects the divided and subdivided 

 streamlets, first by capillary ramifications into larger branch- 

 es, secondly, by these [branches into trunks ; and thus re- 

 turns the blood (almost exactly inverting the order in which 

 it went out) to the fountain from whence its motion pro- 

 ceeded. All which is evident mechanism. 



