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arise, each having an average diameter of 0,2 mm. These take various 
directions. While most of them are more or less parallel with the 
terminal bronchus others take almost a recurrent course. If the section 
lies in such a plane that we look into a vestibule it appears as a di- 
minutive terminal bronchus, as the opening is surrounded with a ring 
of smooth muscle fibres. That these fibres do not extend beyond the 
opening is shown in sections cut at right angles to the vestibule (fig. 1), 
and in such specimens we find a bundle of smooth muscle fibres cut 
transversely on either side of the mouth of the vestibule. 
The diameter of the atrium is slightly more than half that of 
the air-sac, and unlike it, each atrium has three or more openings. 
These openings communicate on the one hand with the air-sacs (sac- 
passages), on the other side with the terminal bronchus (vestibule). 
The walls of the atrium resemble those of the air-sac, being thin, 
and having enclosed in them the capillary net-work of blood-vessels. 
In sections it is often very difficult to determine which openings are 
atria. If the section is thick, 0,1 mm, we can usually distinguish them 
as nearly circular openings surrounded by several large irregular 
openings — the air-sacs. If the section lies in such a plane that 
the lobule is cut transversely we may distinguish the atria as a circle 
of round openings surrounded by large irregular openings — the 
air-sacs. 
The diameter of the air-sac passage connecting the atrium with 
the air-sac is somewhat smaller than that of the vestibule, averaging 
0,143 mm. The air-sac passage (figs. 1, 2, 3 P.) can always de 
distinguished from the vestibule by the absence of smooth muscle 
fibres in its walls. 
The air-sacs are quite irregular in shape, adapting themselves 
to the space they have to occupy (fig. 2). They have an average 
long diameter of 0,511 mm, a short diameter of 0,313 mm and often 
have partitions extending for some depth into their cavity. The irre- 
gular projection of one air-sac fits into a corresponding depression in 
an adjoining one. In this way they are dove-tailed together and we 
can now understand why it is impossible to pull a complete wax, or 
Woon’s metal corrosion apart. The walls of the air-sac are thin, and 
two adjoining air-sacs have a common wall. They are principially 
made up of the capillary net-work of the blood-vessels and connective 
tissue. In sections lying either parallel with or perpendicular to the 
pleura, the air-sacs may be recognized as large irregular openings 
having thin walls, and often nearly surrounded by openings about one 
fourth their size — the air-cells. 
