2. Vena cava superior ([Fig. 129], b).

The superior vena cava is the great vein returning the blood from the head, fore-limb, and cranial part of the trunk. It extends from the level of the first rib on the right side of the vertebral column to the right auricle. Its caudal end lies dorsad of the aortic arch. Opposite the right rib it is formed by the junction of the two innominate veins (m).

Branches of the superior vena cava:

1. V. azygos (d).—The first branch of the superior vena cava is the azygos vein, which enters the vena cava on the right side a centimeter or less craniad of the root of the right lung. It is formed in the abdominal cavity by the confluence of two or three small veins, which collect the blood from the muscles of the dorsal wall of the abdomen. The small median trunk thus formed enters the thoracic cavity between the crura of the diaphragm and lies on the ventral surface of the centra of the thoracic vertebræ, slightly to the right of the middle line. The azygos receives the intercostal veins (n), which correspond to the intercostal arteries and have the same course and distribution. In the caudal part of the thoracic cavity the intercostals enter the azygos separately, but the intercostals of the cranial intercostal spaces usually unite two or three together and enter the azygos by one or more common trunks. The azygos also receives bronchial and œsophageal veins, corresponding to the similarly named arteries.

[Fig. 129].—Thoracic Blood-vessels, from the Right Side.

a, aorta; b, vena cava superior; c, vena cava inferior; d, V. azygos; e, A. subclavia dextra; f, A. mammaria interna; g, V. mammaria interna; h, costocervical axis; i, united trunk of vertebral and costocervical veins; j, A. and V. axillaris; k, V. jugularis externa; l, V. jugularis interna; m, the two innominate veins; n, intercostal arteries and veins; o, thyrocervical axis. 1, heart; 2, trachea; 3, œsophagus; I-XI, cut ends of the first eleven ribs.

2. V. mammaria interna (g).—The internal mammary veins follow the corresponding arteries. The two veins unite to form a common trunk which, after receiving a branch from the cranial part of the sternum, enters the vena cava opposite the third rib.

3. Vv. anonymæ ([Fig. 129], m; [Fig. 115], p).—The innominate veins unite opposite the first intercostal space to form the vena cava superior. Each extends from the cranial end of the vena cava superior craniolaterad to a point a short distance craniad of the first rib and is there formed by the union of the external jugular ([Fig. 129], k) and subclavian (j) veins. The innominate vein has the following branches:

A and B. Vv. vertebralis and costocervicalis (i).—The vertebral and costocervical veins unite to form a single trunk which enters the innominate about opposite the first rib. These two veins follow the corresponding arteries. (Sometimes the vertebral and costocervical veins enter the innominate separately, and in other cases one or more of the components of the costocervical veins (e.g., the transversa colli) may enter the axillary vein. In other cases the entire common trunk of the vertebralis and costocervicalis may enter the superior vena cava directly.)