252 



NORMAL HISTOLOGY. 



Within the irregularly round air-sacs the epithelial lining consists 

 of the large flat endothelioid plates, or respiratory epithelium, 

 between which elements diminutive groups of the smaller polygonal 

 cells appear. In the air-cells, 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 several 

 atria, and the subsequent origin from these of the air-sacs and 

 the alveoli, 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 fields, 

 made up of many 

 smaller areas which 

 correspond to the 

 groups of the alve- 

 oli, are often defined 

 with great distinct- 

 ness by the pigment 

 accumulated within 

 the connective tis- 

 sue separating the 

 adjacent lobules. 



Section of human lung: a, atria and air-sacs cut in various direc- 1 he Ultimate aiT- 



tions; b, alveoli separated by interacinous partitions (c) ; d, masses cells Ol" alveoli of 

 of connective tissue containing accumulations of pigmented par- , . 



tides (#). the lung represent 



theaciniof racemose 



glands, the similarity being especially marked in the uninflated 

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

 cavity of the expanded air-sacs and the less uncertain atria, the 



FIG. 288. 



