The cardiac skeleton, sometimes called the fibrous skeleton of the heart, is the structure of dense connective tissue in the heart that separates the atria from the ventricles. The cardiac skeleton establishes electrically impermeable boundaries to autonomic influence within the heart. The anchored Collagen framework of four valves in a plane has the unique property of being electrically inert. In normal anatomy the AV node is the only electrical conduit from the atria to the ventricles through the cardiac skeleton.
General form and anatomy 
The cardiac skeleton consists of four dense bands of dense connective tissue that encircle the bases of the pulmonary trunk, aorta, and heart valves. While not a "true" skeleton, it does provide structure and support for the heart, as well as isolating the atria from the ventricles. In youth, this collagen structure is free of calcium adhesions and is quite flexible. With aging some calcium can accumulate on this skeleton. This accumulation contributes to the AV node and AV bundle delay of the depolarisation wave in geriatric patients.
Fibrous rings 
|Fibrous rings of heart|
|Transverse section of the heart showing the fibrous rings surrounding the valves|
|Latin||anulus fibrosus dexter cordis, anulus fibrosus sinister cordis|
|Gray's||subject #138 536|
|Latin||trigonum fibrosum dextrum cordis, trigonum fibrosum sinistrum cordis, trigona fibrosa|
|Gray's||subject #138 536|
The right and left fibrous rings of heart (anulus fibrosus cordis) surround the atrioventricular and arterial orifices, and are decidedly stronger upon the left than on the right side of the heart. Physiologically, this is readily appreciated in light of the pressure differential between the right and left circuits. The right fibrous ring is known as the anulus fibrosus dexter cordis, and the left is known as the anulus fibrosus sinister cordis.
The upper chambers (atria) and lower (ventricles) are electrically divided by the properties of collagen proteins within the rings. The valve rings, central body and skeleton of the heart consisting of collagen are impermeable to electrical propagation. The only channel allowed (barring accessory/rare preexcitation channels) through this collagen barrier is represented by a sinus that opens up to the Atrioventricular node and exits to the Bundle of His. The muscle origins/insertions of many of the cardiomyocytes are anchored to opposite sides of the valve rings.
The left atrioventricular ring is closely connected, by its right margin, with the aortic arterial ring; between these and the right atrioventricular ring is a triangular mass of fibrous tissue, the Fibrous trigone, which represents the os cordis seen in the heart of some of the larger animals, as the ox and elephant.
Each ring receives, by its ventricular margin, the attachment of some of the muscular fibers of the ventricles; its opposite margin presents three deep semicircular notches, to which the middle coat of the artery is firmly fixed.
From the margins of the semicircular notches the fibrous structure of the ring is continued into the segments of the valves.
The middle coat of the artery in this situation is thin, and the vessel is dilated to form the sinuses of the aorta and pulmonary artery.
See also 
- -1026555847 at GPnotebook
- Description at cwc.net
- Diagram of skeleton at vanderbilt.edu
- Another illustration at vanderbilt.edu
- Overview at vanderbilt.edu
- 261750843 at GPnotebook, – "left fibrous trigone"
- 798621756 at GPnotebook, – "right fibrous trigone"
- Histology (see slide #96)