104 NATURAL THEOLOaY. 



CHAPTER X. 



OF THE VESSELS OF ANIMAL BODIES. 



The circulation of the blood tlirougli 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 lym- 

 phatic system, or the nervous system, may be more subtle 

 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 answer- 

 ed by it — the distributing to every part, every extremity, 

 every nook and corner of the body, the nourishment v/hich 

 is received into it by one aperture. What enters at the 

 mouth finds its way to the fingers' ends. A more difficult 

 mechanical problem could hardly, I think, be proposed, 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 bloodvessels, that is, the laying of the pi^es ; 

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

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



I. The disposition of the bloodvessels, as far as regards 

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

 city, namely, large and main trunks blanching ofi^by small- 

 er 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 office 



