The heart-wall consists mainly of peculiar striped muscle, the
myocardium, which is covered externally by the visceral part of the
serous pericardium or epicardium, and is lined by the endocardium.
The =epicardium= is in general closely attached to the muscular wall,
but is loosely attached over the coronary vessels and the associated
subepicardial fat. It consists of a layer of flat polygonal cells,
resting on a membrane of white and elastic fibers.
The =myocardium= consists of planes of fibers arranged in a somewhat
complicated manner. The muscular tissue of the atria is almost
completely separated from that of the ventricles by the fibrous rings
around the atrio-ventricular orifices.
In the atria the muscle bands fall naturally into two groups—superficial
and deep. The former are common to both atria, the latter special to
each. The =superficial= or =common fibers= for the most part begin and
end at the atrio-ventricular rings, but some enter the interatrial
septum. The =deep= or =special bundles= also form two sets. Looped
fibers pass over the atria from ring to ring, while annular or spiral
fibers surround the ends of the veins which open into the atria, the
auricles, and the fossa ovalis.
The muscular wall of the ventricles is much stronger than that of the
atria. That of the left ventricle is in general about three times as
thick as that of the right one, but is thin at the apex. The
=superficial fibers= are attached above to the atrio-ventricular fibrous
rings and pass in a spiral toward the apex. Here they bend upon
themselves and pass deeply upward to terminate in a papillary muscle of
the ventricle opposite to that in which they arose. The loops so formed
at the apex constitute a whorl, the =vortex cordis=. The =deep fibers=,
although they appear to be proper to each ventricle, have been shown by
MacCallum to be in reality almost all common to both. Their arrangement
is scroll-like. They begin on one side, curve around in the wall of that
ventricle, then pass in the septum to the opposite side, and curve
around the other ventricle. There is a layer of deep fibers which is
confined to the basal part of the left ventricle; it is attached to the
left atrio-ventricular ring.
Four =fibrous rings= (Annuli fibrosi) surround the orifices at the bases
of the ventricles. The atrio-ventricular rings separate the musculature
of the atria from that of the ventricles. Those which surround the
origins of the pulmonary artery and aorta are festooned in conformity
with the attached borders of the valves. The aortic ring contains on the
right side a plate of cartilage (Cartilago cordis), which frequently
becomes more or less calcified in old animals. Sometimes a smaller plate
is present on the left side.
The =endocardium= lines the cavities of the heart and is continuous with
the internal coat of the vessels which enter and leave the organ. Its
free surface is smooth and glistening and is formed by a layer of
endothelial cells. The latter rest on a thin layer of connective tissue,
which is connected with the myocardium by a subendocardial elastic
tissue containing vessels and nerves.
=Vessels and Nerves.=—The heart receives its blood-supply through the
two =coronary arteries= which arise from the aorta opposite to the
anterior and left cusps of the aortic valve. Most of the blood is
returned by the =coronary veins=, which open into the right atrium by
the coronary sinus.[157] A few small veins open directly into the right
atrium, and others are said to open into the left atrium and the
ventricles. The =lymph vessels= form a subepicardial network which
communicates through stomata with the cavity of the pericardium. There
is a less distinct subendocardial network. The vessels converge usually
to two trunks which accompany the blood-vessels in the atrio-ventricular
grooves and enter the glands at the bifurcation of the trachea. The
=nerves= are derived from the =vagus= and =sympathetic= through the
cardiac plexus.