141 



of the superficies of all the cells, independent of that 

 of the bronchia, equal to 2 1 907 square inches, or 

 more than ten times greater than that of the whole 

 body *. The irregularity in the shape and size of the 

 cells, and the want of adequate means correctly to 

 determine their number, render an accurate estimate 

 of their entire surface extremely difficult, if not al- 

 together impossible : it is sufficient for our purpose 

 to have shewn, that they exceed, in extent, by many 

 times, the surface of the human body. The ramifi- 

 cations of the pulmonary artery and veins, running 

 over these cells, must be minute and extensive, in 

 proportion : and " by this admirable contrivance," 

 says Dr Hales, " the blood is spread out into a vast 

 expanse, commensurate to a very large surface of 

 air, from which it is divided by thin partitions, so very 

 thin as thereby to bring the blood and air within the 

 sphere of each other's attraction f." Such a struc- 

 ture, how favourable soever it may be to an extensive 

 contact of surfaces, is but little fitted to promote a 

 mixture of substances by absorption. 



SECTION II. 



115. IN the former section, we have endeavoured 

 to shew the necessity of atmospheric air to the con- 

 tinuance of living action in the superior animals, 



Tentam. Med. Phys. p. 80. 

 Statical Essays, vol. i. p. 247- 



