STKUCTUKE OF THE LUNGS. 189 



substance of the organ, divide and subdivide into a number 

 of smaller and smaller branches, which penetrate to every 

 part of the organ, until at length they end in the smaller 

 subdivisions of the lung called lobules. All the larger 

 branches have walls formed of tough membrane, contain- 

 ing portions of cartilaginous rings, by which they are held 

 open, and unstriped muscular fibres, as well as longi- 

 tudinal bundles of elastic tissue. They are lined by 

 mucous membrane, the surface of which, like that of the 



Fig. 57-* 



larynx and trachea, is covered with vibratile ciliary epi- 

 thelium (fig. 58). 



As the bronchi divide they become smaller and smaller, 



* Fig. 57. A diagrammatic representation of the heart and great 

 vessels in connection with the lungs . The pericardium has been 

 removed, and the lungs are turned aside, i, right auricle ; 2, vena cava 

 superior ; 3, vena cava inferior ; 4, right ventricle ; 5, stem of the pul- 

 monary artery ; a a, its right and left branches ; 6, left auricular 

 appendage ; 7, left ventricle ; 8, aorta ; 9, 10, the two lobes of the left 

 lung ; n, 12, 13, the three lobes of the right lung ; b b, right and left 

 bronchi : v r, right and left upper pulmonary veins. 



