Part 166

THE SYSTEMIC ARTERIES

The =aorta= is the main systemic arterial trunk. It begins at the base of the left ventricle and is practically median at its origin. It passes upward and slightly forward between the pulmonary artery on the left and right atrium on the right. It then curves sharply backward and upward and inclines somewhat to the left, forming the =arch of the aorta= (Arcus aortæ), and reaches the ventral surface of the spine at the eighth or ninth thoracic vertebra. After passing backward along the ventral aspect of the bodies of the vertebræ between the lungs it traverses the hiatus aorticus and enters the abdominal cavity, where it lies below the vertebral bodies and the psoas minor, just to the left of the median plane. It divides under the fifth lumbar vertebra into the two internal iliac or hypogastric arteries.

From the bifurcation a small vessel, the middle sacral artery (A. sacralis media), sometimes passes backward on the pelvic surface of the sacrum. It becomes lost in the periosteum or joins the coccygeal artery, or in exceptional cases is traceable to the sphincter ani externus.

The caliber of the aorta is greatest at its origin, which is termed the =bulbus aortæ=. Here it forms three pouch-like dilatations, the =sinuses of the aorta= (or of Valsalva). These correspond to the cusps of the aortic valve, and the coronary arteries arise from the left posterior and anterior sinuses. At the arch the diameter is about two inches (ca. 5 cm.), and beyond this it diminishes gradually in width.

FIG. 426.—CARDIAC VESSELS OF HORSE, RIGHT SIDE.

Veins are black, arteries white. ]

It is convenient to divide the aorta into thoracic and abdominal parts. The =thoracic aorta= (Aorta thoracica) lies within the pericardium to the point of attachment of the ligamentum arteriosum, and is inclosed with the pulmonary artery in a prolongation of the epicardium. Beyond this it is between the two pleural sacs. It is crossed on the right by the œsophagus and trachea, on the left by the left vagus nerve. The left recurrent nerve winds around the concavity of the arch from left to right, and the vena azygos and thoracic duct lie along the dorsal part of its right face. The trachea causes it to deviate to the left; but beyond this it becomes median. The =abdominal aorta= (Aorta abdominalis) (Fig. 450) is related above to the lumbar vertebræ, the inferior common ligament, and the left psoas minor muscle; in the hiatus aorticus it is related to the cisterna chyli. On its right is the posterior vena cava, and on its left the left kidney and ureter.

BRANCHES OF THE THORACIC AORTA

1. CORONARY ARTERIES

The two coronary arteries, right and left, are distributed almost entirely to the heart, but send some small twigs to the origins of the great vessels.

The =right coronary artery= (A. coronaria dextra) arises from the anterior sinus of the aorta. It passes forward between the conus arteriosus and the right auricle (appendix) to the coronary groove, in which it curves around to the right and backward. It then descends in the right ventricular groove almost to the apex of the heart.

FIG. 427.—CARDIAC VESSELS OF HORSE, LEFT SIDE.

The dotted lines indicate part of the left coronary artery which is concealed by the left auricle. ]

The =left coronary artery= (A. coronaria sinistra) arises from the left posterior sinus of the aorta, passes to the left behind the origin of the pulmonary artery, and divides into two branches. The =descending branch= (Ramus descendens) passes down the left ventricular groove toward the apex. The =circumflex branch= (Ramus circumflexus) runs backward in the coronary groove, in which it winds around to the right side.

2. COMMON BRACHIOCEPHALIC TRUNK OR ANTERIOR AORTA

The =common brachiocephalic trunk= or =anterior aorta= (Truncus brachiocephalicus communis) is a very large vessel which arises from the convexity of the arch of the aorta within the pericardium. It is directed forward and upward. Its length in horses of medium size is usually about two inches (ca. 5 to 6 cm.), but it is sometimes only half an inch or less (ca. 1 cm.). It is crossed on the left by the left vagus and cardiac nerves, and the left recurrent nerve runs between it and the trachea. It divides opposite to the second intercostal space or third rib into the brachiocephalic and left brachial arteries.

The =brachiocephalic artery= (A. brachiocephalica) is directed forward and a little upward in the anterior mediastinum, beneath the trachea. Opposite the first rib it gives off the bicarotid trunk and is continued as the =right brachial artery=. The latter (A. subclavia dextra) turns downward and bends around the anterior border of the first rib and the insertion of the scalenus muscle above the brachial vein. Its course and branches beyond this point will be described with the vessels of the thoracic limb.

FIG. 428.—TOPOGRAPHY OF THORACIC CAVITY OF HORSE, LEFT SIDE, AFTER REMOVAL OF LEFT LUNG.

The ninth, fifteenth, and eighteenth ribs are retained. a, Pericardium; b, thoracic aorta; c, brachiocephalic trunk; d, d″, dorsal artery; d′, subcostal artery; e, e′, superior or deep cervical artery; f, vertebral artery; g, g′, left brachial artery; h, inferior cervical artery; i, internal thoracic artery; k, common carotid artery; l, jugular vein; m, brachial vein; m′, inferior cervical vein; n, anterior vena cava; o, thoracic duct; p, œsophagus; q, costal part of diaphragm; q′, tendinous center of same; q″, left crus of same; r, anterior mediastinal lymph glands; s, trachea; t, bronchial lymph glands; u, root of left lung; v, anterior mediastinum; v′, posterior mediastinum; w, longus colli; x, sterno-cephalicus; y, scalenus; z, transversalis costarum; 1, longissimus; 2, splenius; 3, spinalis et semispinalis; 4, 4′, rhomboideus; 5, left phrenic nerve; 6, left vagus; 6′, 6″, œsophageal continuations of vagi; 7, dorsal branch of right vagus; 8, cardiac nerve; 9, left recurrent nerve; 10, cervical trunk of sympathetic; 11, first thoracic ganglion of sympathetic; 12, thoracic trunk of sympathetic; 13, brachial plexus. (After Ellenberger-Baum, Top. Anat. d. Pferdes.) ]

