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).

[Fig. 115].—Heart, Ventral View.

[Fig. 116].—Heart, Dorsal View.

Fig. 115.—a, right ventricle; b, left ventricle; c, right auricular appendage; d, left auricular appendage; e, conus arteriosus; f, pulmonary artery; g, aortic arch; h, thoracic aorta; i, vena cava superior; j, innominate artery; k, left subclavian artery; l, left common carotid; m, right common carotid; n, right subclavian; o, azygos vein; p, the two innominate veins; q, coronary artery.

Fig. 116.—a, right ventricle; b, left auricle; c, right auricle; d, vena cava inferior; e, vena cava superior; f, aorta; g, h, i, groups of pulmonary veins (g, dorsal group; h, sinistral group; i, dextral group); j, pulmonary artery (division into two); k, innominate artery; l, left subclavian; m, left common carotid; n, right common carotid; o, right subclavian; p, azygos vein.