EARTH 3 



Recognizing that we have the kind of internal environ- 

 ment we have because we have the kind of kidneys that 

 we have, we must acknowledge that our kidneys con- 

 stitute the major foundation of our physiological free- 

 dom. Only because they work the way they do has it 

 become possible for us to have bones, muscles, glands, 

 and brains. Superficially, it might be said that the func- 

 tion of the kidneys is to make urine; but in a more con- 

 sidered view one can say that the kidneys make the stuff 

 of philosophy itself. 



This is the story of how our kidneys work, and of how 

 they came to work the way they do— which is the story 

 of the evolution of the vertebrates, of which man is the 

 most notable and intelligent species and the only phi- 

 losopher. But the story of the evolution of the vertebrates 

 is one with the story of the earth, and it is with the story 

 of the earth that we begin this book. 



The planet Earth seems fairly sohd beneath our feet, 

 but under the examination of the geologist it is revealed 

 to be an inhomogeneous and unstable sphere that has 

 had a most agitated history. Students of cosmogony are 

 not in unanimous agreement how the solar system was 

 formed, but it is generally agreed that the sun and 

 planets had a common origin from interplanetary dust 

 at a date not much over five thousand million years ago, 

 and about a billion years later the earth acquired its pres- 

 ent stratified structure. In cross section, this structure 

 presents several more or less distinct, concentric spheres 

 that differ from each other substantially in composition 

 and temperature. (Figure i.) The outermost sphere, 

 called the crust, varies in thickness from 2 to 3 miles 

 under the deep oceans, to some 30 to 50 miles in the 

 continental masses. Underneath the crust is what is 

 called the mantle, the outer layer of which is perhaps a 

 basaltic rock, while the inner layer is presumed to con- 

 sist of metallic sulfides and oxides, the entire mantle ex- 

 tending to a total depth of some 1800 miles. The con- 



