2 r 2 NORMAL HISTOLOGY. 



Within the infundibulum the epithelial lining consists principally 

 of the large flat endothelioid plates, or respiratory epithelium, be- 

 tween which elements diminutive groups of the smaller polygonal 

 cells appear. In the air-sacs, presently to be described, the large 

 plate-like elements of the respiratory epithelium chiefly constitute 

 the lining. 



THE LUNGS. 



The lungs, with their system of air-tubes, correspond in plan of 

 structure and in development to racemose glands, the excretory 

 ducts being represented by the bronchial tubes, and the glandular 

 tissue by the pulmonary parenchyma. The latter is built up of 

 groups of air-sacs enclosed by connective tissue to form lobules, 

 which are associated in larger groups ; these latter in turn are united 

 into the lobes. All these divisions are connected by alveolar 

 tissue, the external surface being additionally covered by the 

 pleura. 



By the division of the terminal bronchial tube into the alveolar 

 ducts, and the subsequent origin from these of the infundibula and 

 the air-sacs, the part of the pulmonary parenchyma in communica- 

 tion with a single terminal bronchiole forms a pyramidal mass, 

 whose apex corresponds to the terminal bronchus, and whose 

 base, when reaching the free surface, appears as one of the polygo- 

 nal areas marking 

 the exterior of the 

 lung. These larger 

 polygonal fi e 1 d s, 

 made up of many 

 smaller areas which 

 correspond to the 

 compressed infundi- 

 buli, are often defined 

 with great distinct- 

 ness by the pigment 

 accumulated within 

 the connective tissue 

 separating the adja- 

 cent lobules. 



The air-sacs, air- 

 cells, or alveoli of 

 the lung represent 

 the acini of racemose 



glands, the similarity being especially marked in the uninflated 

 organ, which still retains its glandular character. Opening into the 

 common passages of the alveolar ducts and the infundibula, the air- 



FIG. 288. 



Section of human lung : a, infundibula cut in various directions ; 

 b, air-sacs separated by interacinous partitions (c) ; d, masses of 

 interlobular tissue containing accumulations of pigmented par- 

 ticles (e). 



