NATURAL THEOLOGY. 1G5 



pure ingredient, and at the same time gives out 

 the effete or corrupt air which it contained, and 

 which is carried away, along with the halitus, 

 every time we expire. At least, by comparing 

 the air which is breathed from the lungs with the 

 air which enters the lungs, it is found to have lost 

 some of its pure part, and to have brought away 

 with it an addition of its impure part. Whether 

 these experiments satisfy the question as to the 

 need which the blood stands in of being visited 

 by continual accesses of air, is not for us to in- 

 quii-e into, nor material to our argument : it is 

 sufficient to know, that, in the constitution of 

 most animals, such a necessity exists, and that 

 the air, by some means or other, must be intro- 

 duced into a near communication with the blood.^ 

 The lungs of animals are constructed for this 



*The most simple view, and the best supported, is this — thnt 

 the dark venous blood which is returning from the circulation 

 through the body is loaded with carbon. When it is carried to 

 the right side of the heart, and from that into the lungs, the 

 branches of the pulmonary artery are distributed in great minute- 

 ness on cells infinite in number. These cells communicate with 

 the extreme branches of the windpipe ; and as the atmosphere is 

 received into these cells, the circulating blood comes to be ex- 

 posed to its influence ; for neither the coats of the minute ves- 

 sels which contain the blood, nor the fine membrane of the cells 

 which contain the air, prevents the influence of the atmosphere 

 upon the blood. The carbon of the blood meeting the oxygen in 

 •the atmosphere, forms carbonic acid gas ; and the air expelled in 

 expiration, thus loaded, carries away, of course, a portion of mois- 

 ture by exhalation. (See the Dissertation entitled — On the Cir- 

 culation. Appendix.) 



