202 UNCONSCIOUS NERVOUS OPERATIONS 



of food by the chemical action of the digestive juices into 

 soluble substances ready to be acted upon by the absorb- 

 ents; absorption, or the passage of selected elements from 

 the digested food through the walls of the blood vessels 

 and lymphatics (lacteals); assimilation, or the conversion 

 of the new particles of food into the substance of the 

 tissues of the body; and the breaking down of the sub- 

 stance of the tissues to form waste. The building-up 

 processes constitute what is called anabolism, the break- 

 ing down of tissue Jcatabolism, while the whole double 

 operation receives the scientific name of metabolism. We 

 are now to study anabolism. 



284. Object of Nutrition. The various processes of nu- 

 trition take place in order to supply the human machine 

 with material in suitable form for the production of 

 energy. The oxidizable substances are distributed to 

 the tissues and are there submitted to the action of the 

 oxygen in the blood. Combustion then takes place, that 

 is, the substances in the tissue cells are burned. The 

 complex chemical compounds there found are decomposed, 

 and new and simpler compounds are formed, which being 

 of no further use to the body are removed from it by 

 mechanisms prepared for the purpose. 



285. Forms of Energy in the Body. The force set free 

 in the tissues by the combustion of their particles takes 

 many forms within the body, but as it leaves the body it 

 appears in two forms only : as heat or as work done by 

 the muscles. Just as matter may be changed from one 

 form to another, but cannot be destroyed, so energy, or 

 force, may be changed from one form to another, but can- 

 not be destroyed. The heat driven off in the combustion 

 of coal may be changed througli the agency of steam into 

 the motion of machinery, which may in turn be con- 



