This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The base of the heart is opposite the upper border of the third costal cartilage. It is here that the superior vena cava ends and the aorta begins. It extends from 1.25 cm. (1/2 in.) to the right of the sternum to 2.5 cm. (1 in.) to the left of the sternum.
The right border of the heart extends from 1.25 cm. (1/2 in.) to the right of the sternum at the upper border of the third costal cartilage in an outwardly curved line to the junction of the seventh rib and the sternum. In the fourth interspace it may reach 2.5 cm. (1 in.) beyond the right edge of the sternum.
The lower border passes from the seventh right chondrosternal junction across the sternoxiphoid joint outward in the fifth interspace to the apex beat, which is 4 to 5 cm. (1 1/2 in. to 1 3/4 in.) below and to the inner side of the nipple and about 8.75 cm. (3 1/2 in.) to the left of the median line. This marks the extreme left limit of the heart. In children the apex is higher - it is in the fourth interspace. In old people it is lower.
The left border arches upward from the apex beat, as just given, in an inward and upward direction to 2.5 cm. (1 in.) to the left of the sternum at the upper border of the third costal cartilage.
The atrio- (auriculo-) ventricular groove or line of junction between the atria (auricles) and ventricles runs from the sixth right chondrosternal junction upward and to the left to the third left chondrosternal junction. The atria lie above and to the right of this line and the ventricles below and to the left.
The right atrium (auricle) and right ventricle lie anteriorly and the left atrium and left ventricle lie posteriorly. In the right atrioventricular groove runs the right coronary artery. As it lies on the anterior portion of the heart it is liable to be injured in stab-wounds and give rise to fatal bleeding, as may also the interventricular branch of the left coronary artery as it passes down near the left border of the heart between the right and left ventricles.
When the lungs are distended the right lung covers the heart to the median line. The left lung leaves the median line at the level of the fourth costal cartilage and curves outward and downward to about the apex beat in the fifth interspace, 2.5 cm. to the inner side of the nipple line. At this point a small piece of the lung, the lingula, sometimes curves around in front of and below the extreme tip of the heart. As the air leaves the lungs they retract and their anterior borders hardly reach the edges of the sternum.
The area of cardiac dulness corresponds to the area uncovered by lung and in contact with the chest-wall. This is the area of absolute dulness. It begins opposite the fourth costal cartilage and extends down the sternum, between the median line and left edge, to the liver dulness below opposite the sixth costal cartilage. Toward the left side it arches from the fourth left costo-sternal junction to the apex beat. The area of so-called relative dulness caused by overlapping of the lungs extends along the right edge of the sternum to opposite the upper border of the third rib above, and to the left follows parallel to the left border of the heart to the tip of its apex. Below it blends with the liver dulness (Fig. 221).
In an early stage of pericardial effusion and also in aneurism there may be an extension of the area of dulness upward. In a later stage of pericarditis the lateral area of dulness becomes increased.
Fig. 220. - Relations of the heart, its valves, and the great blood-vessels to the surface of the chest.
This is the angle formed by the right border of the heart as it meets the liver. It is a more or less resonant area in the fifth right intercostal space. Below it is the liver dulness and above and towards the median line is the heart.