THE EPITHELIAL TISSUES. 



53 



2. Pavement-epithelium. — This variety of epithelium consists of 

 thin cells arranged edge to edge to form a single layer. With the 

 exception of certain regions on the surfaces of the pulmonary 

 alveoli, the cells are more cytoplasmic and granular than are those 

 of endothelium which this tissue in other respects closely resembles. 

 During feetal life the smaller air-passages and alveoli of the lung 

 are lined by a pavement-epithelium, the cells of which are nearly 

 as thick as those of some varieties of cubical epithelium. When, 

 however, the lung is expanded by the respiratory acts following 

 birth, many of the cells lining the alveoli become greatly extended 

 and flattened until their bodies are thin and membranous and their 

 nuclei inconspicuous or even destroyed (Fig. 30). These greatly 

 flattened epithelial cells are found covering those portions of the 



Fig. 30. 



Pavement-epithelium. Surface view of the lining of a pulmonary alveolus ; man. (Kolliker.) 

 «, membranous cell without a nucleus ; 6, nucleated granular cell ; c, cut surface of the 

 vertical wall of the alveolus, the structure of which is not represented. 



alveolar walls in which the capillary bloodvessels are situated and 

 permit a ready interchange of gases between the air in the alveolar 

 cavities and the blood circulating in their walls. Many of the 

 epithelial cells covering the tissues in the meshes between the 

 capillaries retain the cytoplasmic and granular character possessed 

 before birth and appear capable of multiplying and, perhaps, 

 replacing such of the thinner cells as may be thrown off or 

 destroyed. 



It will be evident, from the foregoing descriptions, that there 



