328 CHIKANOSUKE OGAWA 
ducts or the dividing parts of such ducts. Since the alveolar 
ducts divide frequently along their periphery, the lengths of 
branches are accordingly short. These do not take the tubular 
form. In other words, the formation of these non-tubular spaces 
is the inevitable result of frequent ramification. I further see 
in my model that some air-sacs arise directly from alveolar ducts, 
and that, on the other hand, there are non-tubular air spaces 
in the more central part of the alveolar ducts. 
Thus I think the term atrium lacks enough significance to 
justify its use, at least for the human lung. 
ALVEOLI 
a. Structure. ‘The shape of alveoli has been called polygonal, 
half spherical, or irregular. Alveoli are always polygonal in 
lungs which have been adequately injected with fixing fluid by 
way of the trachea or injected through the artery while the lungs 
are still in position within the unopened thorax. Because of 
the irregular shape of alveoli, it is often difficult to determine 
how many surfaces they have. Schulze calls attention to the 
fact that in several mammals the surfaces rarely number over 
twelve, and vary in general from five to twelve. My models 
show that the human lung has alveoli with five surfaces in most 
and in a comparatively few cases there are six. Although 
the shape of alveoli is irregular, the formation of alveolar walls 
seem to conform to fixed types. I shall classify alveoli in 
three types. Figure 8 represents diagrams of sectioned alveoli. 
Thin lines denote alveolar septa and the thick ones the alveolar 
walls beside the septa. 
Type A. This kind of alveolus is formed by the projection 
of the walls of alveolar ducts and it lacks special septa. 
Type B. This kind of alveolus is enclosed by alveolar septa 
alone, except at the base. 
Type C. This type is an intermediate form between the other 
two and presents variations. a) One side may consist of the 
concave wall of the alveolar duct and the other of the alveolar 
septum. 6) One side may consist of the concave wall and the- 
