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From this description it will be seen that the air-sacs do not 
communicate directly with the terminal bronchus, as is usually de- 
scribed, but between each air-sac and terminal bronchus there is 
a cavity, constant in all portions of the lung, which I shall term 
atrium. The communication between the atrium and the bronchus 
I shall call vestibulum; that between the atrium and air-sac, air-sac 
passage, or, simply sac passage. 
The terminal bronchus has an average diameter of about 0,4 mm. 
It may be recognized on transverse section by its walls being made 
up of smooth muscle fibres, lined with cylindrical epithelial cells. 
Sometimes the section will lie in such a plane that one or more vesti- 
bules will be seen leading off into the atria, or in rare instances we 
may obtain a section through all parts (fig. 1). Besides the air-cells 
Fig. 1. Camera tracing of a section through the terminal bronchus, atrium and 
air-sac. Enlarged about 75 diameters. 
B. terminal bronchus; A. atrium; S. air-sac3; V. vestibule; P. sac-passage; C. air 
cells arising from terminal bronchus, atrium and air-sac; M. muscle fibres in the wall 
of the terminal bronchus. The arteries are in black; the veins in heavy outline. 
of the air-sacs the terminal bronchi communicate with small air-cells 
having a diameter of 0,047 mm as described by KöLLıkEer. The 
muscular wall of the bronchus does not enter these cells but only 
surrounds their openings into the bronchus. 
From the tip of the terminal bronchus three to six vestibula 
