NATURAL THEOLOGY. 159 



it by one aperture. What enters at the mouth finds 

 its way to the fingers' ends. A more difficult me- 

 chanical problem could hardly, I think, be propos- 

 ed, than to discover a method of constantly repair- 

 ing 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 construc- 

 tion of the engine at the centre, viz., the heart, fur 

 di-iving 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 tov»^ards 

 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 

 thins necessary to the blood, which is not wanted 



CD v' ' 



for the water ; and that is, the carrying of it back 

 again to its source. For this office, a reversed sys- 

 tem of vessels is prepared, which, uniting at their 

 extremities with the extremities of the first system, 

 collects the divided and subdivided streamlets, 

 by first, capillary ramifications into larger branches, 



3'' We must refer our reader to the Dissertation in the Appendix 

 On the Circulation of the Blood and its Uses. 



