344 RESPIRATION. [CH. xxiv. 



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 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. 314), divides and sub- 

 divides ; 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 provided with cilia. At the same time they are 

 altered iu 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 (fig. 314, b). Such a 

 funnel-shaped terminal branch of the bronchial tube, with its 

 group of pouches or air-sacs, has been called an infundilndum 

 (figs. 314, 315), 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 walls are nearly in contact, and they vary from 

 -sVth to Toth of an inch ('5 to '3 mm.) in diameter. Their walls 

 are formed of fine membrane, like those of the intercellular 

 passage ; this membrane is folded on itself so as to form a sharp- 

 edged border at each circular orifice of communication between 

 contiguous air-vesicles, or between the vesicles and the bronchial 

 passages. Numerous fibres of elastic tissue are spread out between 

 contiguous air-sacs, and many of these are attached to the outer 

 surface of the fine membrane of which each sac is composed, 

 imparting to it additional strength and the power of recoil after 

 distension. The vesicles are lined by a layer of pavement 

 epithelium (fig. 316) not provided with cilia. Outside the air- 

 vesicles a network of pulmonary capillaries is spread out so densely 

 (fig. 317) that the interspaces or meshes are even narrower than 

 the vessels, which are, on an average, g-^oth of an inch (8/*) in 



