THE STRUCTURE OF THE HEART

The thick middle layer of the wall (myocardium) is composed of cardiac muscle, which is a variety of stri­ated muscle peculiar to this organ. It is covered exter­nally by the visceral pericardium (epicardium) and internally by the endocardium, a thin smooth-surfaced layer continuous with the lining of the blood vessels.

The atrial and ventricular parts of the muscle are separated by a fibrous skeleton that is mainly formed by the conjunction of the rings that encircle the four heart orifices. The skeleton contains islands of fibrocartilage in which nodules of bone (ossa cordis) may develop (Figure 7-13/5). Although these bones appear preco­ciously in the hearts of cattle, they are not confined to this species as is sometimes suggested. The fibrous skel­eton is perforated in one place (near the entrance of the coronary sinus) to allow passage to the atrioventricular bundle of specialized tissue that conducts the impulse to contract and constitutes the only direct connection between the atrial and ventricular muscles. Delicate extensions of the fibrous tissue also provide the cores of the cusps of the various valves.

The atrial muscle is thin—indeed, the auricular wall may be translucent between the pectinate ridges. It is arranged in superficial and deep bundles; some of the former are common to both atria, but the remainder, and all of the deep bundles, are confined to one. It has been postulated that the fascicles that surround the various venous inlets, both systemic and pulmonary, act as throttles to oppose reflux of blood into the veins during atrial systole.

The much thicker ventricular muscle is also arranged in superficial and deep bundles. Some superficial bundles coil around both chambers, utilizing the septum to com­plete a figure-of-eight course. Others, like the deeper bundles, encircle only the one chamber. The arrange­ment of the muscle is actually very complicated, and analyses of the contraction mechanism still leave much obscure.

The inherent rhythm of the heart is controlled by a pacemaker, a small, richly innervated sinuatrial node of modified cardiac fibers (nodal myofibers) that provide the conducting tissue (Figure 7-15, A). This node, which is not apparent to the naked eye, lies below the epicardium of the right atrial wall ventral to the cranial caval opening (Figure 7-3/11). With each heart cycle a wave of excitation, which arises in the sinuatrial node and spreads throughout the atrial muscle, reaches the atrioventricular node (Figure 7-14 and Figure 7-15, B-C). In ungulates, specialized conductive tissue is present subendocardially in the atrium, mainly on the pectinate muscle. From the atrioventricular node an excitatory stimulus passes rapidly throughout the whole ventricular myocardium via the atrioventricular bundle, largely composed of Purkinje fibers, modified cardiac muscle fibers that conduct impulses much more rapidly than those of the common sort (see Figure 7-14). The atrioventricular node consists of modified nodal and Purkinje fibers and is found within the interatrial septum, cranial to the opening of the coronary sinus; it is richly innervated. This node gives origin to the atrio­ventricular bundle, which penetrates the fibrous skeleton before dividing into right and left limbs (crura) that straddle the interventricular septum (Figure 7-16, A-B). Each limb continues ventrally close to the endocardium and branches to reach all parts of the heart muscle; part of the right bundle travels to the outer wall by way of the septomarginal band. The main conducting struc­tures are not difficult to display by dissection of the beef heart.