Clinical Anatomy of the European Hamster 
chyma, the rings lose their cartilaginous support 
when the main bronchi divide into the lobar bronchi 
{bronchi lobares) (Fig. 4-25) before entering the 
right lung. 
The interior of the bronchi is lined with pseudostratified ciliated 
columnar epithelium (Figs. 4-26, 4-27^4-28, 4-29, 4-30). 
4.5.2 The Intrapulmonary System 
The lung is structurally the sum of all bronchial 
branchings. All of the lobar bronchi divide into 
several smaller intrapulmonary segmental bronchi 
{bronchi segmentales) which ultimately aerate a 
circumscribed conical region, the "bronchopul- 
monary segment," of the lungs (Fig. 4-25). A bron- 
chopulmonary segment is theoretically a subsection 
of the lung to which any particular segmental 
bronchus is distributed, including even the second- 
ary lobule and the terminal (lobular) bronchiole 
which aerates it. Practically, however, the broncho- 
pulmonary segment is defined as that part of the 
lung supplied by direct branches of the lobar bron- 
chi, or segmental bronchi in the case of the left 
hamster lung. These bronchopulmonary segments 
are as discrete as the lobes and can be teased apart 
by following the connective tissue planes. The fact 
that most of the extra fissures in the lung follow 
planes of separation between segments tends to con- 
firm their morphogenetic and morphometric reality. 
The intrapulmonary bronchi become increasing- 
ly smaller by continuous division and subdivision 
of the bronchial tubes in the lung parenchyma, 
from the segmental level down to the terminal, or 
lobular, bronchioles. This transition is mainly 
characterized by diminishing luminal diameter 
rather than qualitative change in structure. 
Throughout this interval of division each bronchiole presents a 
mucous layer lined by cuboidal epithelium with a few ciliated and 
many non-ciliated (Clara) cells, on a basement membrane surrounded 
by a smooth layer invested by fibrous tissue, with more or less regular 
mucous acini. 
The middle layer of smooth muscle remains more 
or less continuous with the segmental bronchi and 
subsegmental bronchioles. 
Each terminal, or lobular, bronchiole ventilates 
a secondary lobule, which is an aggregate of pri- 
mary lobules and is separated from neighboring 
secondary lobules by varying amounts of areolar 
and fibrous tissue. Secondary lobules are the basic 
structural units of the lung parenchyma. Terminal 
bronchioles divide within the secondary lobule into 
respiratory bronchioles, each of which aerates a 
primary lobule or basic functional unit of the lung. 
The respiratory bronchiole divides into several 
alveolar ducts, each of which contains increasingly 
large numbers of alveoli, or clusters of alveolar (air) 
cells, as the duct approaches its terminus in an 
evaginated blind cavity of alveolar cells, or atria. 
From each atrium arises a variable number of ex- 
panded alveolar sacs, whose walls are lined by 
alveolar cells. The ramification of the bronchial 
tree coincides with the findings of Ehard (1973) in 
the Syrian golden hamster. 
The alveoli, or luminal evaginations of the respiratory bronchiole, 
alveolar duct and atrium are lined by a continuous layer of epithelial 
cells; the nuclei of these cells extend into the air space and the cyto- 
plasm is extended into thin sheets along a basement membrane. The 
alveolar wall also includes capillary endothelium, with its basement 
membrane, but the alveolar and capillary layers are not adherent and 
can be distinguished by varying depths of connective tissue elements. 
The blood-air barrier, therefore, is at least two cell layers thick. The 
alveolar epithelium is of the two following types: Type I is a littoral 
cell with thin cytoplasmic extensions along the alveolar lumina lying 
back-to-back with cytoplasmic extensions of capillary endothelium; the 
Type II ("niche" or "septal") cell has a larger, rounder cell body, 
larger nucleus, more cytoplasmic inclusions and is usually found in the 
interstices of the capillary networks, often at the termini of Type I 
extensions. 
The right and left pulmonary arteries arising 
from the pulmonary trunk divide into branches 
which accompany the bronchi, relating especially 
to their dorsal surface. The main intrasegmental 
branches, which are usually single, follow the seg- 
mental bronchi. Unlike segment aeration, however, 
segment vascularization is likely to overlap, with 
each artery to one segment supplying branches to 
another. The arterial network is continuous with 
the capillary plexus forming part of the epithelial 
barrier in the alveolar system. The capillary plexus 
is also continuous with the venule anastomosis 
carrying aerated blood from the lung, but subse- 
quent venous arborization is independent of the 
arterial layer. 
The bronchial arteries, which arise from the 
aorta (or intercostal arteries) and which vascularize 
the lung parenchyma, often form capillary plexuses 
which link with those from the pulmonary artery, 
forming small venous trunks that are one source of 
the pulmonary vein. Most of the blood supplied by 
the bronchial arteries is believed to return via the 
pulmonary, and not by the bronchial veins, which 
arise in, and drain, the hilar area only before empty- 
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