AIR SPACES IN LUNG OF CAT 471 
by an oval opening 0.17 x 0.12 mm. Two sacculi alveolares 
arise from this atrium. 
Atrium 1b comes off from the ductulus alveolaris proximal to 
atrium la and communicates with the ductulus alveolaris through 
a nearly circular opening in its lower wall; its direction is almost 
directly downward. This atrium measures 0.30 x 0.23 x 0.31 
mm. and the opening from the ductulus alveolaris 0.14 x 0.13 
mm. Five sacculi alveolares arise from this atrium. 
Atrium 1c arises from the roof of the ductulus alveolaris prox- 
imal to both of the preceding atria; it extends upward from and 
to the right of the ductulus alveolaris. The atrium measures 0.30 
x 0.24 x 0.17 mm. and the opening by which it communicates 
with the ductulus alveolaris, 0.09 x 0.11 mm. Three sacculi 
alveolares are connected with this atrium. 
Lobule II. Ductulus alveolaris II, and the lobule connected 
with it, hes to the right of lobule I. The distal end of the duc- 
tulus alveolaris shows the usual dilatation. Four atria are con- 
nected with this ductulus alveolaris and are designated 2a, 2b, 
2c, 2d (plates 1, 2). 
Atrium 2a arises from the left side of the distal end of the duc- 
tulus alveolaris and extends to the left and forward. The atrium 
measures 0.24 x 0.24 0.20 mm. and the oval opening by which it 
communicates with the ductulus alveolaris measures 0.19 x 0.12 
mm. ‘Two sacculi alveolares are connected with this atrium. 
Atrium 2b comes from the right side of the distal end of the 
ductulus alveolaris and extends to the right and slightly upward. 
This atrium measures 0.33 x 0.34 x 0.23 mm. and the oval open- 
ing from the ductulus alveolaris measures 0.16 x 0.12 mm. 
Four sacculi alveolares are connected with this atrium. 
Atrium 2c opens out from the floor of the ductulus alveolaris 
and extends downward. Its communication with the ductulus 
alveolaris is nearly round, measuring 0.08 x 0.09 mm. The 
atrium itself is 0.26 x 0.25 x 0.16mm. Three sacculi alveolares 
are connected with this atrium. 
Atrium 2d arises from the right side of the ductulus alveolaris, 
proximal to the other three atria, and extends to the right and 
upward. It measures 0.25 x 0.22 x 0.18 mm. and the rounded 
