ANATOMY AND PHYSIOLOGY 



as the vapor exhaled from vegetables has no taste, this 

 idea is no more probable than the other; add to this 

 that in most weathers they do not appear to perspire 

 or exhale at all. 



"The internal surface of the lungs or air-vessels in 

 men is said to be equal to the external surface of the 

 whole body, or almost fifteen square feet; on this sur- 

 face the blood is exposed to the influence of the re- 

 spired air through the medium, however, of a thin 

 pellicle; by this exposure to the air it has its color 

 changed from deep red to bright scarlet, and acquires 

 something so necessary to the existence of life that 

 we can live scarcely a minute without this wonderful 

 process. 



"The analogy between the leaves of plants and the 

 lungs or gills of animals seems to embrace so many 

 circumstances that we can scarcely withhold our con- 

 sent to their performing similar offices. 



" i. The great surface of leaves compared to that of 

 the trunk and branches of trees is such that it would 

 seem to be an organ well adapted for the purpose of ex- 

 posing the vegetable juices to the influence of the air; 

 this, however, we shall see afterwards is probably per- 

 formed only by their upper surfaces, yet even in this 

 case the surface of the leaves in general bear a greater 

 proportion to the surface of the tree than the lungs of 

 animals to their external surfaces. 



"2. In the lung of animals the blood, after having 

 been exposed to the air in the extremities of the pul- 

 monary artery, is changed in color from deep red to 

 bright scarlet, and certainly in some of its essential 

 properties it is then collected by the pulmonary vein 



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