STETTCTUKE OF TRACHEA 317 



one on each side of the heart, and are held together at the 

 top by the two bronchi or main branches of the trachea. 

 If we inflate these two bags with the aid of the glass 

 tube and then tie the trachea tightly, the lungs may be 

 separated from the cavity and floated upon water. This 



Fig. 147 Bibs and muacles ; a, ribs ; 6, cartilages ; c, junction of bone and carti- 

 lage ; (Z, bony portions, and e, cartilaginous portions of breastbone ; A^ external 

 intercostal muscle ; B, internal intercostal muscle. In tbe middle the external 

 muscle has been removed. 



power to float has earned them the name of " lights " 

 from the butcher. The dissection of these separated lungs 

 shows the following structural features. 



Structure of the trachea. — The trachea is a stiff, cylin- 

 drical tube covered on the outside with a layer of fibrous, 

 connective tissue in . which are embedded the stiffening 

 rings of cartilage. The interior of the tube is lined with 

 a single layer of mucous membrane, the innermost cells of 



