RESPIRATION. 



175 



endothelial tubes, and take origin in the lymph-canalicular system of the 

 pleura proper. Scattered bundles of unstriped muscular fibre occur in 

 the pulmonary pleura. They are are especially strongly developed on 

 the anterior and internal surfaces of the lungs, the parts which move 

 most freely in respiration : their function is doubtless to aid in expira- 

 tion. The structure of the parietal portion of the pleura is very similar 

 to that of the visceral layer. 



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 lob- 

 ules. Each pulmonary lobule may be considered a lung in miniature, 

 consisting, as it does, of a branch of the bronchial tube, of air-cells, 

 blood-vessels, nerves, and lymphatics, with a sparing amount of areolar 



tissue. 



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



FIG. 150. 



FIG. 151. 



FIG. 150. Terminal branch of a bronchial tube, with its inf undibula and air cells, from the mar- 

 gin of the lung of a monkey, injected with quicksilver, a, terminal bronchial twig; 6 6, inf undib- 

 ula and air-cells, x 10. (F. E. Schulze.) 



FIG. 151. Two small infundibula or groups of air-cells, a a, with air-cells, 6 6, and the ultimate 

 bronchial tubes, c c, with which the air-cells communicate. From a new-born child. (Kolliker.) 



which has been just described (a, Fig. 151), divides and subdivides ; its 

 walls at the same time becoming thinner and thinner, until at length 

 they are formed only of a thin membrane of areolar and elastic tissue, 

 lined by a layer of squamous epithelium, not provided with cilia. At the 

 same time, they are altered in shape ; each of the minute terminal 

 branches widening out funnel-wise, and its walls being pouched out ir- 

 regularly info small saccular dilatations, called air-cells (Fig. 151, b). 

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

 group of pouches or air-cells, has been called an infundibulum (Figs. 

 150, 151), and the irregular oblong space in its centre, with which the 

 air-cells communicate, an intercellular passage. 



