316 THE HUMAN BODY. 



tain the substance called chloropUyl (leaf green) have the power 

 of utilizing energy in the form of light for the performance 

 of chemical work, just as a steam-engine can utilize heat for the 

 performance of mechanical work. Exposed to light, and re- 

 ceiving carbon dioxide from the air, and water and ammonia 

 (which is produced by the decomposition of urea) and other 

 simple nitrogen compounds 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. 



Non-oxidizable Foods. Besides our oxidizable foods, a 

 large number of necessary food-materials are not oxidizable, 

 or at least are not oxidized in the Body. Typical instances 

 are afforded by water and common salt. The use of these is 

 in great part physical : the water, for instance, dissolves ma- 

 terials 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, is insoluble in pure 

 water, but dissolves readily if a small quantity of neutral salts 

 is present. Besides such uses the non-oxidizable foods have 

 probably others, in what we may call machinery formation. 

 In the salts which give their hardness to the bones and teeth, 

 we have an example of such an employment of them : and to 

 a less extent the same may be true of other tissues. The 

 Body, in fact, is not a mere store of potential energy, but 

 something more it is a machine for the disposal of it in cer- 

 tain ways; and, wherever practicable, it is clearly advanta- 

 geous to have the purely energy-expending parts made of 

 non-oxidizable matters, and so protected from change and 

 the necessity of frequent renewal. The Body is a self -build- 

 ing and self-repairing machine, and the material for this 



