Page 413 - Atlas of Histology with Functional Correlations

The outer epicardium (13, 16) of the atrium (13) and the ventricle (16) is

continuous and covers the heart externally with mesothelium. A subepicardial
layer (17) contains connective tissue, adipose tissue (15), and numerous coronary
blood vessels (15). The epicardium (13, 16) also extends into the coronary
(atrioventricular [AV]) sulcus and the interventricular sulcus of the heart.

Between the atrium (1) and the ventricle (19) is a layer of dense fibrous

connective tissue called the annulus fibrosus (4). A bicuspid (mitral) AV valve
separates the atrium (1) from the ventricle (19). The cusps of the AV (mitral)
valve (5) are formed by a double membrane of the endocardium (6) and a dense
connective tissue core (7) that is continuous with the annulus fibrosus (4). On

the ventral surface of each cusp (5) are the insertions of the connective tissue
cords, the chordae tendineae (8), which extend from the cusps of the valve (5)
and attach to the papillary muscles (11) that project from the ventricle wall. The
inner surface of the ventricle also contains prominent muscular (myocardial)

ridges called trabeculae carneae (10) that give rise to the papillary muscles
(11). The papillary muscles (11) via the chordae tendineae (8) hold and stabilize
the cusps in the AV valves of the right and left ventricles during ventricular
contractions.

The Purkinje fibers (18), or impulse-conducting fibers, are located beneath
the thin endocardium in the subendocardial connective tissue (2). They are
distinguished from cardiac muscle fibers by their larger size and lighter-staining
properties. The Purkinje fibers are illustrated in greater detail and higher
magnification in Figures 10.14 and 10.15.

The large coronary artery (12) is found in the subepicardial connective
tissue (17). Below the coronary artery is the coronary sinus (14), a blood vessel
that drains the heart. Entering the coronary sinus (14) is a coronary vein (14)
with its valve. Smaller coronary blood vessels (15) are in the subepicardial

connective tissue (17) and in the connective tissue septa of the myocardium (19).

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