Clinical Anatomy of the European Hamster 
the right is usually larger than the left. The thin- 
walled right ventricle (Fig. 4-14), in contrast to the 
thick-walled left ventricle, does not extend to the 
apex of the heart. Internally, the left atrium is sepa- 
rated from the left ventricle by a bicuspid valve 
{valva bicuspidalis), of which the papillary muscles 
(mm. papillares) are a very prominent part (Fig. 
4-16). The right atrioventricular valve is a tricuspid 
valve {valva tricuspidalis), the cusps of which are 
named angular (cuspis angular.is), parietal (cuspis 
parietalis) and septal (cuspis septalis). The angular 
and parietal cusps normally fuse in adults to form 
one large cusp. The third cusp is smaller and as- 
sumes a caudodorsal attachment to a fibrous ring 
(annulus fibrosus). All three papillary muscles 
originate from the ventricular septum. The aortic 
(valva aortae) and pulmonary (valva truncae pul- 
monis) valves exist in the form of three crescentic 
semilunar valves (valvulae semilunares) (Fig. 
4-16). 
The heart consists of 3 layers: epicardium, mvocardium and endo- 
cardium. The epicardium is the outer covering of the heart and the 
great vessels of the heart; it is the serous visceral layer of pericardium 
intimately applied to the heart. The myocardium constitutes the mus- 
cular body of the heart, with fibers Vk-hich are transversely and longi- 
tudinally situated and are intricately interlaced. They can be subdi- 
vided into atrial fibers, ventricular fibers and fibers of the conduction 
system. The endocardium is a thin smooth glistening membrane which 
is composed of endothelial cells placed upon a stratum of connective 
tissue and elastic fibers. 
4.4.2.1.2 The Great Vessels 
The ascending aorta (aorta ascendens) has its 
origin from the left ventricle on the dorsal side of 
the heart at the level of the second thoracic verte- 
bra. It is oriented slightly to the left, ascending 
cranially and dorsally along the cranial vena cava — 
to which it is not fully applied, since parts of 
thymus tissue intervene between the two vessels — 
and finally crosses over the pulmonary artery at the 
level of the third thoracic vertebra. 
At the sternal end of the second costochondral 
joint the brachiocephalic trunk (truncus brachio- 
cephalicus) arises at the initial curvature of the 
aortic arch (arcus aortae), or 7 mm distal to the 
origin of the aorta (Figs. 3-11,4-17, 4-18). There- 
after the aorta continues cranially along the cranial 
part of the ventral mediastinum where it changes 
direction caudally at the level of the first rib pair to 
complete the arch, which curves from a right ventral 
to a left dorsal direction. The left pulmonary hilus 
is caudal to the site where the aortic arch disappears 
dorsal to the left lobe of the lung. 
The length of the brachiocephalic trunk in the 
cranial part of the ventral mediastinum is about 
4 mm; it represents the largest branch from the 
arch (Figs. 3-11, 4-18). Initially it runs cranially 
along the right side of the trachea and then dorsally 
and to the right of the right internal jugular vein 
before dividing into the right subclavian and right 
common carotid arteries. The right common carotid 
(a. carotis communis dextra) runs cranially while 
the right subclavian (a. subclavia dextra) diverges 
to the right brachium. The left common carotid, the 
second and smallest branch from the arch, origi- 
nates at the level of the second thoracic vertebra 
ventral to the left wall of the trachea and runs 
cranially in close proximity to the tracheal wall 
(Figs. 3-11,4-17,4-18, 4-19). A third branch, the 
left subclavian artery, arises 2 mm distal to the left 
common carotid and continues from the aortic arch 
in a cranial direction dorsal to the left internal 
jugular vein until it turns into the upper brachium 
dorsal to the clavicle (Fig. 4-19). The left internal 
thoracic artery (a. thoracica interna) is given off 
from the subclavian artery 1 mm distal to its origin; 
it runs caudoventrally close to the lateral borders 
of the sternum and just dorsal to the costal carti- 
lages, between the intercostal spaces. 
After the caudal turn at the level of the third 
thoracic vertebra, the aortic arch continues as the 
descending aorta (aorta descendens) down the 
dorsal mediastinum ventrally and laterally along 
the thoracic column to the aortic hiatus of the 
diaphragm. 
The pulmonary artery (a. pulmonalis) (Fig. 4- 
17) is the most dorsal of the great vessels leaving 
the heart in the caudal ventral mediastinum. It orig- 
inates from the right ventricle ventral to the proxi- 
mal aorta and dorsal to the right auricle, whose 
medial edge covers it, then extends to the left be- 
tween the ascending aorta and the left auricle in 
alignment with the hilus of the lungs where it meets 
and partially winds around the bronchus, continu- 
ing along the medial wall of the aorta before doub- 
ling back dorsally. About 7 mm from its origin, at 
the level of the fourth thoracic vertebra and ventral 
to the tracheal bifurcation, it divides into a right and 
90 
