GENERAL ANATOMY OF THE HEART
Figure 7-6 Schematic drawings to show the position of the canine heart, based on radiographs.
A, Left lateral view; the caudoventrally sloping long axis (straight line) of the heart is indicated. B, Dorsoventral view showing the asymmetrical position of the heart.right and left atria combine in a continuous U-shaped formation that embraces the origin of the aorta; the formation is interrupted craniosinistrally where each atrium ends in a free blind appendage, the auricle (Figure 7-7, A/1), which overlaps the origin of the pulmonary trunk. The margins of the atria are often crenated.
The ventricles provide a much larger part of the heart that is also much firmer because of the greater
Figure 7-7 Left (A) and right (B) views of the heart. A, 1, Left auricle; 2, pulmonary trunk; 3, right ventricle; 4, left ventricle; 5, left azygous vein. B, 1, Right atrium; 2, caudal vena cava; 3, aorta; 4, right azygous vein (opening into the cranial vena cava).
thickness of the walls. Although the ventricles merge externally, their separate extents are defined by shallow grooves that descend toward the apex. The paraconal (left) groove runs close to the cranial aspect of the heart (Figure 7-7, A); the subsinuosal (right) groove runs close to the caudal aspect (Figure 7-7, B). Both convey substantial vessels that follow the edges of the interventricular septum; together they reveal the asymmetrical disposition of the ventricles. The right chamber lies as much cranially as to the right of the left one (see Figure 7-10). Additional branches of the coronary vessels extend some distance over the ventricular surface in a less constant pattern, but these apart, the external surface is smooth and featureless.
Although it is not apparent externally, a fibrous skeleton separates the atrial from the ventricular muscle mass.The Right Atrium
This chamber lies mainly on the right, although the auricular cul-de-sac extends to the cranial face of the pulmonary trunk to appear on the left side. The greater part forms a chamber (sinus venarum) into which the principal systemic veins discharge (Figure 7-8/1). The caudal vena cava enters the caudodorsal part of this chamber, above the opening of the much smaller vein (coronary sinus) that drains the heart itself. The cranial vena cava opens craniodorsally at the terminal crest (Figure 7-8/7). An azygous vein enters variously. When a right azygous is present (as in the horse, dog, and ruminants), it enters dorsally, either by joining the cranial vena cava (Figure 7-8/6) or discharging between the caval openings; when a left azygous is present (as in ruminants and the pig), it joins the coronary sinus close to its termination after winding around the caudal aspect of the base from the left side (Figure 7-9, A/12).
The interior of the atrium is smooth between the vein entrances, which are unobstructed by valves. Its roof dips between the caval openings, being indented by the passage of pulmonary veins returning across the right atrium to enter the left atrium. The ridge (intervenous tubercle; Figure 7-8/5) produced by the indentation prevents confrontation between the caval streams by deflecting both ventrally, toward the atrioventricular ostium (Figure 7-8/3) that occupies much of the floor. A depressed membranous area (fossa ovalis; Figure 7-8/8) of the septal wall is present caudal to the tubercle; it corresponds to the foramen ovale of fetal life. In sharp contrast, the interior of the auricle (Figure 7-8/1') is made irregular by a series of ridges (musculi pectinati) that branch from the terminal crest that marks the boundary between the auricle and the main compartment.
The Left Atrium
This has a generally similar form.
It receives the pulmonary veins, which enter, separately or in groups, at two or three sites: craniosinistral, craniodextral, and in some species, caudal (Figure 7-9/11,11'). The septal wall may present a scar marking the position of the valve of the fetal foramen ovale. The auricle resembles that of the right side.The Right Ventricle
This chamber, crescentic in transverse section, is wrapped around the right and cranial aspects of the left ventricle (Figure 7-10). It is incompletely divided by a
Figure 7-8 Overview of the interior of the right atrium and right ventricle of the equine heart. 1, Right atrium; 1', right auricle; 2, right ventricle; 3, right atrioventricular valve; 4, caudal vena cava; 5, intervenous tubercle; 6, cranial vena cava; 6', right azygous vein; 7, terminal crest; 8, fossa ovalis.
stout muscular beam (supraventricular crest) that projects from the roof cranial to the atrioventricular ostium. The main part of the chamber lies below this large elongated opening while the extension to the left, the conus arteriosus (Figure 7-12), leads directly to the much smaller circular exit into the pulmonary trunk.
