In this stage the artery is contained in the antero-inftrior triangle of the neck. The fol lowing structures here constitute its anterior relations, and are described in the order in which they present themselves in dissection : —The skin, and subcutaneous cellular stra tum, with one sheet of the cervical fascia, being divided, the tendon of the sterno-mas toid muscle (sternal origin) is exposed : be hind it is a cellular interval of constant ex istence, though of variable extent, where anastomosing veins and small arterial twigs ramify. The outer edges of the sterno-hyoid and of the sterno-thyroid muscles are more deeply placed ; the latter extends much fur ther outwards than the former, and hence it more directly overhangs and conceals the subclavian artery. These muscles are con tained in distinct sheaths (furnished by the deep cervical fascia), which isolate them, as well from the superficial as from the deeper seated parts : the layer of fascia which forms the back of the sheath of the sterno-thyroid muscle is the deepest lamina of the cervical aponeurosis ; towards the middle line it in vests the front of the trachea, and the deep thyroid veins; externally, it covers the carotid and subclavian arteries, with their accompany ing veins, and is connected to the scalenus anticus muscle, whilst, inferiorly, it is attached to the clavicle, to the sternum, and to the " thoracico•cervical septum," through the in tervention of which it is connected with the fibrous layer of the pericardium. On the removal of this fascia from the region under consideration, the immediate anterior relations of the subclavian artery are exhibited ; they are as follow :— a. The right vena innominata, its satellite vein, lies anterior to the artery in the first stage of its course, but on a lower plane, and separated from it by the phrenic and vagus nerves, and by the internal mammary artery.
b. The internal jugular vein, which passes downwards and outwards into the subclavian vein, in order to form with it the right vena innominata, crosses the front of the artery nearly at right angles : an interval (the result of the inclination outwards of the internal jugular vein) occurs between this vein and the common carotid artery ; and here e. The vagus nerve passes over the sub clavian artery ; whilst internal to this point the subclavian artery is enveloped in a nervous sheath, formed of d. The cardiac filaments of the sympathetic nerve.* e. Lastly, the phrenic nerve constitutes an important anterior relation of the subclavian artery, anterior to the very last por tion of the first stage of the vessel, though not in contact with it. The trajet of the nerve is external to that of the internal jugular vein, which, as before mentioned, crosses the same aspect of the artery. There is no actual contact between the phrenic nerve and the subclavian artery, because the nerve, as it leaves the inner margin of the scalenus an ticus muscle, lies on and crosses the origin of the mammary artery, which thus separates the nerve in question from the trunk of the subclavian. The relation of f. The vertebral vein, to the subclavian ar tery, is subject to much variety. According to the descriptions of most anatomists, it passes posterior to the artery; but the writer has found this vein nearly as often in front of the artery as behind it ; and occasionally he has seen the vertebral vein terminate in two branches, of which one passed on the anterior, and the other on the posterior aspect of the subclavian artery, so as to encircle that vessel before they opened into the vena innominata.
When the vertebral vein is single, and passes over the front of the subclavian artery, it usually lies internal and parallel to the in ternal jugular vein.
The anterior relations of the first stage of the right subelavian artery, are, therefore, the fol lowing:— 1. Integument, subcutaneous areolar tissue, fascia.
2. Sterno-mastoid muscle, sterno-clavi cular articulation.
3. Sterno-hyoid and thyroid muscles, iso, lated from one another, and from the last-named muscle, by layers of the cervical fascia.
4. The deep layer the cervical ap °neurosis.
5. The phrenic and vagus nerves, and the cardiac filaments of the sympathetic nerve.
G. The right innominate, and internal jugu lar veins, and sometimes 7. The vertebral vein.
The remaining relations of the artery in its _first stage are the following:— recurrent nerve, which sometimes has an an terior relation to the artery also, in consequence of its arising from the pneumogastric at an unusual height in the neck.
Superiorly, it gives off the vertebral artery opposite a triangular interval, between the scalenus anticus and longus colli muscles.
Posteriorly, it corresponds from within out wards, to the transverse process of the seventh cervical vertebra, to the inferior cardiac fila ments and inferior cervical ganglion, of the sympathetic nerve, to the recurrent nerve, and finally, to the cone of the pleura.
Nearly the outer half of the posterior sur face of this portion of the subelavian artery is thus closely related to this serous sac, being usually that part of the vessel which is ex ternal to the origin of the vertebral branch, and covered on its anterior surface by the internal jugular vein.
First stage of the left subelavian artery. The subclavian artery on the left side arises within the thorax, from the termination of the second stage of the arch of the aorta, at the level of the second dorsal vertebra ; it thence assumes a direction nearly vertically upwards, to the inner margin of the scalenus anticus muscle, and lies to the left side of, and anterior to, the two upper dorsal vertebra?, from which it is'but a short way distant.
Whilst in the cavity qf the thorax, the left subclavian artery corresponds, posteriorly, to the inferior cervical ganglion, and cord of the sympathetic nerve, to the thoracic duct, and more remotely to the longus colli muscle and spinal column. Anteriorly, it is in relation with the pleura, and more immediately with the left pneumogastric and phrenic nerves (the latter descends parallel to the artery, but the former crosses it very obliquely, from above down wards and inwards), with the vertebral vein, the confluence of the internal jugular and left subclavian veins, and the great venous trunk which they form by their confluence, viz. the left vena innominata. Internal and posterior to the artery are placed the oesopha gus and the left recurrent nerve ; internal and anterior to it, is the left common carotid artery ; whilst external to the artery, through out the entire of its thoracic stage, are the left lung and pleura.