ORGANS OF RESPIRATION. 247 



ternal to the pavement epithelium, giving great 

 elasticity to the air sacs and functions in the expul- 

 sion of air during normal breathing. The term in- 

 fundibulum is sometimes applied to the distal air 

 passage lined by pavement epithelium. 



The following is a resume* of the tissues found in 

 the walls of the respiratory passages : 



_ Non-nucleated 

 epithelial cell. 



Nucleated epi- 

 thelial cell. 



Fig. 190. Inner surface of human alveolus treated with silver nitrate, 

 showing respiratory epithelium (after Kolliker). 



1. Epithelium extends the whole length. In the 

 trachea and bronchi this is stratified ciliated. In the 

 atrium and air sacs it is simple pavement and made 

 up of two kinds of cells, namely, nucleated and non- 

 nucleated. 



2. Elastic fibers extend the whole length. In the 

 respiratory parts the fibers form a reticulum just 



