BLOOD-VASCULAR SYSTEM OF THE HORSE—THE HEART 617 
BLOOD-VASCULAR SYSTEM OF THE HORSE 
THE PERICARDIUM! 
The pericardium is the fibro-serous sac which surrounds the heart, and also 
the great vessels in connection with it to a greater or less extent. Its form is in 
general similar to that of the heart. The fibrous layer is relatively thin, but 
strong and inelastic. It is attached dorsally to the large vessels at the base of the 
heart, and is continued in part up to the longus colli muscle. It is firmly attached 
ventrally to the middle of the posterior half of the thoracic surface of the sternum. 
The serous layer is a closed sac, surrounded by the fibrous pericardium, and invag- 
inated by the heart. It is smooth and glistening, and contains a small amount of 
clear serous fluid, the liquor pericardii. Like other serous membranes, it may be 
regarded as consisting of two parts, parietal and visceral. The parietal part lines 
the fibrous layer, to which it is closely attached. The visceral layer covers the 
heart and parts of the great vessels, and is therefore also termed the epicardium. 
The serous pericardium is composed of a connective-tissue membrane, rich in 
elastic fibers, and covered on its free surface by a layer of flat mesothelial cells. 
The pericardium is covered by the pericardiac part of the mediastinal pleura 
(Pleura pericardia), and is crossed laterally by the phrenic nerves. Its lateral 
surfaces are related chiefly to the lungs, but the lower part is in partial contact with 
the chest-wall. On the left side the area of contact is from the third rib to the 
sixth rib and intercostal space. On the right side the contact is smaller and is 
chiefly opposite the ventral part of the third and fourth intercostal spaces and inter- 
vening rib. The anterior end of its base is opposite to the second intercostal space 
or third rib, and the posterior border is opposite to the sixth rib and space. The 
base is related to the great vessels, the trachea and its bifurcation, the bronchial 
lymph glands, the cesophagus, and the vagus, left recurrent, and cardiac nerves. 
The extent of contact of the pericardium with the lateral wall of the thorax is determined by 
the cardiac notch of the lung. On the left side the anterior margin of the notch is at the third 
rib and the posterior margin is at the sixth rib dorsally and sixth intercostal space ventrally. The 
greatest height of the notch is at the fourth rib and intercostal space, where it is a handbreadth 
or more (ca. 10-12 em.) above the sternal end of the rib. The notch is quadrilateral, but much 
narrower above than below. On the right side the notch is usually triangular and is much smaller. 
Its anterior margin begins at the third mtercostal space, about three inches (ca. 7-8 cm.) above the 
sternal ends of the third and fourth ribs and runs obliquely to the sternal end of the third rib. The 
posterior margin begins at the same point and extends to the ventral end of the fourth intercostal 
space or of the anterior border of the fifth rib. 
The two parts of the serous pericardium are, of course, continuous with each other at the 
line of reflection on to the great vessels. The latter are covered in varying degree by the visceral 
layer. The aorta and pulmonary artery are enclosed in a complete common sheath as far as the 
bifurcation of the latter. The membrane passes inward between the pulmonary artery and the 
left auricle, and is continued between the right auricle and the aorta, thus forming the transverse 
sinus of the pericardium (Sinus transversus pericardii). The posterior vena cava is covered 
on the right and ventrally for a distance of an inch or a little more (ca. 3 em.). The pulmonary 
veins have practically no serous covering. The epicardium is closely adherent to the muscular 
tissue of the heart, but is attached to the vessels by areolar tissue and fat, and hence is easily dis- 
sected off them. 
THE HEART 
The heart (Cor) occupies the greater part of the middle mediastinal space. 
Tts shape is that of an irregular and somewhat flattened cone. It is attached at 
its base by the great vessels, but is otherwise entirely free in the pericardium. It 
is decidedly asymmetrical in position, about two-fifths being to the right of the 
median plane and three-fifths to the left. The long axis (from the middle of the 
base to the apex) is directed ventrally and backward. The base of the heart (Basis 
cordis) is directed dorsally and its highest part lies about at the junction of the 
nf 1 The pericardium is described before the heart, since it must be examined first in the labor- 
atory. 
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