Thorax 
ing in the azvgos or intercostal system. 
The nonparenchymatous part of the lung is 
composed of an external serous membrane, the 
visceral pleura, under which there is an areolar 
layer of mainly elastic fibers investing the whole 
lung surface and extending into the parenchyma to 
invest the secondary lobules. 
4.5.3 Lungs and Pulmonary 
Topography 
Each lung (pulmo) is roughly a half-cone with its 
base resting on the diaphragm and its apex occupy- 
ing the cervical dome of the pleura and reaching the 
level of the first rib (Figs. 3-11, 4-17). The medial 
or concave surface lies against the mediastinum and 
the outer convex wall against the rib cage. The 
lungs are separated by a complete interpulmonary 
septum, the mediastinum, extending from sternum 
to spine. The soft, sponge-like tissue is molded 
against the walls of the chest cavity and bears the 
impression of the structures to which it is related. 
In adult male hamsters the lungs have an average 
weight of 2. 1 ±0.7 g and in adult females 1 .8±0.4 g. 
Measured by water displacement, the volume of the 
organ is 2.6±0.6 ml in adult males and 2.0±0.5 ml 
in adult females (Tables 2, 5) (Reznik, et ai, 1973). 
In an exsanguinated hamster, the lungs are pinkish 
white in color and have an elastic consistency. When 
perfused, the lungs become nearly white (Fig. 4- 
31, 4-32). After in situ fixation, the lung turns dark 
brown and on the surface, especially at the margins, 
small and verv delicate lobulations are visible (Fig. 
4-33). The lungs fioat in water and crepitate when 
handled, due to the presence of air in the alveoli. As 
the left thoracic cavity houses not only the heart but 
also a large part of the accessory lobe of the right 
lung (Figs. 3-11, 4-17, 4-25, 4-34), the left lung is 
relatively small and is not divided into lobes (Figs. 
4-25, 4-34, 4-35). 
The right lung is divided into five lobes {lobi) by 
deep interlobar fissures that extend from the mar- 
gins to the main bronchi; these lobes are completely 
isolated from one another, and there exists no par- 
enchymatous communication between them except- 
ing only some consolidation between accessory 
lobes. The apex of the right lung ends at the cranial 
margin of the first rib. The sharp edges (margo 
acutus) of the left lung extend from the diaphragm 
only to the midpoint of the first rib. The right mid- 
dle lobe lies caudolaterally between heart and ster- 
num. The diaphragmatic, or most caudal, lobe is 
the largest lobe and has the form of a right-angle 
triangle; it is related to the diaphragm, extending 
from the fifth or sixth intercostal space to the 
twelfth intercostal space. The accessory lobe and 
the intermediate accessory lobe begin cranial to the 
diaphragmatic lobe and immediately dorsal to the 
caudal vena cava, the mesentery of which separates 
the accessory lobes from the diaphragmatic lobe. 
The accessory lobes are connected by a broad pa- 
renchymatous fusion and are separated only by a 
short fissure. The intermediate accessory lobe lies 
partially dorsal to the heart and is displaced to the 
left lateroventrally and caudally. Most of the lateral 
surface of the accessory lobe is overlapped by the 
left lung (Fig. 4-36). 
The pulmonary root {radix pulmonis) contains 
several pulmonary veins, the pulmonary artery, 
bronchi, bronchial arteries of small size, nerves in 
the form of the pulmonary plexus and bronchial 
lymph nodes (Inn. bronchiales) and lymphatics, 
bronchial glands and the origin of the pulmonary 
ligament — all bound in a cuff of pleura and con- 
nective tissue. 
The costal, or lateral, surface of the lungs is 
smooth and convex, adapting accurately to the chest 
wall. The mediastinal surface is irregular and con- 
cave; its most important part corresponds to the 
pulmonary hilum and its primary relation is with 
the heart and pericardium. After reflecting off the 
sides of the vertebral bodies, where it is separated 
by the narrow dorsal mediastinum from the pleura 
on the opposite side, the mediastinal pleura passes 
to the side of the pericardium, where it is closely 
applied for a short distance before investing the 
dorsal part of the root of the lung. 
From the caudal margin of the lung root, a dorsal 
and ventral layer of pleura come into opposition 
and are prolonged caudally to the diaphragm as one 
distinct fold, the pulmonary ligament {lig. pulmon- 
ale). From the ligament the ventral layer is reflected 
on the pericardium and dorsal surface of the ster- 
num. From the dorsal margin of the lung root the 
pleura can be traced over the costal surface of the 
lung, apex and base, sides of the interlobar fissures, 
onto the mediastinal surface and ventral margin of 
the root. 
95 
