AXILLARY ARTERY ( arteria axillaris ). This artery, which is the continuation of the subclavian trunk, commences at the outer border of the first rib, beneath the lower mar gin of the subclavius muscle : lying at first on the external surface of the superior part of the thorax, it traverses the axillary space, applies itself to the internal side of the upper extre mity, and terminates at the lower edge of the tendon of the teres major muscle. The ave rage length of this vessel is about six inches : when the arm hangs by the side it describes a curve in its course, the concavity of which is downwards and inwards, but it is brought to a nearly horizontal right line by raising the arm to a right angle with the trunk, and it may be made to describe a curve, the concavity of which is turned upwards, by raising the arm to the greatest possible extent of elevation.
The depth of this artery from the surface is greatest at its commencement, whence to its termination it gradually becomes more superfi cial.
Relations.—Anteriorly the axillary artery is covered by the following parts; on first emer ging from under the margin of the subclavius muscle, it is covered by the costo-coracoid li gament, beneath which the anterior thoracic nerves coming from the brachial plexus cross it in their course to the pectoral muscles, the vessel then passes under the pectoralis minor muscle, from the lower edge of which to its termination the coraco-brachialis lies in front of it. Posteriorly it rests at its commence ment on the first intercostal muscle, then, with the interposition of a considerable quan tity of cellular tissue, on the first digitation of the serratus magnus, which separates it from the external surface of the second rib, it next crosses the tendon of the subsca pularis muscle, from the lower edge of which to its termination it lies on the anterior sur face of the tendon of the teres major. Ex ternally it is bounded by the lowest cord of the brachial plexus, until it arrives at the supe rior edge of the subscapularis, and for the re maining part of its course by the commence ment of the external cutaneous nerve. Inter nally it is bounded by the axillary vein, which is in contact with it in the whole of its course, except while crossing the subscapularis, where the internal root of the median and the ulnar nerve separate the vein from the artery.
The lesser pectoral muscle, in crossing the axilla at the lower part of the upper third of that region, divides the axillary artery into three stages. The first extends from the clavicle to
the upper edge of the lesser pectoral ; in this stage the most important relation which the artery has, is to the vein, which lies upon its inner side and upon a plane anterior to it, so that in a state of distension the vein would overlap the artery. All the nerves are behind and external to it. In the second stage, which is that concealed by the lesser pectoral, the vein, still on the thoracic side and more an terior, is separated from the artery by the nerves, which, forming the axillary plexus, are so closely applied to it, behind and on each side, as to form, as Velpeau remarks, a complete nervous sheath. In the third stage, which is below the lesser pectoral and in immediate connexion with the subscapularis muscle, the artery is still in the midst of nerves, having on each side a root of the median, together with the external cutaneous nerve on the outside and the internal cutaneous and. ulnar on the inside, the circumflex and musculo-spiral being posterior to it. In this stage the vein is in ternal and superficial to the artery, but sepa rated from it by the nerves which lie on its internal side.
A ligature cannot be placed on the axillary artery in any stage of its course without endan gering parts of great importance; in the second stage, however, the connexion of the artery with the axillary plexus is so intimate as com pletely to preclude the possibility of tying it there without incurring the greatest risk of serious injury. Hence there are but two situ ations in which it can be deemed prudent to expose this artery. Of these the operation in the third stage may be accomplished with greater facility, because the artery is here much more superficial, and although its relations are numerous, and in some degree complicated, they admit of being separated from the artery to such a distance as will guard them from injury. To tie the artery in the first stage, however, is much more difficult, chiefly in consequence of the great depth at which it lies, the necessity there is for cutting through large muscles, and the almost certainty of troublesome and unavoidable venous haemor rhage. The principal part which the surgeon has to avoid in applying the ligature needle is the vein.