Thorax 
left branch. The left branch enters the root of the 
left lung close to the tracheal bifurcation, where it 
runs between and cranial to the left main bronchus 
and the bronchial artery. It continues on the dorsal 
surface of the bronchus, finally ramifying into three 
parts within the left lobe of the lung. The right 
branch of the pulmonary artery extends between 
the trachea and the right main bronchus, where it 
gives off one branch to the right cranial lobe. Dis- 
tally the right pulmonary artery gives off a second 
branch to the right middle lobe and then a third 
branch to the right caudal lobe. Accordingly, a 
fourth and fifth is given off to the right accessory 
and intermediate accessory lobes, respectively. 
The pulmonary veins {vv. pulmonales), two from 
each lung, arise from pulmonary capillaries that 
coalesce into increasingly larger branches running 
through the parenchyma of the lung, finally form- 
ing a single venous trunk (vena pulmonalis). Their 
network is generally independent of the pulmonary 
bronchi and arteries. The pulmonary veins are the 
only veins which carry oxygenated blood. The left 
pulmonary vein accompanying the left bronchus, 
which lies dorsal to the left auricle, runs trans- 
versely and cuts ventral to the esophagus, collecting 
the branches originating from the right accessory 
and intermediate accessory lobes; it takes up a right 
branch from the right accessory lobe during the 
terminal fourth of its length, continuing obliquely 
and cranially to the left and lying ventral to the 
entry of the bronchus into the lung. The course of 
the right venous branches conforms roughly to that 
described for the right pulmonary arteries, both 
vessel layers lying in the caudal part of the ventral 
mediastinum. The left and right pulmonary veins 
join before emptying into the left auricle of the heart 
immediately after crossing the cranial vena cava 
(Fig. 4-17). 
The European hamster has two cranial venae 
cavae {venae cavae craniales), one caudal vena cava 
{vena cava caudalis) and one azygos vein {vena 
azygos sinistra). 
At the level of the eleventh thoracic vertebra the 
caudal vena cava is admitted to the thorax from the 
abdominal cavity via the aperature of the vena cava 
in the diaphragm. At the level of the third thoracic 
vertebra, the caudal vena cava bends ventromedi- 
ally and is applied to the side of the esophagus. 
emptying into the right auricle along with the 
cranial vena cava. 
The right cranial vena cava arises from the 
internal jugular vein and a venous trunk composed 
of the right subclavian vein, which runs along the 
superficial pectoral muscle, and the external jugular 
veins, which run caudally just below the cervical 
superficial fascia before crossing the clavicle ven- 
trally. It enters the thoracic cavity at the caudal 
border of the clavicle, dorsal to the insertion of the 
sternomastoid muscle (m. sternomastoideus) on the 
manubrium, and runs a short distance along the 
right side of the trachea in the dorsal mediastinum, 
crossing the right subclavian artery (immediately 
distal to the origin of the brachiocephalic trunk) 
into the cranial part of the ventral mediastinum and 
finally emptying into the dorsal wall of the right 
auricle. The left cranial vena cava is similarly posi- 
tioned on the left side, originating from the left 
subclavian junction ventral to the clavicle and the 
left internal jugular vein. The left external jugu- 
lar — left subclavian trunk anastomoses distally 
with branches of the right internal jugular vein, 
which crosses the left subclavian artery 2 mm before 
emptying into the left cranial vena cava. The left 
cranial vena cava then crosses the left subclavian 
artery, passes ventral to the left branch of the pul- 
monary artery and the left bronchus and empties 
into the dorsal wall of the right atrium near the left 
side of the caudal vena cava. At the level of the third 
rib, the left cranial vena cava takes up the azygos 
vein {v. azygos) which runs cranially along the 
vertebral column, taking up the intercostal veins. 
4.4.2.1.3 Thymus and Lymphatic 
Tissue 
The thymus, part of the lymphatic system, lies 
intrathoracically at the ventrolateral surface of the 
auricles of the heart (Fig. 4-20). This two-lobed 
organ is about 8 mm long and each lobe has a width 
of about 2 mm. The thymus is proportionally much 
larger in young than in adult animals. The entire 
organ is invested by a fibrous capsule from which 
originate several septae which subdivide the two 
lobes into various, irregular lobules. 
Histologically, epithelial reticular cells, lymphocytes and corpuscles 
of Hassall can be distinguished. 
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