Cardiac fibrous skeleton

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The fibrous skeleton of the heart is a complex set of collagenous rings that connect annuli of all four cardiac valves. Between the four annuli are two trigones (right and left) and the membranous portions of the interatrial, interventricular, and atrioventricular septa. The annuli of the two semilunar valves are attached in a manner that resemble a figure of 8, which lies obliquely, almost vertically oriented in the sagittal plane.

There are several vital functions of the skeleton:

anchors the bases of the cardiac valves
keeps the atrioventricular and semilunar orifices patent
anchors the myocardium of the walls of all four cardiac chambers
allows independent atrial and ventricular contraction by electrically insulating the atria above the fibrous skeleton from the ventricles below