36 HUMAN PHYSIOLOGY 



to be further degraded, so far as its nitrogenous group- 

 ings are concerned, by the bacteria of the soil. No 

 mention has been made in this discussion of the abound- 

 ing atmospheric nitrogen. The free element does not 

 arise in any great quantity from organic decompositions 

 nor does it generally enter into the economy of living 

 matter directly from the air. In a few cases, which are 

 highly important, nitrogen is captured by bacteria and 

 " fixed" by them as a part of their own substance. 



