130 THE HUMAN BODY. 



pipe, is a kind of a box made of cartilage. It is wider 

 than the rest of the wind-pipe, and projects in front, 

 making Adam's apple. It is the voice-box. In it are 

 the vocal cords, or bands, by which sound is made. It 

 will be more fully described hereafter. 



9, The trachea is a tube four or five inches long, and 

 from half an inch to an inch in diameter, extending from 

 the larynx down into the chest. It is made of a series of 

 flat rings of cartilage, sixteen or twenty in number, which 

 do not quite come together behind, but are somewhat like 

 a horseshoe. These are connected, and covered with mem- 

 brane and muscular fibers. They serve to keep the tube 

 open. 



10, The trachea divides into two branches, called bron- 

 chi. One goes to the right, and one to the left, lung. 

 They are made just like the trachea, and are one or two 

 inches long. 



11, Each bronchus, as it enters the lung, divides into 

 smaller tubes, called bronchial tubes; and these keep on 

 dividing until they are not more than T J-^ of an inch in 

 diameter. They then end in the air-cells. 



BREATHING. 



SECTION IV. 1. We have studied the lungs, and the 

 passages leading to them. We have now to study the 

 process by which air goes in and out of them. It is not 

 enough that the passage is open. The lungs are not like 

 a house, with windows and doors, through which the 

 breeze plays freely. They are, rather, like a deep well, 

 or a mine, into which fresh air will not go, unless, in 

 some way, a current is made. We make this current by 

 breathing. 



