296 THE HUMAN BODY. 



plex unstable bodies, capable of yielding energy when again 

 broken down. However, to do such work, to break up 

 stable combinations and make from them less stable, needs 

 a supply of kinetic energy, which disappears in the process 

 being stored away as potential energy in the new compound; 

 and we may ask whence it is that the plant gets the supply 

 of energy which it thus utilizes for chemical construction, 

 since its simple and highly oxidized foods can yield it none. 

 It tas been proved that for this purpose the green plant 

 uses the energy of sunlight: those of its cells which contain 

 the substance called chlorophyl (leaf green) have the power 

 of utilizing energy in the form of light for the perform- 

 ance of chemical work, just as a steam-engine can utilize 

 heat for the performance of mechanical work. Exposed to 

 light, and receiving carbon dioxide from the air, and water 

 and ammonia (which is produced by the decomposition of 

 urea) from the soil, the plant builds them up again, with 

 the elimination of oxygen, into complex bodies like those 

 which animals broke down with fixation of oxygen. 

 Some of the bodies thus formed it uses for its own growth 

 and the formation of new protoplasm, just as an animal 

 does; but in sunlight it forms more than it uses, and the 

 excess stored up in its tissues is used by animals. In the 

 long run, then, all the energy spent by our Bodies comes 

 through millions of miles of space from the sun; but to 

 seek the source of its supply there would take us far out of 

 the domain of Physiology (see Astronomy). 



Non-Oxidizable Foods. Besides our oxidizable foods, a 

 large number of necessary food materials are not oxidiza- 

 ble, or at least are not oxidized in the Body. Typical in- 

 stances are afforded by water and common salt. The use 

 of these is in great part physical: the water, for instance, 

 dissolves materials in the alimentary canal, and carries the 

 solutions through the walls of the digestive tube into 

 the blood and lymph vessels, so that they can be carried 

 from part to part; and it permits interchanges to go on by 

 diffusion. The salines also influence the solubility and 

 chemical interchanges of other things present with them. 

 Serum albumen, the chief proteid of the blood, for example, 



