Chemical Changes in Animal Organism. 269 



tion of providing for the energy needs of the organism and 

 for the repair of bodily waste. By far the greater part of 

 the food is employed for the first-named of these two func- 

 tions, only relatively small parts being required, at any 

 rate in the case of the adult, for the formation of tissue. 

 By far the largest amount of food ingested undergoes 

 oxidative destruction, and the foremost problem for the 

 biological chemist is to elucidate the chemical mechanism 

 of the oxidative processes. With regard to the chemical 

 mechanism of tissue formation practically nothing is known, 

 but within recent years much work has been done in de- 

 termining the food materials which are necessary for tissue 

 repair and growth. 



The first stages in the investigation of metabolism 

 consist in the examination of the initial and the end pro- 

 ducts of the metabolic processes in drawing up what, at 

 the heading of this section, has been designated the 

 balance-sheet of the organism. 



When food is ingested it passes into the alimentary 

 tract, in which it undergoes various degradative processes, 

 some of which will be considered later ; the degraded pro- 

 ducts are absorbed into the blood stream, which is kept 

 circulating by means of the heart pump, and are conveyed 

 to the various organs of the body, where they undergo 

 other changes. The oxygen supply for the combustion is 

 carried also in the blood in combination with the haemoglobin 

 (oxyhaBmoglobin), the oxygenation taking place during 

 inspiration by the lungs. The blood also carries away 

 the products of catabolism of the organs, of which the 

 carbon dioxide, one end product of oxidation, is excreted by 

 the lungs during expiration, whereas the nitrogenous pro- 

 ducts of catabolism are eliminated by the kidneys in the 

 urine. There are in addition certain other waste products 

 which are not absorbed but which pass out through the 