The right atrioventricular (tricuspid) valve is composed of three flaps or cusps that attach to a fibrous ring that encircles the opening. The cusps are fused at their attachment but part toward the center of the opening, where their free margins are thick and irregular, especially in later life. Each cusp is joined by fibrous strands (chordae tendineae) that descend into the ventricular cavity to insert on projections from the walls (papillary muscles). Generally, three of these muscles are present, and the chordae tendineae are so arranged that they connect each cusp to two muscles and each muscle to two cusps (Figure 7-12/2,3). The arrangement prevents eversion of the cusps into the atrium during ventricular contraction (systole).
The lumen of the ventricle is crossed by a thin band of muscle (trabecula septomarginalis) that passes from the septal to the outer wall (see Figure 7-16, B/2). It provides a shortcut for a bundle of the conducting tissue, thus ensuring a more nearly simultaneous contraction of all parts of the ventricle (see Figure 7-3). A further modification of the muscle is provided by the many irregular ridges (tra-
Figure 7-9 Left (A) and right (B) views of the bovine heart. 1, Right ventricle; 2, left ventricle; 3, left auricle; 4, paraconal interventricular branch of left coronary artery; 4', circumflex branch of left coronary artery; 4", subsinuosal interventricular branch of left coronary artery; 5, pulmonary trunk; 6, right auricle; 7, aorta; 8, ligamentum arteriosum; 9, cranial vena cava; 10, 10', left and right pulmonary arteries; 11, 11', left and right pulmonary veins; 12, left azygous vein; 13, right azygous vein; 14, caudal vena cava; 15, right coronary artery.
Figure 7-10 Transverse section through the ventricles. Note the different thicknesses of the walls of the right and left ventricles. 1, Most cranial point; 2, right ventricle; 3, interventricular septum; 4, left ventricle.
Figure 7-12 Cranioventral view of the interior of the right ventricle. 1, Cusp of right atrioventricular valve; 2, chordae tendineae; 3, papillary muscles; 4, pulmonary valve; 5, right auricle.
Figure 7-11 Section of the heart (cow). 1, Right auricle; 2, left atrium; 3, left atrioventricular valve; 4, interventricular septum; 5, aorta.
beculae carneae) that give the lower part of the wall a spongy appearance.
These are confined to the “inflow” part of the cavity and are thought to reduce blood turbulence.The opening into the pulmonary trunk lies at a more dorsal level than the atrioventricular ostium and is cra- niosinistral to the origin of the aorta. It is closed during ventricular relaxation (diastole) by the backflow of blood forcing together the three cusps that arise around its margin and constitute the pulmonary valve (Figure 7-13/4). The cusps are semilunar and deeply hollowed on the arterial side, fitting together tightly when the valve is closed; thickenings of the contact areas, sometimes pronounced in older animals, improve the seal.
The Left Ventricle
This chamber is circular in section (see Figure 7-10) and forms the apex of the heart as a whole. Except toward the apex, its wall is much thicker than that of the right ventricle in conformity with the greater work it must perform; however, the impression that the chamber is also much smaller is illusory. The left atrioventricular (bicuspid or mitral) valve that closes the atrioventricular ostium generally has only two major cusps but is otherwise comparable to that of the right side. It lies largely to the left of the median plane (Figure 7-11/3 and 7-13/2). The exit to the aorta takes a more central position within the heart.
Figure 7-13 Dorsal view of the base of the bovine heart after removal of the atria. The ossa cordis on both sides of the aortic valve have been exposed. 1, Right atrioventricular valve; 2, left atrioventricular valve; 3, aortic valve; 4, pulmonary valve; 5, ossa cordis; 6, left coronary artery; 7, right coronary artery.
The aortic valve, generally resembling the pulmonary valve, shows a different orientation of its cusps (Figure 7-13/5). The nodular thickenings in the free margins of the aortic cusps are conspicuous.