The Arteria Innominata, or Brachio-cephalic artery, arises from the arch of the aorta at the commencement of its second stage, and corresponding to the termination of the great sinus of Morgagni; it lies on the front of the trachea, a little to the left side of the middle line, and on a level with the cartilage of the second rib. From its origin it proceeds upwards, backwards, and to the right side, to terminate behind the right sternoclavicular articulation by dividing into the right subclavian and right carotid arteries. If a needle be passed directly backwards and immediately on a level with the top of the sterno-clavicular articulation of the right side, it will be found to pass between the two origins of the sterno-mastoid muscle and through the angle formed by the bifurcation of the arteria innominata into the right subclavian and carotid arteries. The arteria innominata varies in length from an inch to about an inch and a half: it may be dissected either from the neck or from the interior of the thorax; and the student is recommended to adopt both of these methods. On dissecting from the neck downwards to the thorax, the following parts will be found related to the artery. Anteriorly, after removing the integuments and fascia of the neck, we see the sternal origin of the sterno-cleido-mastoid muscle, the first bone of the sternum, the sterno-clavicular articulation, and the sterno-hyoid and sterno-thyroid muscles: near the origin of the artery the left vena innominata, with which it is connected by the descending layer of the thoracic fascia, crosses in front of it; and still higher up, in the young subject, the thymus gland. Posteriorly, the artery rests upon the trachea: on its left side we find the middle and inferior thyroid veins, and occasionally a middle thyroid artery, which separates it from the left carotid. On its right side, and on a plane anterior to it, we observe the right vena innominata, and between the two vessels the pneumogastric nerve runs in close relation to the bifurcation of the artery : still more externally than the vagus, the phrenic nerve may be seen lying behind the right vena innominata; and in its passage to the outside of the superior vena cava, still lower down, the vessel is accompanied by the inferior cardiac nerve or nerves: the superior part of the parietal division of the right pleura is situated inferior and external to the artery. We have spoken of a fascia in connection with the left vena innominata, as it passes across the arteria innominata: this fascia will be found to connect not merely these two vessels with one another, and to afford them coverings, but by a deeper-seated process to connect the artery with the trachea, to which latter tube also it furnishes an investment. This fascia has been described by Sir A. Cooper as enveloping these vessels, connecting them with the bones which form the opening of the thorax, and continuous with the fibrous portion of the pericardium. He also describes this fascia as continuous above with the deep-seated fascia of the neck described by Burns.*

Mr. Godman, of Philadelphia, also described this the thoracic fascia, and its continuity with the pericardium and fascia of the neck.

The anatomy of the great venous trunks in relation to the arteria innominata, next demands our attention. The left vena innominata will be seen crossing obliquely above the middle portion of the arch of the aorta, in front of the left carotid, trachea, and arteria innominata, downwards and towards the right side of this latter vessel, a distance of about three inches. The right vena innominata will be seen passing in a more vertical direction, but taking a shorter course, and ranging below the level of the first stage of the right subclavian artery. The two venae innominatse unite to form the vena cava descendens, upon a plane anterior and to the right of the arteria innominata, and about half an inch below its bifurcation. An intervascular space will be found in this situation, formed superiorly and internally by the trunk of the arteria innominata and part of the right subclavian artery; inferiorly and to the right side by the right vena innominata; internally the interval is closed by the termination of the left vena innominata in the vena cava descendens; and superiorly by the internal jugular uniting with the subclavian vein to form the right vena innominata. This interval will be found to contain a quantity of loose areolar tissue, the vagus nerve, and the origin of its recurrent branch or inferior laryngeal nerve, which may be seen in this situation passing underneath the right subclavian artery: the inferior cardiac nerve will be found here also : the layer of fascia, already described as continuous with the deep layer of the cervical fascia, covers all these parts. It is this space which the surgeon's aneurism-needle must traverse in the operation of tying the arteria innominata.

* Anatomy of the Thymus Gland, p. 24.

Anatomical Investigations, by John D. Godman, in Philadelphia Journal, 1824.

If the dissection of the artery be made from the chest, the apex of the right lung should be drawn downward; the finger may be then passed upwards into the summit of the supra-clavicular region, so as to pass behind the middle stage of the right subclavian artery; it will be then found that the parietal layer of the pleura will ascend from the thorax into this region, forming the apex of the cone of the pleura. If the finger be now pressed internally and anteriorly, the under surface of the arteria innominata may be felt, through the pleura.

If a vertical section of the arteria innominata and arch of the aorta be made, the right wall of the former vessel will be observed to form nearly a directly continuous surface with the convexity of the arch; whilst its left wall will be seen forming a spur-like projection into the aorta: a considerable amount of the column of blood issuing from the heart will be thus directed into the arteria innominata. The same observation will apply to the origins of the left carotid and left subclavian arteries, though in these vessels the arrangement is not so distinctly seen.