Chap. XIII] 



RESPIRATORY SYSTEM 



239 



their walls become thinner, the small plates of cartilage cease, 

 the fibrous tissue disappears, and the finer tubes are composed 

 of only a thin layer of muscular and elastic tissue lined by mucous 

 membrane. 



LUNGS 



The lungs are cone-shaped organs which occupy almost all of the 

 cavity of the thorax that is not taken up by the heart, the large 

 blood-vessels, the lymphatics, and the oesophagus. Each lung 



RIGHT LUNG 



LEFT LUNG 



Fig. 144. — Bronchi and Bronchioles. The lungs have been widely sepa- 

 rated and tissue cut away to expose the air-tubes. (Gerrish.) 



presents an outer surface which is convex, a base which is concave 

 to fit over the convex portion of the diaphragm, and a summit or 

 apex which rises half an inch above the clavicle. On the inner 

 surface is a vertical notch called the hilum, which gives passage 

 to the bronchi, blood-vessels, lymph-vessels, and nerves. 



The right lung is the larger and heavier ; it is broader than the 

 left, owing to the inclination of the heart to the left side ; it is 

 also shorter by one inch, in consequence of the diaphragm rising 