The =left brachial artery= (A. subclavia sinistra) is longer than the right one and rises to a higher level. It forms an almost semicircular curve, the concavity being ventral. It is related internally to the œsophagus, trachea, and thoracic duct, and the left vagus, phrenic, and cardiac nerves cross under its origin. It emerges from the thorax like the artery of the right side. There is thus a difference at first between the trunks of opposite sides, but beyond this their course and distribution are similar.

The brachial and brachiocephalic arteries within the thorax give off the dorsal, superior cervical, vertebral, and internal thoracic arteries. At the first rib they give off the external thoracic and inferior cervical arteries.

1. The left =dorsal= or =dorso-intercostal artery= (A. costo-cervicalis) passes dorsally across the left face of the trachea and œsophagus toward the second intercostal space. The right artery arises usually by a common trunk with the superior cervical, crosses the right face of the trachea and has no contact with the œsophagus. Both detach small branches to the trachea, lymph glands, and pleura, and divide on reaching the longus colli into two branches. Of these the =subcostal artery= (A. intercostalis suprema) is the smaller. It passes backward under the costo-vertebral joints with the sympathetic trunk. It gives off the second, third, and fourth intercostal arteries, and ends at the fifth space, where it anastomoses with the first aortic intercostal artery, or constitutes the fifth intercostal artery, or dips into the longissimus muscle. It also gives off spinal branches and twigs to the longus colli and the pleura. The other branch (A. transversa colli) is the direct continuation of the trunk. It emerges through the upper end of the second intercostal space, passes across the transversalis costarum and longissimus toward the withers, and divides into several diverging branches. An anterior branch passes upward and forward between the splenius and complexus and anastomoses with branches of the superior cervical artery; the others are directed upward under the serratus magnus and rhomboideus to the withers, supplying the muscles and skin of this region.

FIG. 429.—TOPOGRAPHY OF THORACIC CAVITY OF HORSE, RIGHT VIEW.

The ninth, fifteenth, and eighteenth ribs are retained. a, Pericardium; b, posterior vena cava; c, vena azygos; c′, œsophageal vein; d, e, anterior vena cava; f, brachial vein; g, inferior cervical vein; h, jugular vein; i, vertebral vein; k, superior cervical vein; l, dorsal vein; m, trachea; n, root of right lung; o, œsophagus; p, mediastinal lobe of right lung; q, mediastinum; r, bronchial lymph glands; s, mediastinal lymph glands; t, costal part of diaphragm; t′, tendinous center of same; u, intercostal muscle; v, longus colli; w, posterior deep pectoral muscle; w′, anterior deep pectoral muscle; x, superficial pectoral muscle; y, sterno-cephalicus; z, scalenus; 1, 5, right brachial artery; 2, dorso-cervical trunk; 2′, superior or deep cervical artery; 3, dorsal artery (cut); 3′, subcostal artery; 4, internal thoracic artery; 6, external thoracic artery; 7, inferior cervical artery; 8, bicarotid trunk; 10, aorta; 11, œsophageal artery; 12, thoracic duct; 13, right phrenic nerve; 14, brachial plexus; 15, right vagus; 15′, 15″, œsophageal continuations of vagi; 16, right recurrent nerve; 17, cervical trunk of sympathetic; 18, thoracic trunk of sympathetic; 20, inferior cervical ganglion; 21, first thoracic ganglion of sympathetic; 22, right cardiac nerve; 23, longissimus; 24, transversalis costarum; 25, spinalis; 26, multifidus; 27, ligamentum nuchæ. (After Ellenberger-Baum, Top. Anat. d. Pferdes.) ]

The left dorsal artery sometimes arises with the superior cervical by a common trunk. On the right side there may be a common stem for the dorsal, superior cervical, and vertebral arteries. Occasionally the artery arises from the anterior aorta. Sometimes it emerges through the third space. The subcostal may arise independently behind the dorsal or from the superior cervical artery.

2. The =superior= or =deep cervical artery= (A. cervicalis profunda) arises in front of the dorsal or by a common trunk with it. It crosses the œsophagus (left side), the trachea (right side), and the longus colli, and emerges from the thoracic cavity by passing through the space behind the first costo-transverse articulation. In the thorax it gives off a small branch (A. mediastini cranialis) to the mediastinum and the pericardium; also the first intercostal artery (A. intercostalis prima), a very small vessel which passes down in the first intercostal space. After leaving the thorax the artery passes upward and forward on the spinalis muscle and the lamellar part of the ligamentum nuchæ, covered by the complexus. Its terminal branches anastomose with branches of the occipital and vertebral arteries in the region of the axis. Numerous collateral branches are detached to the extensor muscles of the head and neck, the ligamentum nuchæ, and the skin, and anastomoses occur with the dorsal artery above and the vertebral artery below.

The artery sometimes emerges through the second intercostal space.

FIG. 430.—VERTEBRAL ARTERY OF HORSE. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]