Thorax 
eludes the heart, pericardium, ascending aorta, 
caudal half of the cranial vena cava (with the azygos 
vein opening), the pulmonary arteries (all of which 
are intrapericardial), the right and left pulmonary 
veins and the phrenic nerves. 
4.4.2.1.1 Heart 
The heart {cor) (Figs. 3-11 , 4-7, 4-12 to 4-19) 
is a hollow muscular organ with the form of a trun- 
cated cone and lies in the caudal part of the ventral 
mediastinum between the second and fifth rib pairs 
(Fig. 3-10). It is enclosed within a fibroserous sac, 
the pericardium, which consists of a visceral and 
parietal portion {lamina visceralis et parietalis). 
The pericardium is formed from collagenous fib- 
rous tissue. 
The base of the heart {basis cordis) is situated 
predominately in the right half of the thorax, while 
the apex {apex cordis) is oriented caudally and to 
the left (Figs. 3-10, 3-11). The horizontal position 
of the heart is maintained by the phrenicopericar- 
dial ligament {ligamentum phrenicopericardiacum) , 
which is composed of two small sagittal ligaments 
(Kittel, 1953) drawn from the apex of the peri- 
cardium to the diaphragm. During expiration of the 
lungs, the heart is covered dorsolaterally only by the 
pulmonary lobes. It does not contact the diaphragm. 
The tracheal bifurcation is craniodorsal to the heart 
and the vertebral column; caudodorsal to the heart 
are the esophagus, aorta {aorta thoracica) and vagus 
nerves {nn. vagi). The pericardial surface also is 
applied to the visceral pleura of the right accessory 
lobe. Cranially, the pericardium reaches to the 
height of the second intercostal space where it 
curves and lies adjacent to the thoracic artery. The 
surfaces of the heart are named according to their 
relation to adjacent organs. The sternocostal surface 
{facies sternocostalis) is formed by a large part of 
the cranial wall of the right ventricle and by the 
medial part of the right atrium. The vertebral side 
of the heart is primarily formed by the dorsal wall 
of the left atrium and the smaller cranial portion of 
the wall of the left ventricle. The greatest portion of 
the cardiac surface is made by the right and left pul- 
monary surfaces (jacies pulmonales dextra et sinis- 
tra). The former is represented by the dorsal wall of 
the right ventricle and the largest part of the right 
atrium with its auricle; the latter is formed by the 
main part of the left ventricular wall and the dorsal 
wall of the left atrium with its auricle. The sterno- 
costal surface is flattened while the rest of the car- 
diac surface is rounded; in this way, a dull edge is 
formed on the dorsal side of the heart. 
The right atrium {atrium dextrum) lies in the 
second intercostal space and is bordered laterally by 
the right lung. It is bent ventrally before the origin 
of the ascending aorta. The dorsal wall of the right 
atrium extends caudally to the level of the fourth 
thoracic vertebra. The right ventricle {ventriculus 
dexter) lies in the second and third intercostal space 
and borders the right medial and right diaphrag- 
matic lobes (Figs. 3-10, 3-11) of the lung. The left 
atrium {atrium sinistrum), lying at the level of the 
fourth thoracic vertebra, assumes the space dorsal 
to the fourth costal cartilage and lies adjacent to the 
left pulmonary lobe. The left ventricle {ventriculus 
sinister) is in the fourth intercostal space and pro- 
jects cranially and caudally (Fig. 4-7, 4-18). It is 
bordered by the left and right accessory lobes. The 
heart measures approximately 19 mm from base to 
apex. Its width is about 1 1 mm and 10 mm and its 
weight approximately 1.46 g and 1.30 g for males 
and females, respectively (Tables 2, 5). 
The heart is divided into four major chambers, 
the left and right atria and left and right ventricles. 
Externally, the atria are separated from one another 
by a very indefinite, vertical, interatrial sulcus; and 
the coronary sulcus {sulcus coronarius) separating 
the atria from the ventricles is also quite obscure 
(Fig. 4-15). Within these grooves lie the trunks of 
the coronary vessels. The shallow longitudinal 
grooves do not coincide with the ventricular boun- 
daries (Fig. 4-15). Even though a right descending 
longitudinal groove {sulcus interventricularis) of 
the right ventricular wall is present in most cases, a 
left descending longitudinal sulcus is rarely recog- 
nizable; rather, in the area of the latter longitudinal 
groove, an oblique vascular sulcus is usually formed 
which runs caudodorsally (Fig. 4-15). In the area 
of the interatrial, interventricular and coronary 
grooves, very little or no adipose tissue is visible. 
The paths of the coronary vessels vary greatly from 
animal to animal. 
Externally, the atria are recognizable only by the 
triangular auricles {auriculae) (Fig. 4-14), of which 
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