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42 



PULMONARY ARTERIES Sect. IV. I. 3. 



teen fquare feet. On this furface the blood is expofed to the influ- 

 ence of the refpired air, through the medium of a thin moid pel- 

 licle. By this expofure to the air it has its colour changed from deep 

 red to bright fcarlet, and acquires fomething fo neceflary to the ex- 

 igence of life, that we can live fcarcely a minute without this won- 

 derful procefs. 



In aquatic animals, as fifh, the blood is expofed to the air, which is 



diffufcd in the water by the gills ; the furface of which is probably 

 greater in proportion to the external furface of their bodies, than that 

 of the air-veffels of the lungs of aerial animals to their external fur- 



faces. 



Through thefe gills, 



or aquatic lungs, a current of water is 

 made perpetually to pafs by the gaping of the 6fh, as it moves, like 

 the air in refpiration ; and from this water it is probable the fame 

 material is acquired by the gills of fifh as from the air by the lungs 

 of aerial animals. 



3. The great furface of the leaves compared to that of the trunk 

 and branches of trees is fuch, that it would feem to be an organ well 

 adapted for the purpofe of expofmg the vegetable juices to the influ- 

 ence of the air. This however we (hall fee afterwards is probably 

 performed only by their upper furfaces, which are expofed to the light 



as well as air, and on that account acquire greater oxygenation, as 

 will be fhewn hereafter : yet even in this cafe the upper furfaces of 

 the leaves muft bear a greater proportion to the furface of the bark of 



r 



the tree than that of the air-cells of the lungs of animals to their ex- 

 ternal furfaces. 



Aerial or aquatic animals, by their mufcular exertions, produce a 



current of air or water reciprocally to and from their lungs, and can 

 occafionally change the place, where they refpire, when the air or wa- 

 ter becomes vitiated. But as vegetables have but little mufcular 

 power to move their leaves, except in a few inftances 3 and as the air 

 or water is frequently nearly flationary, where they exift, it feems to 



have been neceflary to expofe their fluids to the air or water on a 



greater 



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