ORGANS OF RESPIRATION. 245 



cells, or alveoli. Simple pavement epithelium lines 

 the atria and air sacs, consisting of two varieties of 

 cells: (i) small nucleated elements, and (2) larger 

 non-nucleated plates. The latter line the alveoli 

 and are applied directly against the blood capillaries, 

 while the nucleated elements intervene at the free 

 margin of the alveoli. Before birth the air sacs are 

 collapsed and all the pavement epithelium is com- 

 posed of nucleated cells. With the first breath of 

 air the air sacs become distended, not uniformly, 

 but to form vesiculated walls, the small vesicles being 

 the alveoli. The walls of the latter suffer greater 

 distention, due to a less resistance offered by the 

 opposing blood capillaries, and the nucleated cells in 

 this region become changed to non-nucleated plates, 

 through which an exchange of gases takes place dur- 

 ing the functional activity of the lung. The atria 

 and air sacs vary in size according to their distention 

 with air. An average diameter is i .o mm. for the 

 atria, and i.o by 1.5 mm. for the air sacs. Each air 

 sac has from six to eight air cells, or alveoli, that also 

 vary greatly in size, an average diameter being 0.25 

 mm. One system of atria and air sacs constitute a 

 lobule and is separated from adjacent lobules by in- 

 tervening areolar tissue. The base of each lobule is 

 directed toward the surface of the lung and the apex 

 towards its root. These lobules can be seen in a 

 macroscopic surface examination of the lung. 



The elastic fibers of the membrana propria, de- 

 scribed in the wall of the trachea and bronchi, ex- 

 tend to the distal end of the air passages, where they 

 spread out to form a thin reticular fabric just ex- 



