The<unk>  margin of the heart<unk>  is long, and is formed by the<unk>  atrium above and the<unk>  ventricle below.
The atrial portion is rounded and almost vertical; it is situated behind the third, fourth, and fifth<unk>  costal cartilages about 1.25 cm. from the margin of the sternum.
The ventricular portion, thin and sharp, is named the acute margin; it is nearly horizontal, and extends from the sternal end of the sixth<unk>  coastal cartilage to the apex of the heart.
Right border of heart
Left margin of heart
Diaphragmatic surface of heart
Left sternal border
Sternocostal surface
Apex of the heart
Sternal plane
Mediastinal surface of lung