The heart lies in the mediastinum, enclosed in the pericardial sac, and
projects rather more toward the left than toward the right. It is an
ovoid or pear-shaped organ, with its long axis directed approximately
craniocaudad. Its caudal end or apex is, however, directed slightly
ventrad and to the left, while the larger cranial end or base faces
slightly dorsad as well as craniad.
Laterally and dorsally the heart is largely covered by the lungs. The
ventral side and a considerable portion of the lateral surface are,
however, not thus covered, so that they lie against the thoracic wall.
The heart extends from about the fourth or the fifth to the eighth rib,
and its apex touches the diaphragm.
The cavity of the heart is divided by a longitudinal dorsoventral septum
into lateral halves--a right and a left side. Each side is again divided
by a transverse dorsoventral septum (=auriculoventricular=) into two
chambers, one of which, the =auricle=, lies at the base; the other, the
=ventricle=, lies at the apex of the heart. There are thus right and
left auricles and right and left ventricles. An external groove, partly
filled with fat, separates the auricular portion of the heart from the
ventricular part; this groove is known as the =sulcus coronarius=.
Each auricle or ventricle has a single set of blood-vessels either
leaving it or entering it. Thus the blood enters the right auricle by
the =inferior= and =superior= venæ cavæ (Fig. 116, _d_ and _e_) (præcava
and postcava). It passes thence into the right ventricle, and from the
right ventricle to the lungs by a single =pulmonary artery= (Fig. 115,
_f_). It returns to the left auricle by the numerous =pulmonary veins=
(Fig. 116, _g_, _h_, _i_), and passes thence to the left ventricle.
From the left ventricle it passes to the body by a single =aorta= (Fig.
115, _g_).
When the heart is viewed from the ventral surface (Fig. 115), a
considerable blood-vessel, the =coronary artery= (_q_), running from the
base to a point a little to the right of the apex, indicates the
position of the septum (ventricular septum), between the right (_a_) and
left (_b_) ventricles. The apex thus belongs to the left ventricle. From
the cranial end of the right ventricle the pulmonary artery (_f_) is
seen passing obliquely craniad and toward the left. Beneath the
pulmonary artery the aorta (_g_) appears rising from the middle of the
base of the heart and passing directly craniad. At the base appears a
part of the auricular appendage of the left auricle (_d_), and at the
right a part of the right auricular appendage (_c_). The former (_d_) is
larger and bent at right angles, so that its free end is directed toward
the apex.
In the dorsal view (Fig. 116) the position of the ventricular septum is
not indicated. On the surface of the left ventricle a short distance
from the ventricular septum and nearly parallel to it are seen branches
of the coronary artery and the coronary vein passing toward the apex.
Craniad of the left ventricle is seen the left auricle (_b_) with the
pulmonary veins (_g_, _h_, _i_) opening into it. Craniad of the right
ventricle (_a_) is the right auricle (_c_) with the venæ cavæ (_d_ and
_e_) opening into it near the middle. Coming from beneath the auricles
are seen the pulmonary artery (_j_) and the aorta (_f_).
The =chambers of the heart=.
1. The =right auricle= (atrium dextrum) (Figs. 115 and 116, _c_).
Externally the right end of the right auricle projects so as to form its
=auricular appendage= (Fig. 115, _c_) which lies at the right of the
base of the aorta (Fig. 115, _g_). The superior vena cava (Fig. 116,
_e_) is seen entering near the cranial end of the auricular septum
opposite the base of the aorta. The inferior vena cava (Fig. 116, _d_)
enters the auricle near the coronary sinus and close to the auricular
septum. The netted appearance which the wall, especially that of the
appendage, presents externally is due to the muscular thickenings of the
wall (=musculi pectinati=).
[Illustration: FIG. 115.--HEART, VENTRAL VIEW.