The innominate artery (brachiocephalic artery a. anonyma), the largest of the vessels which proceed from the arch of the aorta, arises from the commencement of the transverse portion of the arch, before the left carotid. From this point the vessel ascends obliquely towards the right, until it arrives opposite the sterno-clavicular articulation of that side, nearly on a level with the upper margin of the clavicle, where it divides into the right subclavian, and the right carotid artery. Its place of bifurcation would, in most cases, be reached by a probe passed backwards through the cellular interval between the sternal and clavicular portions of the sterno-mastoid muscle. The length of the innominate artery is very variable, but usually ranges from an inch and a half to two inches.
This artery lying within the thorax for the most part is placed behind the first bone of the sternum, from which it is separated by the sterno-hyoid and sterno-thyroid muscles and a little lower down by the left innominate vein, which crosses the artery at its root. The innominate artery lies in front of the trachea, which it crosses obliquely; on its left side is the left carotid artery, with the thymus gland or its remains ; and to the right, is the corresponding innominate vein and the pleura.
In the ordinary condition no branches arise from this vessel.
Peculiarities of the innominate artery
The length of the innominate artery now and then exceeds two inches, and occasionally it measures only one inch or less. Its place of division is a point of surgical interest, inasmuch as upon it in a great measure depends the accessibility of the innominate itself in the 'neck, as well as the length of the right subclavian artery. Though usually bifurcating nearly on a level with the upper margin of the clavicle at the sternal end, it has been sometimes found to divide at a considerable distance above that bone, and, but less frequently, below it also.
Lastly, though usually destitute of branches, this vessel has been observed to supply a thyroid branch, and sometimes a thymic branch, or one (bronchial) which descends in front of the trachea. Sometimes there is no innominate artery, the right subclavian arising as a separate trunk from the aorta. The innominate artery has not unfrequently been seen to give origin to the left carotid. In cases of transposition of the aorta, an innominate artery, as might be expected, exists on the left instead of the right side.
From Quain's Anatomy.