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 

 smaU 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 cm.) in a subject of medium size. It is related, 

 dorsally to the trachea, the right vagus and cardiac nerves, and anterior medias- 

 tinal Ij^mph 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 



jnusculo-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 tl^''^ 

 crura of the diaphragm. It passes forward along the right side of the bodies of the 

 thoracic vertebrae, 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 oesophagus, and 

 opens into the right atrium opposite the fourth intercostal space. Its tributaries 

 are: 



