10-25. FIBROUS SKELETON OF THE HEART
There is an FCT structure within the substance of the heart. This structure is
known as the fibrous skeleton of the heart. This fibrous skeleton serves two general
purposes: (1) as sites of attachment for muscle tissues and (2) as supporting structures
for the cardia valves. All of the fibrous structures are continuous and form the fibrous
skeleton of the heart.
a. Fibrous Portion of the Interventricular Septum. The uppermost portion
(also called the membranous portion) of the interventricular septum is a part of the
fibrous skeleton of the heart.
b. Atrioventricular (AV) Rings. Each atrioventricular valve of the heart is
surrounded by a dense fibrous ring. This ring maintains the valve opening.
c. Cylinders at Bases of Great Arteries. Each of the semilunar valves of the
heart is located within a short fibrous cylinder. This cylinder maintains the structure and
function of the valve.
10-26. WALL STRUCTURE
The walls of the chambers of the heart are in three layers.
a. The chambers themselves are lined with a simple epithelium known as the
b. Likewise, a simple epithelium surrounds the outside of the heart. It is known
as the epicardium. The epicardium is the same as the visceral pericardium, which we
shall discuss later.
c. By far the most important is the myocardium, the middle layer. It is made up
of cardiac muscle tissue.
(1) Cardiac muscle tissue consists of fibers formed by the fusion of many
individual cells (syncytium). These cardiac fibers are striated and branched.
(2) The myocardium is thicker in the walls of the ventricles than the atria.
This is because greater pressures are needed for the ventricles to perform their
function. The wall of the left ventricle is especially thick, since it has to drive the blood
throughout the body.
(3) The inner surfaces of the ventricular walls have ridges of muscle known
as the trabeculae carneae, with spaces between them.
(4) When the musculature within a chamber wall contracts, the lumen
(cavity) decreases in diameter. This is particularly true of the left ventricle. There is