CH. XXVL] 



THE LUNGS 



351 



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

 prevented. 



The pleura, like other serous sacs, is frequently the scat of intlam- 

 matory changes (pleurisy) ; the pleural cavity then becomes enlarged 

 by an increase in the amount of fluid lymph which it contains. The 

 increase is accompanied by corresponding collapse of the lungs. A 

 formation of fibrin may take place in the exuded fluid ; this adheres 

 to the pleura and causes its surfaces, originally smooth, to become 

 rough, and painful friction between the two surfaces, or even their 

 adhesion to one another, may supervene. 



Each lung is partially subdivided into separate portions called 

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

 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 minia- 

 ture, consisting, as it does, of a 

 branch of the bronchial tube, of air- 

 sacs, blood-vessels, nerves, and lym- 

 phatics, with a sparing amount of 

 areolar tissue. 



On entering a lobule, the small 

 bronchial tube, the structure of 

 which has just been described 

 (fig. 287) 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, 

 called air-sacs. Such a funnel-shaped terminal branch of the 

 bronchial tube, with its group of pouches or air-sacs, is called an 

 infundibulum, and the irregular oblong space in its centre, with 

 which the air-sacs communicate, an intercellular passage. 



The air-sacs, or air-vesicles, may be placed singly, like recesses 

 from the intercellular passage, but more often they are arranged in 

 groups, or even in rows, like minute sacculated tubes ; so that a short 

 series of vesicles, all communicating with one another, open by a 

 common orifice into the tube. The vesicles are of various forms, 

 according to the mutual pressure to which they are subject ; their 



FIG. 287. 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 communicate. From a new- 

 born child. (Kolliker.) 



