Page 332 - Anatomy and Physiology of Farm Animals, 8th Edition

Anatomy of the Cardiovascular System / 317

closed cavity (the pericardial space) that endocardium, and a thick muscular layer
called myocardium.
contains only a small amount of fluid to
VetBooks.ir allow frictionless movement of the beating visceral layer of pericardium. The endocar-
The epicardium is the same as the
heart. The heart is invaginated into the
pericardium much like a fist thrust into an dium is a layer of simple squamous
inflated balloon (see Fig. 1‐9). This arrange- endothelial cells that lines the chambers of
ment results in two distinct layers of peri- the heart, covers the heart valves, and is
cardium (Fig. 17‐2). The inner layer, which continuous with the lining of the blood
is intimately adherent to the outer surface vessels. The myocardium consists of car-
of the heart, is called visceral pericar- diac muscle, described in Chapter 9.
dium, or epicardium. The outer layer, The heart is divided into right and left
called parietal pericardium, is continu- sides, which correspond to the low‐pressure
ous with the visceral layer at the base of the (pulmonary circulation) and high‐pressure
heart and is reinforced by a superficial (systemic circulation) systems mentioned
fibrous layer (the fibrous pericardium), earlier. Each side has two chambers: (1) an
which in turn is covered by a layer of medi- atrium, which receives blood by way of
astinal pleura (also called pericardial large veins and which contracts to fill; (2) a
pleura). The parietal pericardium, fibrous ventricle, which pumps blood from the
pericardium, and mediastinal pleura heart through a large artery (Figs. 17‐3 and
together form the pericardial sac, which 17‐4). The atria are thin‐walled chambers,
is grossly identifiable as a thin but tough each of which features an appendage called
tissue surrounding the heart. an auricle.
In cattle, the apex of the heart con- The myocardium of the ventricles,
tacts the dome of the diaphragm, and the which pump blood back into vascular beds,
reticulum in the abdominal cavity lies on is much thicker than that of the atria. The
the caudal side of the diaphragm. Sharp wall of the left ventricle is also thicker than
metallic objects (most commonly, bits of that of the right; blood ejected from the left
wire) that are swallowed often accumu- side during its contraction is under higher
late in the reticulum. The contractions of pressure than that ejected from the right
this organ can cause these foreign bodies ventricle. The right ventricle does not quite
to penetrate the adjacent diaphragm and reach the apex of the heart, as the apex is
the pericardial sac, resulting in an infec- formed entirely by the more muscular left
tion of the sac called traumatic pericar- ventricle. The myocardium between the
ditis, one manifestation of hardware two chambers is the ventricular septum.
disease. The tissues of the pericardium Between the atrium and the ventricle of
thicken, and fluid builds up within the each side is an atrioventricular valve, or
pericardial sac, which leads to heart A‐V valve (Fig. 17‐4). The left A‐V valve is
failure in affected cattle. Hardware occasionally called the bicuspid valve,
disease is usually prevented by adminis- because in humans it has two distinct flaps,
tering a magnet by mouth; the magnet, or cusps. Another more commonly used
which tends to remain in the reticulum, synonym is mitral valve, because of its
gathers swallowed metallic objects and fancied resemblance to a bishop’s miter, or
prevents them from migrating through two‐sided hat. The right A‐V valve is also
the wall of the forestomach. called the tricuspid valve because in
humans it has three flaps or cusps. The
thin valve leaflets are attached to the inner
Cardiac Anatomy wall of the ventricle at the junction of
atrium and ventricle. The free margins of
The heart wall consists of three layers: a the cusp are tethered to the interior of the
thin, outer serous covering called epicar- ventricular wall by fibrous cords called
dium; a thin, inner endothelial lining called chordae tendineae. The chordae tendineae