zoo 



RESPIRATION. 



As the bronchi divide they become smaller and smaller, 

 and their walls thinner -, the cartilaginous rings especially 



Fig. 57.* 



becoming scarcer and more irregular, until, in the smaller 

 bronchial tubes, they are represented only by minute and 

 scattered cartilaginous flakes. And when the bronchi, by 

 successive branches, are reduced to about -^ of an inch 

 in diameter, they lose their cartilaginous element alto- 

 gether, and their walls are formed only of a tough, fibrous, 

 elastic membrane, with traces of circular muscular fibres ; 

 they are still lined, however, by a thin mucous membrane, 

 with ciliated epithelium. 



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

 bronchi ; v v, right and left upper pulmonary veins. 



