CHAPTER I 



EARTH 



Nearly a century has elapsed since Claude Bernard, 

 among the most notable of nineteenth-century physiolo- 

 gists, first pointed out that the true mediimi in which 

 we live is neither air nor water, but the plasma or Uquid 

 part of the blood that bathes all the tissue elements. This 

 'internal environment,' as he later called it, is so isolated 

 from the world that atmospheric disturbances cannot al- 

 ter it or penetrate beyond it: It is as though the organism 

 had enclosed itself,' he said, m a kind of hothouse where 

 the perpetual changes in external conditions cannot 

 reach it/ It was Bernard's view that we achieve a free 

 and independent life, physically and mentally, because 

 of the constancy of the composition of our internal en- 

 vironment. 



In Bernard's time the chemistry of living organisms 

 was poorly laiown and afforded only a meager insight 

 into the complexity of the internal environment. But as 

 the modem sciences of biochemistry and physiology 

 have added chapter after chapter on this subject, this 

 new knowledge has only emphasized the importance of 

 his generalization. 



Apart from the red and white blood cells and other 

 'formed' elements, the extracellular fluid (i.e., blood 

 plasma and interstitial fluid) of all animals contains many 

 different organic and inorganic substances. In the or- 



