LUNGS 303 



(2) spaces bounded by alveoli (alveolar sacs, atria and alveolar ducts, the 

 ducts having muscle fibers in their walls); (3) small bronchioles having 

 scattered alveoli along their walls, and therefore presenting a mixed epithe- 

 lium (respiratory bronchioles); and (4) bronchioles with no respiratory 

 epithelium. 



The study of sections of the adult lung is facilitated by comparison 

 with those from an embryonic lung. Comparable sections, including 

 the pleura, and drawn at the same scale of magnification, are shown in 

 Figs. 300 and 301. In the lung of the embryo of four months, the terminal 

 branches of the bronchioles are found in the centers of lobules, one of which 

 is shown in Fig. 300 (bounded by b. v. and lym.~). The axial bronchioles 

 break up into ramifying tubules lined with cuboidal cells, and at birth 

 the alveoli which are found at the end of these structures are also lined with 

 cuboidal epithelium. The main arteries run with the axial bronchioles 

 in the centers of lobules; and the large veins and lymphatic vessels are at 

 their periphery. This arrangement is retained in the adult (Fig. 296). 

 Deep in the lung, the small bronchi are surrounded by considerable con- 

 nective tissue, containing arteries, veins and large lymphatic vessels. 



After respiration has been established, the alveoli become greatly 

 distended, so that the connective tissue containing the capillary vessels 

 is flattened out in very thin layers. These layers are bounded on either 

 side by the respiratory epithelium of adjacent alveoli (Fig. 301). In 

 producing this epithelium, the cells not only become flattened but they are 

 transformed into thin structureless plates, and those "from several cells 

 may fuse to form large plates. In amphibia, nuclei in small amounts of 

 protoplasm are found attached to the basal or connective tissue side of the 

 plates, often associated in groups. In addition to these cells, the alveolar 

 walls contain the endothelial cells of the capillaries, connective tissue cells, 

 wandering cells, and many elastic fibers. These fibers surround the 

 alveoli and encircle their outlets; the alveolar walls are so elastic that in 

 inspiration they may expand to three times the diameter to which they 

 return during expiration (o.i to 0.3 mm.). Pores have been described 

 leading from one alveolus to another (Fig. 297, B). 



The richness of the capillary network in the alveolar walls is seen in 

 injected specimens (Fig. 299). Respiration takes place by the transfer of 

 gases between the blood in these vessels and the air in the alveoli, therefore 

 through the endothelial cells and alveolar plates, together with the trivial 

 amount of connective tissue which may intervene. 



The pulmonary and bronchial blood vessels have already been de- 

 scribed, and their relations to the lobule of the lung are shown in Fig. 296. 

 The pulmonary arteries are axial vessels within the lobules, breaking up in- 

 to terminal branches at the atria, and these branches become axial along the 

 alveolar sacs. Each terminal branch has been described as the center of 



