432 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



ments of living organisms taken from the atmosphere 

 and embodied for a brief time are again returned, and 

 the same matter is worked over and over again by an 

 eternal circulation. The circulation of C and O through 

 the atmosphere, plants, and animals back to the atmos- 

 phere is represented in diagram (Fig. 301, repeated from 

 page 5). It is seen that C is taken in the form of C0 2 

 from the atmosphere by plants, is decomposed, the C 

 fixed in organic matter, and oxygen returned to the air. 

 The C is taken from plants by animals as food, is then 

 combined with O from air taken in respiration, and re- 

 turned to the air in its original form as C0 2 , and so on 

 continually. 



The circulation of all the elements of organic matter 

 between the organic kingdom and the atmosphere is 

 represented by the diagram (Fig. 302). The food of 

 plants consists of C0 2 , H 2 0, and NH 3 . These are taken 



fltm"p^»y 



Fig. 301.— Diagram illustrating the 

 circulation of carbon and oxy- 

 gen. 



Fig. 302. — Diagram showing the 

 circulation of the elements of or- 

 ganic matter. 



from the atmosphere and embodied in organic form of 

 both plants and animals. In their death these are again 

 returned to the atmosphere as C0 2 , H 2 0, and NH 3 , to 

 be again taken as before. Thus the same small quan- 



