page_title: Cardiac skeleton
section_title: Fibrous rings
hierarchical_section_title: Cardiac skeleton / Structure / Fibrous rings
context_page_description: The cardiac skeleton, also known as the fibrous skeleton of the heart, is a high density single structure of connective tissue that forms and anchors the valves and influences the forces exerted through them. The cardiac skeleton separates and partitions the atria from the ventricles.
context_section_description: The right and left fibrous rings of heart (anuli fibrosi cordis) surround the atrioventricular and arterial orifices.  The right fibrous ring is known as the anulus fibrosus dexter cordis, and the left is known as the anulus fibrosus sinister cordis. The right fibrous trigone is continuous with the central fibrous body. This is the strongest part of the fibrous cardiac skeleton.
  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 atrioventricular rings serve for the attachment of the muscular fibers of the atria and ventricles, and for the attachment of the bicuspid and tricuspid valves.
  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, such as the ox.
  Lastly, there is the tendinous band, already referred to, the posterior surface of the conus arteriosus.
  The fibrous rings surrounding the arterial orifices serve for the attachment of the great vessels and semilunar valves, they are known as The aortic annulus.
  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.
  The attachment of the artery to its fibrous ring is strengthened by the external coat and serous membrane externally, and by the endocardium internally.
  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.
caption: Transverse section of the heart showing the fibrous rings surrounding the valves
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