384 BIOLOGY FOR BEGINNERS 



Nose. The air system begins with the nose, which is adapted as 

 an entrance for air, 



(1) By the hairs and moist mucus to catch dust. 



(2) By the sense of smell to guard against bad air. 



(3) By its long moist passages which warm and moisten the air. 

 The mouth was not intended as a breathing organ except in 



emergencies, and habitual mouth breathers lose all the advantages 

 mentioned above. 



Trachea. Passing from the nasal cavity to the back of the mouth, 

 the air enters the trachea. This is a large tube which opens into 

 the mouth at the back of the tongue, so that the food passes over 

 it when we swallow. Its upper end is therefore protected by the" 

 base of the tongue and by a sort of self-acting lid (epiglottis) 

 which closes when food is passing on its way to the gullet, which is 

 further back in the mouth cavity. The enlarged upper end of the 

 trachea is the larynx in which are situated the vocal (speech) or- 

 gans, and which may be seen externally as the " Adam's apple." 

 The walls of the trachea are supported by rings of cartilage, which 

 hold it open for free passage of air. 



With the hand on the larynx, swallow a mouthful of food and 

 notice two things, (1) how it rises and contracts inward to meet 

 the epiglottis, (2) how the very base of the tongue moves back and 

 down over the opening. Both these movements are to allow the 

 food to pass over the top of the trachea and into the gullet. 



Bronchi and Air Cells. At its lower end the trachea divides into 

 two branches (bronchi) extending to each lung, where they sub- 

 divide into countless minute bronchial tubes which finally terminate 

 in very thin-walled, elastic air cells of which the lung tissue is 

 largely made. Thus there is provided in one organ (the lungs) 

 enough surface for air osmosis to supply (via blood) the needs of 

 the millions of body cells that have no direct access to air. 



The Lungs. The lungs fill all the body cavity from the shoulders 

 to the diaphragm except the space occupied by the heart and blood 

 vessels. They are very spongy, consisting mainly of the air tubes 

 and cells and a very extensive network of blood vessels and capil- 

 laries, all held together by connective tissue and covered on the 



