174 MILLER. [Vol. VIII. 



bronchus is the " vestibulum " which opens into the "atrium." 

 From this arises the "faux" or air-sac passage which leads 

 into the "peristylum" or air-sac. This air-sac is set about 

 with "cubicula" or air-cells. Cuts 7 and 8 fully elucidate 

 these points. 



From this description it will be seen that the air-sacs do not 

 communicate directly with the terminal bronchus, as is usually 

 described, but between each air-sac and terminal bronchus 

 there is a cavity constant in all parts of the lung (Figs. 17, 18 

 and 19, pink color) which I have already termed the atrium.^ 

 The communication between the atrium and bronchus I have 

 called vestibulum ; that between the atrium and air-sac, air-sac 

 passage, or, simply sac passage. 



The terminal bronchus does not have a smooth cylindrical 

 surface, but we find projecting from all portions small cells, 

 the air-cells of the bronchus. The opening by which these 

 cells communicate with the bronchus is surrounded by the 

 smooth muscle fibres which line the bronchus. In sections of 

 the lung we may recognize the terminal bronchi by their 

 diameter, the thickness of their walls and the presence of 

 smooth muscle fibres. 



Leading out of the distal extremity of the terminal bronchus 

 we find from three to six openings or vestibules. These are cir- 

 cular in outline, or nearly so ; and the smooth muscle fibres lin- 

 ing the bronchus surround them, forming a sort of sphincter 

 muscle. They have an average diameter of 0.2 mm. These 

 openings do not all take the same direction (Fig. 19); usually 

 one of them appears as though it were a continuation of the 

 bronchus, while the others open out at various angles or may 

 take a course nearly recurrent to that of the bronchus, as shown 

 in Fig. 18. In sections which lie in such a plane that we look 

 directly into the vestibules, the presence of smooth muscle 

 fibres about the openings make them seem to be small terminal 

 bronchi. They may, however, be distinguished from a bronchus 

 by their diameters. 



That the smooth muscle fibers found about the vestibules do 

 not extend from the bronchus into the atrium, is shown in 



1 Miller, Anatom. Anz., 1892. 



