The axillary vein returns all the blood from the upper limb : it is formed by the union of the brachial venm comites (with one or other of which, in the arm, the basilic vein is con tinuous), whilst the cephalic vein joins it near its termination. Like its accompanying artery, the axillary vein consists of three stages, and its relations to surrounding parts are identical with those of that vessel. The vein is placed internal and anterior to the artery throughout its entire course. When the arm is abducted, the vein becomes inferior.
In addition to the cephalic vein, the axil lary trunk receives numerous branches which correspond to those of the axillary artery, viz., the circumflex and subseapular veins, the alar, and the long, superior, and acrornial thoracic veins.
The axillary is the last vein of the upper extremity, as we approach the heart, in which valves exist : at the lower edge of the first rib it becomes continuous with the subcla vian vein..
Subclavian great vessel extends from the axillary vein to the inner margin of the scalenus anticus muscle, where it unites with the internal jugular vein, to form the vena innominata. The subclavian vein therefore corresponds to the second and third stages only of the subclavian artery ; and, as the direction of the vein is nearly transverse, whilst that of the artery describes an arch, it necessarily follows that the former vessel is much shorter than the latter. On both sides of the neck the scalenus anticus muscle, and the phrenic and pneumogastric nerves, are, interposed between these associated vessels. The subclavian vein is related anteriorly to the subclavius and sterno-mastoid muscles, to the clavicle, supra-scapular artery, platysma, and superficial structures of the neck, and rests upon the first rib, the scaleni muscles, and the phrenic nerve, which descends be tween the vein and the scalenus anticus mus cle. Contrary to what is observed in the case of the axillary and of most other veins of large size, the branches which join the subclavian vein do not correspond to those which are given off by the subclavian artery. The collateral branches of the subclavian vein, are the external and internal jugular veins; the former joins it on the outer, and the latter on the inner side of the sterno-mastoid mus cle. The subclavian vein not uncommonly receives the vertebral also, although, as has been stated, this vein is most frequently a tributary of the brachio-cephalic trunk.
Brachio-cephalic veins. (Verne innominatre, Meckel.)—These veins, one on each side, are formed by the union of the internal jugu lar with the subclavian vein ; they are appa rently of the same size, but are contrasted in other essential particulars, as they are ex amined on the right and left side : thus they will be found to differ from each other in length and in direction, as well as in their connexions with surrounding parts, as also in the number of collateral branches which they respectively receive.
The right vena innonzinata is comparatively short, and nearly vertical in direction ; it is placed behind the cartilage of the first rib, and is related externally to the right pleura and internally to its corresponding artery, the arteria innominata ; the vein and artery how ever are not in close apposition, and the right pneurnogastric nerve is found in the interval between them, but deeper than both ; fila ments of the cardiac nerves also are inter posed between them.* The left vena innominata, more than twice as long as the preceding, traverses the upper part of the anterior mediastinum almost hori zontally from left to right, but at the same time with an obliquity downwards and slightly forwards. It is placed behind the first bone of the sternum, from which it is separated by the sterno-hyoid and sterno-thyroid muscles, and by the thymus gland ; and bending across the three primary branches of the aorta, slightly overlaps the anterior surface of the arch itself towards the right side. Imme diately below the cartilage of the first rib and on the right side of the mesian line, the innominate veins unite to form the superior vena cava.
Collateral branches.—The vertebral is in general the only vein which terminates in the right vena innominata, for although it does occasionally occur that the inferior thyroid and internal mammary veins join the vena in nominata on their respective sides, yet in the great majority of instances these veins ter minate differently on the right and left side : on the left they almost invariably open into the corresponding vena innominata, whilst on the right they as constantly open into the vena cava superior. It is not very uncom mon for the left vena innominata to receive all the inferior thyroid veins ; and the left superior intercostal vein, which is usually connected by a large collateral branch with this vein, sometimes, but more rarely, termi nates directly in that vessel.