

THE LUNGS 



1099 



The alveoli are lined by a delicate layer of simple Bquamoua epithelium, the cells of which 

 are united at their edges by cement substance. Between the squames are here and there smaller, 



p.v. 



FIG. 975. Schematic longitudinal section of a primary lobule of the lung (anatomical unit); r. b., respiratory 

 ironchiole, al. d.. alveolar duct; at., atria; a. s., alveolar sac; a. alveolus or air cell; p. a. : pulmonary artery; p.v., 

 mlmonary vein; /..lymphatic; I. n., lymph node. (Miller.) 



polygonal, nucleated cells. Outside the epithelial lining is a little delicate connective tissue 

 containing numerous elastic fibers and a close net-work of blood capillaries, and forming a common 

 wall to adjacent alveoli (Fig. 975). 



a 



FIG. 976. Section of lung of pig embryo, 13 cm. long, showing the glandular character of the developing alveoli 

 (J. M. Flint.) X 70. a. Interstitial connective tissue, b. A bronchial tube. c. An Alveolus. I. lymphatic clefts, 

 g. Pleura. 



The f ?tal lung resembles a gland in that the alveoli have a small lumen and are lined by 

 cubical epithelium (Fig. 976). After the first respiration the alveoli become distended, and the 

 epithelium takes on the characters described above. 



