466 WILLIAM SNOW MILLER 
rium bears on its periphery numerous alveoli and opens into a 
variable number (two to five) of sacculi alveolares. The sac- 
culi alveolares present a great diversity of form; they are very 
irregular and adapt themselves to the space they have to oc- 
cupy. ‘The irregular contour of one sacculus fits into correspond- 
ing irregularities of the adjoining sacculi. Each sacculus alveo- 
laris bears on its periphery numerous alveoli, the true alveoli 
pulmonis. 
While the original description applied to the lung of the dog, 
I have found by reconstruction, that it applies to the lung of the 
cat, ox, child and adult man. Flint found that it applied to 
the lung of the pig; Oppel that it could, however, be applied to 
any mammalian lung. 
It has seemed to me, as I have read the papers of those in- 
vestigators who have failed to recognize the presence of the atrium, 
that failure to recognize it is due to one of three causes: study of 
single sections; use of corrosion preparations; over distention. 
This last cause has made me trouble in times past. It was not 
until I learned to fix the lung in situ, that is, in the unopened 
thorax, that I obtained uniform results. Removal of the lungs 
from the thorax and filling them with the fixing fluid can so 
stretch and distort the atria that they may be mistaken for duc- 
tuli alveolares. 
Atria do not possess the muscular walls of the bronchioli, nor 
the scattered muscle of the ductuli alveolares, but resemble the 
sacculi alveolares in structure, and, like them, are very dis- 
tensible. May it not be possible that the atria take some import- 
ant part in respiration, occupying as they do, an intermediate 
position between the ductuli alveolares, on the one hand, and 
the sacculi alveolares on the other? 
NOMENCLATURE 
In the following description of the air spaces I shall use the — 
nomenclature which I have advocated ever since the B. N. A. 
made its appearance in 1895. In 1900 and again in 1902 I wrote 
as follows: 
