The Heart is somewhat pyramidal in shape, its long axis being nearly horizontal, and directed downwards, forwards, and to the left. The anterior surface presents itself when the pericardium is opened from the front, and consists of the right auricle, and occupying most of the surface, the right ventricle ; the tip of the left auricular appendix, and a comparatively small portion of the left ventricle, separated from the right ventricle by the anterior interventricular furrow (which lodges a branch of the left coronary artery, and the great cardiac vein, surrounded by fatty tissue). The other structures exposed from the front are (a) the pulmonary artery, whose valve is situated behind the upper edge of the third left costal cartilage close to the sternum ; (b) the ascending aorta, whose valve is behind the left border of the sternum close to the lower edge of the third cartilage. The trunk ascends behind the second right cartilage, and arches over the pulmonary arteries, the convexity of the arch corresponding to the junction of the manubrium and gladiolus. It gives off the innominate and left carotid arteries opposite the centre of the manubrium, which vessels run up to either sterno-clavicular joint; (c) to the right of the aorta, a small portion of the superior vena cava. The tricuspid valve lies behind the middle of the sternum, about the level of the fourth costal cartilage, and the mitral behind the third intercostal space, 1 inch to the left of the sternum. All the valves, therefore, are so situated that the mouth of an ordinary stethoscope placed over the left margin of the sternum at the third intercostal space will cover a portion of each. It will be remembered that the orifices of the venae cavae are practically valveless. The superior vena cava begins opposite the lower border of the first right cartilage, by the union of the two innominate veins, and descends, curving slightly to the left, to the third right costal cartilage, where it enters the right auricle. These relationships are of importance with reference to wounds of the chest. Where the heart is involved, the right ventricle most frequently suffers, then the left ventricle, and then the right auricle. W^ounds of the ventricles tend to be less serious than those of the auricles, owing to the thicker and more muscular coat, and such wounds have been successfully sutured. In some cases death from cardiac wounds is possibly due to nervous shock rather than to bleeding.

The posterior surface of the heart presents the auricles, particularly the left, and the orifices of the four pulmonary veins and of the venae cavae. It extends from the fifth to the eighth dorsal spine. The lower surface presents chiefly the left ventricle, and small portions of the right ventricle and auricle. In front, near the apex, the heart is in relation with the upper wall of the stomach, the diaphragm and pericardium intervening, while behind it lies over the left lobe of the liver. Congenital displacem:nts of the heart are occasionally met with. In some cases of transposition of the viscera the heart may be displaced to the right (dextrocardia), or, associated with congenital sternal fissure, an ectopia cordis may occur, the heart projecting forwards through the aperture. Pathologically, the heart is most frequently displaced laterally by pleural affections, such as effusion or pneumothorax ; cirrhosis of the lung, by shrinking, tends to drag it toward the affected side, while emphysema depresses the diaphragm, and with it the heart. The heart may also be displaced upwards by gaseous distension of the stomach, ascites, etc. Changes of the heart itself, such as dilatation and hypertrophy, also affect the position, and it should be remembered that the position of the normal heart is influenced by respiration and position of the body.