682 BLOOD-VASCULAR SYSTEM OF THE HORSE 
orifice of the great cardiac vein, so that a common trunk (coronary sinus) does not 
then exist. 
The small cardiac veins (Vv. cordis minores), three to five in number, are 
small vessels which return some blood from the right ventricle and atrium; they 
open into the latter near the coronary groove in spaces between the musculi pec- 
tinati. 
THE ANTERIOR VENA CAVA (Fig. 554) 
The anterior vena cava (V. cava cranialis) returns to the heart the blood from 
the head, neck, thoracic limbs, and the greater part of the thoracic wall. It is 
formed at the ventral part of the thoracic inlet by the confluence of the two jugu- 
lar and two brachial veins. It passes backward in the anterior mediastinum, at 
first median and ventral to the common carotid trunk, then deviates to the right 
of the common brachiocephalic trunk, and opens into the right atrium opposite 
to the fourth rib. It is partly enclosed by the pericardium. The demarcation be- 
tween vein and atrium is not very distinct. It contains no valves except at the 
mouths of its radicles. Its length is about five to six inches (ca. 12-15 cm.) and 
its caliber about two inches (ca. 5 em.) in a subject of medium size. It is related, 
dorsally to the trachea, the right vagus and cardiac nerves, and anterior medias- 
tinal lymph glands. Its right face is crossed by the right phrenic nerve, and on the 
left it is related to the brachiocephalic trunk and artery. The thoracic duct opens 
through the dorsal wall of the origin of the vena cava. It receives, in addition to 
small pericardial and mediastinal veins, the following tributaries: 
1. The internal thoracic vein (V. thoracica interna) is a satellite of the artery 
of that name. It opens into the anterior vena cava at the first rib. The ventral 
intercostal veins (Vv. intercostales ventrales) open into the internal thoracic and 
musculo-phrenic veins. 
2. The vertebral vein (V. vertebralis) corresponds to the homonymous artery. 
On the right side it terminates either in front of the deep cervical vein or by a short 
common trunk with it. On the left side it usually unites with the deep cervical 
and dorsal veins to form a common trunk. 
3. The deep cervical vein (V. cervicalis profunda) corresponds to the artery. 
On the right side it passes downward and backward across the right face of the 
trachea and opens into the vena cava; it may form a common trunk with the dorsal 
or vertebral. On the left side there is usually a common trunk for all three. It 
receives the first intercostal vein. 
4. The dorsal vein (V. costocervicalis) corresponds to the artery. On the 
right side it leaves the artery on entering the thorax, crosses the right face of the 
trachea, and opens into the vena cava behind the deep cervical or by a common 
trunk with it. On the left side it usually joins the deep cervical and vertebral to 
form a short common trunk which crosses the left face of the intrathoracic part of 
the brachial artery opposite the second rib and opens into the anterior vena cava. 
It receives the third, fourth, and fifth intercostal veins by means of the subcostal 
vein on the right side. 
THE VENA AZYGOS 
The vena azygos (Fig. 554) is an unpaired vessel which arises at the level of 
the first lumbar vertebra by radicles from the spinal and psoas muscles and the 
crura of the diaphragm. It passes forward along the right side of the bodies of the 
thoracic vertebre, in contact usually with the thoracic duct, which separates the 
vein from the aorta. At the seventh vertebra it leaves the spine, curves downward 
and forward over the right side of the thoracic duct, trachea, and cesophagus, and 
opens into the right atrium opposite the fourth intercostal space. Its tributaries 
are: 
