STRUCTURE OF THE LUNGS. 



157 



The trachea, or tube through which air passes to the lungs, 

 divides into two branches one for each lung ; and these primary 

 branches, or bronchi, after entering the 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, containing portions of cartilaginous rings, by which 



FIG. 57. 



A diagrammatic representation of the heart and great vessels in connection with 

 the lungs Y & . The pericardium has been removed, and the lungs are turned aside- 

 1, right auricle; 2, vena cava superior, 3, vena cava inferior ; 4, right ventricle ; 5, 

 stem of the pulmonary 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; 11, 12) 

 13, the three lobes of the right lung ; b b, right and left bronchi; v v, right and. left 

 upper pulmonary veins. 



they are held open, and unstriped muscular fibres, as well as 

 longitudinal bundles of elastic tissue. They are lined by mu- 

 cous membrane, the surface of which, like that of the larynx 

 and trachea, is covered with vibratile ciliary epithelium (Fig. 

 Ooj. 



As the bronchi divide they become smaller and smaller, and 

 their walls thinner ; the cartilaginous rings especially becom- 



brane which covers the lung is called the visceral layer of the pleura, 

 and that which lines the walls and floor of the chest the parietal l&yer. 

 The appearance of a cavity or space (Fig. 56) between the visceral 

 layer of pleura (covering the lungs) and the parietal layer (covering 

 the inner surface of the wall of the chest and upper part of the dia- 

 phragm) is only inserted for the sake of distinctness. 



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