HUMAN LUNG 583 
MOUTH OF AN ALVEOLAR DUCT 
I mean by the above expression the mouth of each alveolar 
duct (that is, each branch of alveolar-duct system). Inasmuch 
as the wall of alveolar ducts is not smooth, the mouths of alveolar 
ducts hold no regular shape. Yet on the whole they may be 
considered round. The diameter may be the same or somewhat 
less than that of its respective alveolar duct. In the latter case 
the ducts seem to be narrowed. This fact is in accordance with 
Schulze’s statement that narrowness of mouth is observed, but 
not as a rule. Mouths of alveolar ducts are generally well 
indicated, although they are sometimes outlined indistinctly, 
and usually enclosed by alveolar septa. These therefore border 
on one hand mouths of alveolar ducts and on the other the 
alveoli. In this case there may be alveoli both proximally and 
distally or on one side of the mouths of alveolar ducts. It oc- 
casionally happens that the circumference of the mouths consist 
entirely of alveolar septa. Usually the mouths partially lack 
the alveolar septum and are enclosed by the wall of the alveolar 
duct itself. This is especially true in a wall common to the 
dividing alveolar ducts. 
THE COMMON WALL OF THE DIVIDING ALVEOLAR DUCTS 
Upon the division of alveolar ducts into branches they have 
a common wall for some distance. In most cases this wall con- 
tinues as far as the next division, although the course is not long. 
Accordingly, the branches of alveolar ducts are inclined to form 
groups with closely related branches. If occasionally another 
branch comes in between two branches of alveolar ducts, the 
common wall then continues only a short distance. Upon the 
division into an obtuse angle, or the T form, no common wall 
is formed. Here there exists only the alveolar septum (fig. 7). 
The common wall has at times a different appearance from 
other walls. In the division of the first parts of the alveolar 
ducts, sometimes several shallow scaphoidal cavities exist in 
the common wall along the circumference of the duct. These 
may be considered shallow alveoli. Sometimes the common 
