106 



RESPIRATION 



The alveoli or air-cells vary in diameter from i to J mm. They are 

 composed of epithelium, strengthened with a thick network of elastic 

 connective tissue. The epithelium is of two sorts, a large, flat, thin-celled 

 variety like endothelium, and a kind composed of small flat polygonal 

 nucleated cells. The latter lie singly or in small groups scattered among the 

 cells of the former variety, and are, it is supposed, substitutes ready, after 

 differentiation, to take the places of destroyed cells of the other variety. 

 Otherwise their special purpose is quite unknown. It is possible that they 

 may have some specific osmotic powers, as probably have the different 

 sorts of epithelium of the uriniferous (kidney) tubules. (See page 246). 

 The number of the respiratory alveoli in adult human beings has been 



FIG. 57 



Magnification of the lung-area by the alveoli. Were the lungs plain sacs instead of organs 

 made up of 725,000,000 alveoli, their respiratory capacity would be only about one two-thousandth 

 of what it is. This ratio is that of the small square to the large. 



estimated at 725,000,000. Bearing in mind the vastness of this multitude, 

 it is easier to believe, what appears to be true, that the combined concave 

 area of the alveoli is about 200 square meters, or more than one hundred 

 times the entire superficial area of the body. This is the size of the air- 

 layer exposed on what is functionally the periphery of the lungs. Its great 

 size makes it possible to understand how the gaseous interchange can 

 take place, for, as we shall see, the forces which bring about the osmosis 

 back and forth through the alveolar wall are slight and can have, there- 

 fore, small intensive action. This lack of intensity is made up by 



