2 HUMAN PHYSIOLOGY 



oxygen, oxidizes the organic compounds which compose its 

 body, thereby forming simple compounds (carbonic acid, 

 water, ammonia and simple ammonia derivatives, e.g. urea); 

 on the other hand, it again builds up its body substance 

 from materials of the outer world. The former process is 

 called dissimilation ; the latter, assimilation. 



The power of assimilation varies in different living beings. 

 Plants containing chlorophyll are able, in the presence of 

 sunlight, *o ^assimilate very simple compounds (e.g. carbonic 

 acid, watty^/nito&tibs), oxygen being set free. Certain Bac- 

 teria cafiT'aSsitfiilate free nitrogen. Animals, however, do 

 not fornY:thet.* *Dody substance from inorganic but from 

 organic /compolmds which they obtain as foods from the 

 plants. Tfte animal body is able by reduction and synthesis 

 to form 'higher. organic compounds from the organic food- 

 stuffs. Ch building of fats from carbohydrates is an ex- 

 ample of *iweIF-proven synthetical reduction taking place in 

 the anirfiahbejiy! 



The produote of dissimilation of the animal world can be 



1 * C C c 



reassiniilateci3?y t the plants, the water and the carbonic acid 

 directljv ^ut; 'the ammonia derivatives only after they have 

 been changed to nitrates by certain Bacteria found in the 

 soil. Thus the circulation of the carbon, hydrogen, and 

 nitrogen in the organic world is completed. 



For the physiological combustion a continual supply of free 

 oxygen is not necessary. Frogs can live for a long time in an 

 atmosphere free of oxygen. In this case the organism obtains the 

 necessary oxygen from the oxygen which has been stored up in its 

 body in chemical combination. Many organisms, e.g. anaerobic 

 Bacteria, can generally live in an atmosphere free of oxygen and 

 still produce carbon dioxide, obtaining the oxygen from surround- 

 ing compounds which contain this element. Unlike the intensity 

 of the fire in a furnace, the intensity of the physiological combus- 

 tion cannot be increased by an increased supply of oxygen. 



The theory of the transformation of energy is based upon 

 the law of conservation of energy discovered byj. R. Mayer 

 and H. Helmholtz. This law states that the total amount 

 of energy in the universe always remains constant, that no 



