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.