108 THE HUMAN BODY. 



Respiration. The blood in its course flows through the 

 lungs. It is necessary not merely that food, hut oxygen also 

 should be carried to every part of the body. As the blood 

 traverses the lungs it picks up oxygen from the air in them; 

 this air is then renewed by taking a fresh breath, and so 

 on. The organs concerned in renewing the air in the 

 lungs are the respiratory organs, and the act of renewal is 

 respiration. 



Assimilation. As each organ works it oxidizes; some of 

 its substance is broken down by combination with oxygen 

 brought to it by the blood, and is thus converted into burnt 

 waste matter. The blood, as we have seen, brings, how- 

 ever, not merely oxygen, but also food matters in solution. 

 These ooze through the walls of the blood-vessels, and are 

 taken up by the living tissues and built into new tissues 

 like themselves, to replace the part which has been used 

 up and destroyed. This building and repair of tissues 

 and organs from the dissolved food obtained from the 

 blood is known as assimilation, in plain English, "a 

 making alike." Each living tissue takes from the blood 

 foods which are not like itself, and builds them up into a 

 form of matter like its own. The converse process, which 

 accompanies all vital action, the breaking down into wastes 

 of a living tissue when it works, is called dissimilation, 

 or "a making unlike." 



The Relation of the Circulatory Organs and the Absorb- 

 ents to Excretion. It is as essential to the body that its 

 wastes be carried off from the organs, as that the used-up 



What must be carried to all parts in addition to food ? Where 

 does the blood get oxygen? What is meant by the respiratory 

 organs ? By respiration ? 



What happens when an organ works ? How are oxidized tissues 

 replaced ? What is meant by assimilation ? By dissimilation ? 



What is needful to each organ in addition to a supply of fresh 

 material ? 



