rture, the
auriculo-ventricular orifice, which is furnished with a remarkable
mechanism of valves, allowing the transmission of blood from the
auricles into the ventricles, but preventing a reverse course. The
ventricles are thick-walled cavities, forming the more massive portion
of the heart toward the apex. They are separated by a partition, and are
connected with the great arteries--the pulmonary artery and the
aorta--by which they send blood to all parts of the body. At the mouth
of the aorta and at the mouth of the pulmonary artery is an arrangement
of valves in each case which prevents the reflux of blood into the
ventricles. The auriculo-ventricular valve in the left side is composed
of two flaps, hence it is called the bicuspid valve; in the right side
this valve has three flaps and is called the tricuspid valve. The flaps
which form these valves are connected with a tendinous ring between the
auricles and ventricles; and each flap of the auriculo-ventricular
valves is supplied with tendinous cords, which are attached to the free
margin and under-surface, so as to keep the valves tense when closed--a
condition which is produced by the shortening of muscular pillars with
which the cords are connected. The arterial openings, both on the right
and on the left side, are provided with three-flapped semilunar-shaped
valves, to prevent the regurgitation of blood when the ventricles
contract. The veins emptying into the auricles are not capable of
closure, but the posterior vena cava has an imperfect valve at its
aperture.

The inner surface of the heart is lined by a serous membrane, the
endocardium, which is smooth and firmly adherent to the muscular
structure of the heart. This membrane is continuous with the lining
membrane of the blood vessels, and it enters into the formation of the
valves.

The circulation through the heart is as follows: The venous blood is
carried into the right auricle by the anterior and posterior venae cavae.
It then passes through the right auriculo-ventricular opening into the
right ventricle, thence through the pulmonary artery to the lungs. It
returns by the pulmonary veins to the left auricle, then is forced
through the auriculo-ventricular opening into the left ventricle, which
propels it through the aorta and its branches into the system, the veins
returning it again to the heart. The circulation, therefore, is double,
the pulmonary, or lesser, being performed by the right side, and