After a course within the thorax of about two inches and a half, the left subclavian artery pierces the " thoracico-cervical septum," inclines outwards and a little forwards, and attains the tracheal edge of the scalenus anti cus muscle, when it bends abruptly outwards ; this terminating portion of the first stage of the vessel (which alone belongs to the cervi cal region) has, anterior to it, the left internal jugular vein, the vagus and phrenic nerves, and the thoracic duct, just before its termina tion in the great subclavian vein ; in addition to these relations, the left subclavian has the same coverings as the corresponding portion of the subclavian artery on the right side.
Differences between the right and the left subclavian arteries, in their first stage.—Length. The first stage of the left subclavian artery is much longer than that of the right, which it exceeds by about the length of the Arteria innominata. In the old subject, however, the difference is scarcely so great, for at this period of life, the part of the arch of the aorta, from which the left subclavian artery arises, is higher than that portion of it from which the innominata springs.
left subclavian artery is nearer to the spinal column, the right to the clavicle and sternum ; consequently the depth of the artery from the surface is considerably greater on the left side than on the right, a circumstance which adds greatly to the diffi culty of the operation for securing the left subclavian artery in a ligature.
the left side, the artery in its first stage runs almost vertically, but as it approaches the scaleni, it inclines outwards and forwards, in order to pass between these muscles; this latter part of the vessel is situ ated in the neck, and is the only portion of its first stage found there. On the right side, the subclavian artery runs nearly horizontally outwards, and is placed throughout the whole of its first stage within the limits of the antero inferior triangle of the neck ; for although somewhat concealed at its origin by the sterno clavicular articulation, yet it may be removed from under cover of that joint, by extending the neck, and depressing forcibly the shoulder and clavicle.
pleura is an anterior relation of the commencement of the left, but a posterior relation of the termina tion of the right subclavian artery.
b. Veins. The satellite vein of the right subclavian artery, in its first stage (the right vena innominata), is parallel to the artery, though on a plane anterior and inferior to it, whilst the corresponding vein on the left side crosses the left subclavian at right angles.
2. The internal jugular vein runs parallel to the left, but intersects at right angles the front of the right subclavian artery.
3. The vertebral vein is usually anterior to the artery of the left, but posterior to the artery of the right side.
c. phrenic, vagi, and sympa thetic nerves are necessarily parallel to the left subclavian artery,in the first stage of its course, in consequence of its vertical direction, whilst they run at right angles to the corresponding part of the right, by reason of its transverse course ; in other respects, however, these nerves hold the same relative position to the subclavian arteries at either side, the phrcnic and vagus being anterior, and the sympathetic posterior, to the right and left subclavians respectively. Lastly, the recurrent laryngeal nerve passes behind the right subclavian artery, looping round its under surface, whilst the nerve on the left side encircles the arch of the aorta, internal to the origin of the left sub clavian artery ; the recurrent is consequently related to the inner side of the left subclavian artery, hut at no part of its course does it lie posterior to that vessel.
d. The thoracic duct and (esophagus are connected with the artery of the left side, exclusively. The former holds a double relation to that vessel ; in the thorax it passes behind it, in the neck it rises high above it, and then bends downwards in front of it, to open into the confluence of the great veins. The (eso phagus, as has been mentioned, lies to the right oldie artery.
The differences which have thus been pointed out, are of material importance with reference to the operations upon these two arteries.
In the remainder of their course the sub clavian arteries are perfectly symmetrical, and one and the same description will apply to both.
Subelavian arteries in their second stage. In its second stage each subclavian artery lies in a remarkable intermuscular space, bounded by the scaleni muscles. These muscles (sca lenus anticus and .scalenus posticus) are closely approximated to each other at their attach ment to the tubercles of the cervical trans verse processes ; but in descending to their insertions they diverge, leaving between them tr space truly triangular, of which the base, placed inferiorly, corresponds to the first rib, and to a small portion of the second. In this space the subclavian artery, the brachial plexus of nerves, and the cone of the pleura, are situated. In front, the artery is in contact with the anterior wall of this triangle, consti tuted by the scalenus anticus muscle ; behind, it is separated from the posterior boundary of the triangle by the summit of the cone of the pleura, which ascends thus high into the interval between the scaleni, interposing itself between the scalenus posticus and the subcla vian artery. Towards the summit of this "scalene triangle" the nervous cords which constitute the brachial plexus are placed along the convexity of the artery, superior and external to it.