AIR SPACES IN LUNG OF CAT 473 
While in a complete reconstruction the differentiation of the 
atria is an easy matter, in sections it is sometimes difficult to 
distinguish them; this is especially true if only individual sec- 
tions are available. In sections in which the air spaces and the 
ductulus alveolaris are cut longitudinally the recognition of atria 
is not difficult. ‘The presence of smooth muscle and the charac- 
ter of the epithelium determine the distance to which the duc- 
tulus alveolaris extends and the immediate widening of the air 
spaces marks the position of the atrium (plate 3). In transverse 
or oblique sections through the lobule it is sometimes difficult to 
determine the position of the atria unless one has serial sections 
to study. Plate 4, 1b, illustrates the appearance of a section 
taken transversely through an atrium and the surrounding sac- 
culi alveolares; it also shows an outline tracing of the principal 
air spaces in a section which includes the three lobules entering 
into the reconstruction. 
In their general configuration the sacculi alveolares differ in 
no respect from those I have illustrated in previous contributions. 
Many of them are subdivided by deep clefts. In some instances 
this is undoubtedly associated with the course of the branches of 
the pulmonary artery which are distributed to the sacculi alveo- 
lares. No two sacculi alveolares are of the same size or shape; 
they apparently have grown along the line of least resistance. 
Sacculi alveolares may arise from any of the smaller divisions 
of the bronchial tree. In the present reconstruction small sac- 
culi alveolares were found connected with the ductuli alveolares 
and also with the bronchioli respiratorii. In my reconstruction 
of the lung of the dog I found an atrium, with two sacculi al- 
veolares attached, arising from the same division of the bron- 
chial tree. 
When I first began my work, under the direction of Prof. F. P. 
Mall he taught me that when an artery gave rise to branches of 
various orders, any of the subsequent orders could arise from a 
preceding order. The same statement holds true for the bron- 
chial tree and its ultimate endings. 
