The Upper Border of the Heart
A: Mark a point on the lower border of the left second costal cartilage 1.2 cm away from the sternal margin.
B: Mark a point on the upper border of the right third costal cartilage 1.2 cm away from the sternal margin.
Join these two points with a straight line. The line marks the upper border of the heart which is formed by the atria. Right atrium forms the right one-third and the left atrium forms the left two third of the upper border.
Right Border of the Heart
C: Mark a point on the right sixth costal cartilage 2.5 cm away from the sternal margin.
Connect points B and C a lie which is slightly convex to the right and this line will mark the right border. The right border is formed by the right atrium.
Lower Border of the Heart
D: Mark a point in the left fifth intercostal space 9 cm. from the median plane. Join points C and D by a line which passing through the xiphisternal joint. The right twothirds of the lower border is formed by the right ventricle and the left one third is formed by the left ventricle.