472 WILLIAM SNOW MILLER 
opening into the ductulus alveolaris measures 0.1 x 0.09 mm. 
Two sacculi alveolares arise from this atrium. 
Lobule III. This lobule lies below lobules I and II on a line 
about midway between them. Its ductulus alveolaris extends 
downward and forward and has a decided dilatation at its dis- 
tal end, measuring at its widest point 0.21 mm. Three atria 
designated as 3a, 3b, 3c arise from this ductulus alveolaris (plates 
1 and 2). 
Atrium 3a arises from the distal end of the ductulus alveolaris 
and extends in the same direction. It measures 0.30x 0.41 x 
0.26 mm. and the opening from the ductulus alveolaris measures 
0.20 x 0.19 mm. Four sacculi alveolares open out from this 
atrium. 
Atrium 3b comes off on the left side near the distal end of the 
ductulus alveolaris and extends to the left and downward. It 
measures 0.33 x 0.25 x 0.20 mm. and communicates with the 
ductulus alveolaris by a nearly circular opening 0.13 x 0.11 mm. 
Two sacculi alveolares are connected with this atrium. 
Atrium 3¢ arises from the right side of the ductulus alveolaris 
proximal to the other atria and extends to the right and down- 
ward. It measures 0.30 x 0.23 x 0.22 mm. and the opening into 
the ductulus alveolaris measures 0.09x 0.08 mm. Three sacculi 
alveolares arise from this atrium. . 
The above figures show, as we would naturally expect, that 
there is a gradual diminution in the diameter of the divisions of 
the bronchial tree as we approach the lobule. As soon as the 
lobule is entered the air spaces at once enlarge. The average 
size of the atrium is 0.29 x 0.26 x 0.22 mm. being, in its smallest 
dimension, twice the diameter of the ductulus alveolaris from 
which it arises. In previous communications I have shown that 
the atria are about half the size of the sacculi alveolares. These 
figures differentiate clearly the atria from the connecting air 
spaces. The communication between the ductulus alveolaris 
and the atria I have, from the first, described as being nearly 
circular in outline. The average of the communications in the 
lobules reconstructed gives practically a circular outline: 0.135 x 
0.116 mm. 
