348 



INSPIRATION 



[CH. XXIV. 



If, however, an opening is made so as to permit air or fluid to 

 enter the pleural sac, the lung, in virtue of its elasticity, recoils, and 

 a considerable space is left between it and the chest-wall. In other 

 words, the natural elasticity of the lungs would cause them at all 

 times to contract away from the ribs were it not that the contraction 

 is resisted by atmospheric pressure which bears only on the inner 

 surface of the air-tubes and air-sacs. On the admission of air into 

 the pleural sac atmospheric pressure bears alike on the inner and 

 outer surfaces of the lung, and their elastic recoil is no longer 

 prevented. 



Each lung is partially subdivided into separate portions called 

 lobes ; the right lung into three lobes, and the left into two. Each 



a 



FIG. 323. Terminal branch of a bronchial 

 tube, with its infundibula and air-sacs, 

 from the margin of the lung of a monkey, 

 injected with quicksilver, a, Terminal 

 bronchial twig ; b b, infundibula and air- 

 sacs, x 10. (F. E. Schulze.) 



Fio. 324. Two small infundibula or 

 groups of air-sacs, a a, with air-sacs, 

 b b, and the ultimate bronchial tubes, 

 c c, with which the air-sacs com- 

 municate. From a new-born child. 

 (Kolliker.) 



of these lobes, again, is composed of a large number of minute parts, 

 called lobules. Each pulmonary lobule may be considered to be a 

 lung in miniature, consisting, as it does, of a branch of the bronchial 

 tube, of air-sacs, blood-vessels, nerves, and lymphatics, with a sparing 

 amount of areolar tissue. 



On entering a lobule, the small bronchial tube, the structure of 

 which has just been described (a, fig. 323), divides and subdivides ; 

 its walls at the same time become thinner and thinner, until at 

 length they are formed only of a thin membrane of areolar, muscular, 

 and elastic tissue, lined by a layer of pavement epithelium not pro- 

 vided with cilia. At the same time they are altered in shape ; each 

 of the minute terminal branches widens out funnel-wise, and its 

 walls are pouched out irregularly into small saccular dilatations, 



