General Anatomy of the Heart
The base of the heart is formed by the thin-walled atria, which are clearly separated from the ventricles by an encircling coronary groove that contains the main trunks of the coronary vessels within a concealment of fat.
The 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 (Fig. 7.7A/1), which overlaps the origin of the pulmonary trunk. The margins of the atria are often crenated.The thick walled ventricles provide a much larger and much firmer part of the heart. 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 (Fig. 7.7A); the subsinuosal (right) groove runs close to the caudal aspect (Fig. 7.7B). Both grooves convey substantial vessels that follow the edges of the interventricular septum, and 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 Fig. 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
The right atrium 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 (Fig. 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 (Fig. 7.8/7). An azygos vein enters variously. When a right azygos is present (as in the horse, dog, and ruminants), it enters dorsally, either by joining the cranial vena cava (Fig. 7.8/6) or by discharging between the caval openings; when a left azygos 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 (Fig. 7.9A/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 produced by the indentation (intervenous tubercle; Fig. 7.8/5) prevents confrontation between the caval streams by deflecting both ventrally, toward the atrioventricular ostium (Fig. 7.8/3) that occupies much of the floor. A depressed membranous area (fossa ovalis; Fig. 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 (Fig. 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.
FIG. 7.7 (A) Left view of the heart. 1, Left auricle; 2, pulmonary trunk; 3, right ventricle; 4, left ventricle; 5, left azygous vein. (B) Right view of the heart. 1, Right atrium; 2, caudal vena cava; 3, aorta; 4, right azygos vein (opening into the cranial vena cava).
FIG. 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 azygos vein; 7, terminal crest; 8, fossa ovalis.
The Left Atrium
The form of the left atrium is generally similar to that of the right atrium. It receives the pulmonary veins, which enter separately or in groups at two or three sites: Craniosinistral, craniodextral, and in some species, caudal (Fig. 7.9∕11i and 11'). The septal wall may present a scar marking the position of the valve of the fetal foramen ovale. The auricle resembles that on the right side.
The Right Ventricle
This chamber, Crescentic in transverse section, the right ventricle is wrapped around the right and cranial aspects of the left ventricle (Fig. 7.10). It is incompletely divided by a 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, whereas the extension to the left, the conus arteriosus (Fig. 7.11), 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 papillary muscles are present, and the chordae tendineae are so arranged that they connect each cusp to two muscles and each muscle to two cusps (Fig. 7.11/2 and 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 wall to the outer wall (see Fig. 7.17B/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 Fig.
7.3). The muscle is further modified by the many irregular ridges (trabeculae carneae) that give the lower part of the wall a spongy appearance. These ridges, which are confined to the "inflow" part of the cavity, are thought to reduce blood turbulence.
FIG. 7.9 (A) Left and (B) right views of the bovine heart. 1, Right ventricle; 2, left ventricle; 3, left auricle; 4, paraconal interventricular branch of left coronary artery (a.); 4', circumflex branch of left coronary a.; 4", subsinuosal interventricular branch of left coronary a.; 5, pulmonary trunk; 6, right auricle; 7, aorta; 8, ligamentum arteriosum; 9, cranial vena cava; 10 andl0', left and right pulmonary arteries; 11 and 11', left and right pulmonary veins; 12, left azygos vein (v.); 13, right azygos v.; 14, caudal vena cava; 15, right coronary a.
FIG. 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; L, left; R, right.
FIG. 7.11
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.
Dorsal view of the base of the bovine heart after removal of the atria. The ossa cordis on both
FIG. 7.12
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; L, left; R, right.
The opening into the pulmonary trunk lies at a more dorsal level than the atrioventricular ostium and is Craniosinistral 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 (Fig. 7.12). 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 (Fig. 7.13).
The Left Ventricle
The left ventricle is circular in section (see Fig. 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 demands; however, the impression that the left 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 (Figs. 7.12/2 and 7.14/3). The exit to the aorta takes a more central position within the heart.
FIG. 7.13 Tricuspid valve and the right ventricle of the dog. 1, Cusps of the tricuspid valve, magenta: parietal cusp, blue: angular cusp, green: septal cusp; 2, chordae tendineae; 3, papillary muscle (Mm. papillares parvi); 3”, papillary muscle (M. papillaris magnus); 3* papillary muscle (M. papillaris subarteriosus); 4, pulmonary valve.
The aortic valve, generally resembling the pulmonary valve, shows a different orientation of its cusps (Fig. 7.12/3). The nodular thickenings in the free margins of the aortic cusps are conspicuous.