. The anatomy of the domestic animals . Veterinary anatomy. 626 BLOOD-VASCULAK SYSTEM OF THE HORSE The left atrio-ventricular orifice (Ostium atrio-ventriculare sinistrum) is chiefly opposite to the fifth rib and intercostal space. The orifice frequently extends back opposite to the sixth rib, and its anterior margin may be opposite to the posterior border of the fifth rib. It is about four inches (ca. 10 cm.) above the sternal end of the fifth rib. The plane of the opening is somewhat oblique, and is higher behind than in front. It is almost circular and is guarded by the bicuspid valve (Valv
. The anatomy of the domestic animals . Veterinary anatomy. 626 BLOOD-VASCULAK SYSTEM OF THE HORSE The left atrio-ventricular orifice (Ostium atrio-ventriculare sinistrum) is chiefly opposite to the fifth rib and intercostal space. The orifice frequently extends back opposite to the sixth rib, and its anterior margin may be opposite to the posterior border of the fifth rib. It is about four inches (ca. 10 cm.) above the sternal end of the fifth rib. The plane of the opening is somewhat oblique, and is higher behind than in front. It is almost circular and is guarded by the bicuspid valve (Valvulabicuspidalis).i The two cusps of this valve are larger and thicker than those of the right side of the. Fig. 549.—Ceoss-section of Ventricular Part of Heart of Horse. The section is cut about 5 cm. from the coronary groove. The ventricles are moderately distended. V. Right ventricle; T'. s., left ventricle; S. v., ventricular septum; 1, 1', papillary muscles of right ventricle; 2, large moderator band of right ventricle; 3, 3, 3', papillary muscles of left ventricle; 4, 4', moderator bands of left ventricle; 5, descend- ing branch of left coronary artery and great cardiac vein; 6, right coronary artery and small cardiac vein. The anterior septal papillary muscle of the right ventricle is not visible, and the right one of the left ventricle is double. heart. One is placed in front and separates the atrio-ventricular and aortic open- ings. The other is placed behind and laterally and is usually divided into two or three parts. The aortic orifice (Ostium aorticum) is directed upward and slightly forward. It is chiefly situated opposite to the fourth rib and intercostal space, but may extend forward to the third space or backward to the fifth rib. It is about five inches (ca. 12-13 cm.) above the sternal end of the fourth rib.^ It is guarded by the aortic valve, composed of three semilunar cusps (Valvulae semilunares aortse); one cusp is ' Also termed the mitral valve. ^ I
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