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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
  The alveoli are lined by a delicate layer of simple squamous epithelium, the cells of which are united at their edges by cement substance. Between the squames are here and there smaller, 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).


FIG. 975– Schematic longitudinal section of a primary lobule of the lung (anatomical unit); r. b., respiratory bronchiole; al. d., alveolar duct; at., atria; a. s., alveolar sac; a, alveolus or air cell; p. a.: pulmonary artery: p. v., pulmonary vein; l., lymphatic; l. n., lymph node. (Miller.) (See enlarged image)



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. l. lymphatic clefts. q. Pleura. (See enlarged image)

  The fetal 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.

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