116 



of the absorption of elastic fluids in the human 

 lungs is of so much importance in itself, and has so 

 much divided the opinions of physiologists, as to de- 

 mand from us a more distinct and detailed discus- 

 sion. 



94. When then the air, received into our lungs, 

 is said to obtain admission into the blood-vessels by 

 a process of absorption, it may be proper, in the 

 first place, to inquire into the structure of those or^ 

 gans, at least so far as to ascertain by what means 

 such a process takes place. The lungs, one of which 

 occupies each cavity of the chest, are composed prin- 

 cipally of air-cells and blood-vessels, connected 

 through their whole extent by intervening cellular 

 membrane. The trachea, or windpipe, on its arri- 

 val in the chest, divides into a right and a left 

 branch, which branches again subdivide into smaller 

 ones called bronchia, and these into others still more 

 minute, until at length they lose their cartilaginous 

 texture, become membranous, and expand into a 

 cellular structure, which fills at all times the cavity 

 of the chest. " The cells composing this structure, 

 are purely membranous, of an irregular figure, com- 

 pressed and closely connected, and have a free com- 

 munication with each other. Between the different 

 lobes, lobules, and cells of the lungs, a large quan- 

 tity of common cellular substance, destitute of fat, 

 is interposed, which unites and strengthens them ; 

 but the cells have no communication with this sub- 

 stance ; for, when air is blown into it, the lobules 

 are compressed, but when the air is blown in through 

 a branch of the trachea, the cells are again dis- 

 tended, and the lobules recover their former dimen- 



