THE RESPIRATORY SYSTEM. 63 



a manikin of the body. We will remove the outer 

 muscles of the chest. Now, the ribs are before us. 

 Eemoving these, the contents of the chest are shown 

 precisely in their natural places. These (8, 9,) are 

 the lungs. The heart liesQimmediately under and 

 between them. 



The lungs are very spongy and light, 

 fiie^m! being composed largely of air-cells, 



whose walls are very delicate. These 

 air-cells are all connected with tubes (Turn to (25) 

 third section of lungs) and these tubes unite into 

 this one large air-passage (24) called the trachea 

 or wind-pipe. In the upper part of this wind-pipe, 

 which comes close up to the mouth, the instrument 

 of the voice, called the larynx, is situated. It is this 

 (21, 22, 23), enlarged portion of the trachea. The 

 exact structure of the larynx or voice-organ is shown 

 by special sections above the lungs in the body mani- 

 kin. This arrangement of these air-passages reminds 

 one of an inverted tree. The larynx or voice-organ 

 corresponds to the lower and thicker part of the trunk 

 of the tree ; the trachea to the trunk itself ; the branch- 

 ing air-tubes in the lungs to the branches and twigs, 

 and the air-cells to the leaves. 



The pulmonary artery, which brings 

 use of the ^ e j m p ure blood from the heart into 



jL4ii.ii.ir8* 



the lungs, branches out (18) into many 

 small capillary tubes which wind among and around 

 these numerous air-cells. When the cells of the 



