194 



NORMAL HISTOLOGY. 



bronchioles. Towards the distal ends of the latter partial transition into 

 patches of simple squamous epithelium occurs, these tubules containing, 

 therefore, both cuboidal and plate-like cells. 



The Air-Spaces. The walls of the air-spaces the atria, the alveolar 

 sacs and the pulmonary alveoli have essentially the same structure and 

 consist of a delicate fibre-elastic framework supporting the .blood-vessels 

 and the epithelium. The latter, the respiratory epithelium, is made up of a 

 single layer of polygonal plates, with indistinct nuclei, and includes groups 

 of large and small cells. The number of the smaller cells, as seen in silvered 

 preparations of adult lung (Fig. 243), progressively decreases towards the 

 alveoli, in which they are reduced to small groups of isolated elements sur- 

 rounded by the larger plates. In the foetus and in the still-born child, the 

 alveoli are lined entirely by low cuboidal cells; after inflation of the lung- 



Air-sac 



Passage from 

 atrium into air-sac 



Alveolu 



Terminal bronchus 



Pulmonary artery 



Bronchiole 



Atrium 



Alveolus 



Air-sacs 



FIG. 242. Section of lung, showing relations of terminal divisions of air-tubes. X 50. 



tissue has been completed, the cuboidal cells become expanded into the 

 small plates. The larger plates arise by the subsequent fusion of several of 

 the small ones, groups of the latter, which retain their independence, appear- 

 ing, in decreasing numbers as age advances, as the islands of one or more 

 small cells. 



The framework of the pulmonary alveoli is almost exclusively elastic 

 fibres that are condensed into rings around the openings or bases of the 

 alveoli and elsewhere enclose the spaces with elastic networks. Where these 

 rings come into contact and fuse, the alveoli are separated by partitions of 

 some thickness; beyond these septa the walls between the adjoining air- 

 spaces are very thin and include the two layers of epithelial plates and the 

 dense capillary network supported by the elastic reticulum. Owing to the 

 elastic character of their walls, the alveoli expand during inspiration to two 

 or three times their usual diameter (. i-. 3 mm. ), the lining epithelial plates and 

 the blood-vessels stretching to the necessary degree. The capillary networks 



