Passing transversely through the base of the heart is a fibrous framework or "skeleton" made of dense connective tissue, not bone as the name might suggest. The purpose of this tough, immobile scaffold is to: (1) provide an attachment for the atrial and ventricular myocardium, (2) anchor the four valves of the heart, and (3) electrically insulate the myocardium of the ventricles from the atria.
The supporting framework of the cardiac skeleton (Figs. 15 and 18) provides immobile support for the atrioventricular openings during atrial and ventricular contractions, as well as support for the semilunar valves against the high pressures generated during and after ventricular contractions. The skeleton is a formation of four attached rings, with the opening for the aortic semilunar valve in the central position and the other valve rings attached to it.
The triangular formation between the aortic semilunar valve and the medial parts of the tricuspid and bicuspid valve openings is the right fibrous trigone or the central fibrous body, the strongest portion of the cardiac skeleton. The smaller left fibrous trigone is formed between the aortic semilunar valve and the anterior cusp of the mitral valve. Continuations of fibroelastic tissue from the right and left fibrous trigones partially encircle the atrioventricular openings to form the tricuspid and bicuspid annulus or annulus fibrosus. The annuli serve as attachment sites for the atrioventricular valves as well as atrial and ventricular myocardium. Strong collagenous tissue passes anteriorly from the right and left fibrous trigones to encircle and support the aortic and pulmonary semilunar valve annuli.
The membranous interventricular septum is an inferior extension of the central fibrous body that attaches to the muscular interventricular septum. The membranous septum provides support for the medial (right and posterior) cusps of the aortic semilunar valve and continues superiorly to form part of the atrial septum. The tendon of Todaro is a fibrous extension of the membranous septum that is continuous with the valve (eusta-chian) of the inferior vena cava. The atrioventricular bundle of conduction fibers from the atrioventricular node penetrate the central fibrous body, pass through the membranous septum, and split into left and right bundle branches at the apex of the muscular septum (or the junction of the right and posterior cusps of the aortic semilunar valve).
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.