Fig. 2. Corrosion preparation of an atrium, with a single air-sac attached. En- 
larged 100 diameters. 
A. atrium; S. air-sac; P. an air-sac passage divided transversely; V. vestibule 
divided transversely. 
We may distinguish three forms of air-cells (figs. 1, 3 C.). 
1) Those arising from the bronchus. 
2) Those arising from the atrium. 
3) Those arising from the air-sacs. 
Those arising from the bronchus have already been spoken of: 
they are smaller than those arising from the atrium and from the 
air-sac. We find these air-cells along the smaller bronchi and on the 
terminal bronchus: they have an average diameter of 0,047 mm, while 
the diameter of those of the atrium and of the air-sac is 0,113 mm. 
They arise from all sides of the air-sac, but are most pronounced on 
the distal end. Between the air-cells, on the central side of the air- 
sac, the ramifications of the pulmonary artery are found !). On the 
1) In describing the relations within the lung lobule all directions 
which are parallel with the terminal bronchus will be spoken of as 
