The subclavian artery, B Q, Plate 5, traverses the root of the neck, in an arched direction from the sterno-clavicular articulation to the middle of the shaft of the clavicle, beneath which it passes, being destined for the arm. In general, this vessel rises to a level considerably above the clavicle; and all that portion of the arching course which it makes at this situation over the first rib has become the subject of operation. The middle of this arching subclavian artery is (by as much as the thickness of the scalenus muscle, X, Plate 5) deeper situated than either extremity of the arch of this vessel, and deeper also than any part of the common carotid, by the same fact. So many branches spring from all parts of the arch of the subclavian artery, that the operation of ligaturing this vessel is less successful than the same operation exercised on others. The structures which lie in connexion with the arch of the subclavian also render the operation of tying the vessel an anxious task. It is crossed and recrossed at all points by large veins, important nerves, and by its own principal branches. The vagus nerve, S E, Plate 6, crosses it at B, its root; external to which place the large internal jugular vein, K, Plate 5, lies upon it; external to this latter, the scalenus muscle, X, Plate 5, with the phrenic nerve lying upon the muscle, binds it fixedly to the first rib; more external still, the common trunk of the external jugular and shoulder veins, U, Plate 5, lie upon the vessel, and it is in the immediate vicinity of the great brachial plexus of nerves, P P, which pass down along its humeral border, many branches of the same plexus sometimes crossing it anteriorly. The depth at which the middle of the subclavian artery lies may be learned by the space which those structures, beneath which it passes, necessarily occupy. The clavicle at its sternal end is round and thick, where it gives attachment to the sterno-cleido-mastoid muscle. The root of the internal jugular vein, when injected, will be seen to occupy considerable space behind the clavicle; and the anterior scalenus muscle is substantial and fleshy. The united spaces occupied by these structures give the depth of the subclavian artery in the middle part of its course.