i6 



ELEMENTARY BIOLOGY 



the leg is locomotion ; the function of the ear, to perceive certain 

 disturbances of the air which we call sounds ; the function of a 

 root, to absorb certain materials from the earth ; and so on. In study- 

 ing the behavior of an organ we usually have in mind what it does in 

 relation to the life of the organism as a whole. For there are many 

 structures in living bodies that have no special functions, as the 

 vermiform appendix in our own bodies, or the wing of an ostrich. 

 Or an organ may do something that really has no significance in the 

 working of a living machine, as the wagging of a dog's tail or the 

 movement of a boy's ear. 



25. Chemical composition. On comparing the chemical com- 

 position of living bodies with that of non-living bodies we 



Fig. I. The chemical composition of the human body- 

 In addition to the elements named there are nitrogen (N), calcium (Ca), phosphorus (P), 

 potassium (K), sodium (Na), magnesium (Mg), sulfur (S), chlorin (CI), iron (Fe), and 



traces of iodin, fluorin, and silicon 



shall find that, of the seventy-five or eighty elements that 

 have been described by the chemists (see p. 9), from twelve 

 to fifteen are found in the bodies of very nearly all plants 

 and all animals. 



There is nothing in this list of elements that distinguishes 

 living bodies. Each of these elements occurs in the soil ; 

 some of them occur in the waters of the oceans and lakes ; 

 and a few are found in the air. 



Organic and inorganic. Yet there is an important difference be- 

 tween living and non-living things on the chemical side. Although 

 the elements of the living body are the same as the elements of the 



