166 NATURAL THEOLOGY. 



purpose. They consist of blood-vessels and air- 

 vessels, lying close to each other ; and whenever 

 there is a branch of the trachea or windpipe, 

 there is a branch accompanying it of the vein and 

 artery, and the air-vessel is always in the middle 

 between the blood-vessels.* The internal sur- 

 face of these vessels, upon which the application 

 of the air to the blood depends, would, if collect- 

 ed and expanded, be, in a man, equal to a super- 

 ficies of fifteen feet square. Now, in order to 

 give the blood in its course the benefit of this or- 

 ganization, (and this is the part of the subject 

 with which we are chiefly concerned,) the follow- 

 ing operation takes place. As soon as the blood 

 is received by the heart from the veins of the 

 body, and before that is sent out again into its 

 arteries, it is carried, by the force of the contrac- 

 tion of the heart, and by means of a separate and 

 supplementary artery, to the lungs, and made to 

 enter the vessels of the lungs ; from which, after 

 it has undergone the action, whatever it be, of 

 that viscus, it is brought back by a large vein 

 once more to the heart, in order, when thus con- 

 cocted and prepared, to be thence distributed 

 anew into the system. This assigns to the heart 

 a double office. The pulmonary circulation is a 

 system within a system ; and one action of the 

 heart is the origin of both. 



For this complicated function four cavities 

 become necessary, and four are accordingly pro- 



* Kcill's Anatomy, p. 121. 



