342 



Our Surroundings 



distribution of oxygen to all parts of the body is one of the most 

 important functions of the blood. 



Nose and Other Organs. The nose, pharynx, larynx, 

 trachea, and bronchial tubes, or bronchi, form a continuous pass- 

 age through which the air passes to the air sacs of the lungs. 

 The nose has two openings, called nostrils, and is lined with 



mucous membrane containing 

 blood vessels. The air is 

 warmed as it comes in contact 

 with this membrane. The nose 

 also contains cilia, or tiny hairs, 

 which, aided by the winding 

 course of the air passage, pre- 

 vent the entrance of dust and 

 germs into the lungs. By its 

 structure, the nose is thus 

 adapted to do its work well, 

 protecting the lungs from the 

 entrance of cold air, germs and 

 dust. 



The air moves from the 

 nose into the pharynx, or throat 



THE RESPIRATORY SYSTEM cavit ^ then * P aSS6S thr U ^ 



The short arrows show the path taken by an opening covered by a small 



organ, the epiglottis, which 



raises to allow the air to pass into the trachea, or windpipe. 

 The upper part of the trachea constitutes the larynx, or voice 

 box, in which the vocal cords are located. 



The trachea is a small tube about four inches in length, 

 composed of rings of cartilage held together by connective tis- 

 sue, and lined with mucous membrane. It, like the nose, con- 

 tains cilia which protect the lungs from dust. The bronchi are 

 two divisions of the trachea which in turn divide and subdivide, 

 ending in the minute air sacs in the lungs. 



Adaptation of the Lungs. The lungs are flexible. The air 

 sacs of the lungs have very thin membrane coverings filled with 

 capillaries, thus enabling the oxygen to diffuse readily and to reach 



