The =heart= (Cor) occupies the greater part of the middle mediastinal
space. Its 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 presents two surfaces, two borders, a base,
and an apex for description.
[Illustration:
FIG. 419.—HEART OF HORSE, LEFT VIEW. HARDENED _in situ_.
The dotted line indicates the line of reflection of the serous
pericardium. The epicardium and subepicardial fat have not been
removed from the heart.
]
In =position= the heart is decidedly asymmetrical, 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
downward, backward, and somewhat to the left. The =base= (Basis cordis)
is directed dorsally and lies a little above the middle of the
dorso-ventral diameter of the thorax. It is opposite to the ribs from
the third to the sixth inclusive. The =apex= (Apex cordis) lies above
the last segment of the sternum, and is separated by a very short
interval from the sternal part of the diaphragm. The =anterior border=
(Margo cranialis) is strongly convex and is directed obliquely downward
and backward; its lower part is parallel with the sternum. The
=posterior border= (Margo caudalis) is much shorter, nearly vertical,
and is opposite to the sixth rib or intercostal space. The =surfaces=,
=right= and =left= (Facies dextra, sinistra), are convex and are marked
by =grooves= which indicate the division of the heart into four
chambers, the two atria above and two ventricles below. The left surface
(covered by the pericardium) is in contact with the lower third of the
chest-wall from the third to the fifth intercostal space. On the right
side the cardiac notch of the lung is smaller and further forward, so
that the area of contact is at the third and fourth ribs.
[Illustration:
FIG. 420.—HEART OF HORSE, RIGHT VIEW. HARDENED _in situ_.
Line of reflection of serous pericardium dotted. Epicardium and
subepicardial fat have not been removed from heart. Left ventricle
is considerably contracted.
]
The =coronary= or =atrio-ventricular groove= (Sulcus coronarius)
indicates the division between the atria and the ventricles. It almost
completely encircles the heart, but is interrupted by the origin of the
pulmonary artery. The =longitudinal= or =interventricular grooves=,
right and left (Sulci longitudinales sinister et dexter), correspond to
the septum between the ventricles. The left groove is left-anterior in
position. It begins at the coronary groove behind the origin of the
pulmonary artery, and passes downward almost parallel to the posterior
border, but does not reach the apex. The right groove is right posterior
in position. It begins at the coronary groove below the termination of
the posterior vena cava and passes downward and a little backward,
ending about an inch and a half (ca. 3 to 4 cm.) above the apex. The
grooves are occupied by the coronary vessel and a variable quantity of
fat.
[Illustration:
FIG. 421.—BASE OF HEART OF HORSE WITH LARGE VESSELS, DORSAL VIEW.
SPECIMEN HARDENED _in situ_.
]
=Size and Weight.=—In horses of medium size the heart usually weighs
about seven or eight pounds (ca. 3.5 kg.), or about 0.7 per cent. of the
body-weight. There is, however, great range of variation in apparently
normal specimens.
The following measurements were obtained in medium-sized hearts in
diastole:
Sagittal diameter of base 25 cm.
Greatest width of base 18 to 20 cm.
Circumference at coronary groove 65 to 70 cm.
Distance between origin of pulmonary artery and apex 25 cm.
Distance between termination of posterior vena cava and apex 18 to 20 cm.