The axillary artery [a. axillaris], that part of the artery of the upper limb which intervenes between the subclavian and the brachial portions, lies obliquely upon the upper and lateral part of the thorax, extending from the outer border of the first rib to the lower margin of the tendons of the latissimus dorsi and teres major muscles. In this course it passes through the axilla or axillary space, and its direction varies with the position of the limb : when the arm hangs freely by the side, the vessel describes a curve having its concavity towards the chest; when the arm is at right angles with the trunk, the vessel is nearly straight, and if the limb be still more elevated, the concavity of the curve described by the vessel is directed upwards.
This artery is deeply seated, except towards its termination, near the floor or base of the axillary space, where it approaches the surface, and on the inner side is covered only by the skin and fascia; and here (in the armpit) the flow of blood through the artery may be controlled with the finger. In order to stop the circulation, the pressure should be directed outwards, as the vessel, after leaving the thorax, where it is close to the second rib, lies to the inner side of the humerus.
In front, the axillary artery is covered, after having passed below the clavicle, by the pectoral muscles, (the greater pectoral muscle lying over it in the whole of its course, and the smaller muscle crossing over the middle of the vessel) and beneath those muscles by the costo- coracoid membrane with the thin fascia continued from it, and by the subclavius muscle. On the side of the chest the vessel is immediately in contact with the serratus magnus, which is to its inner side; and after reaching the arm, it rests against the subscapular muscle, the latissimus dorsi, and teres major, (the muscles being behind the vessel); and has before it, and to the outer side, the coraco-brachialis muscle. Towards its lower end the artery is covered, on its inner side, only by the integument and fascia, exclusive of the vein and nerves, the position of which is now to come under consideration.
The axillary vein lies in a great measure in front of the artery with an inclination to the inner or thoracic side. The vein is immediately in contact with the fascia continued from the costo-coracoid membrane over the vessels and nerves ; the fascia is, in fact, adherent to it. Two small veins in some instances run along the surface of the artery in the manner of venae comites. The cephalic vein crosses over the artery near its upper end to terminate in the axillary vein; and some veins' from the neighboring muscles will likewise be found crossing it in the same way.
At the upper part of the axilla the brachial nerves lie to the outer side of the artery; about the middle of the space the plexus of nerves surrounds the artery, the roots of the median nerve crossing before the vessel, and immediately in contact with it. Below this, ihe nerves emanating from the plexus are placed at different sides of the artery, and the position they bear to the vessel may be stated as fol- lows, viz., behind it, the circumflex and musculo-spiral ; to its inner side, the ulnar and two internal cutaneous ; to the outer side, the external cutaneous and median. The external cutaneous and the circumflex nerves leave the artery in the axilla, and at the lower part of the space or in the armpit the median nerve is commonly before the vessel; and in an operation, that nerve would serve as a guide to the position of the artery. It might be distinguished from the other large nerves (ulnar and musculo-spiral) by the circumstance of its being the nearest to the pectoral muscle.
The axillary space
The axilla or axillary space, through which the artery passes, is somewhat of a pyramidal form, the summit or apex being above, at the interval between the insertions of the scaleni muscles, and the base below, closed in by the layer of fascia stretched across between the lower borders of the pectoralis major and latissimus dorsi muscles (The lower part of the axilla is sometimes crossed by a band of muscular fibres). This space is bounded in front by the pectoralis major and pectoralis minor, the latter forming only a narrow part of the anterior wall, which is completed above and below it by the pectoralis major. The posterior boundary is formed by portions of the latissimus dorsi, teres major, and subscapular muscles. On the inner side, the space is bounded by the serratus magnus, which is spread over the side of the thorax; and on the outer side by the subscapular, the coraco-brachialis and biceps muscles, which lie on the humerus. The anterior and posterior boundaries are at a considerable distance from each other upon the thorax, but they converge towards the arm, so that the axilla becomes narrower in that direction.
Through the space thus circumscribed, the axillary artery, accompanied by the axillary vein and the brachial plexus of nerves, and invested with a thin fascia, extends, from apex to base, along the outer and narrower side of the axilla, and is placed nearer to the anterior than the posterior wall. At the fore part of the axilla, in contact with the pectoral muscles, lie the thoracic branches of the main vessel; at the back part is the large subscapular branch; and it is only at the inner side, towards the thorax, that large blood-vessels do not occur. In the space are contained, besides the axillary vessels and the large nerves, a considerable number of lymphatic glands and also much loose cellular membrane, which facilitates the movements of the scapula and its muscles on the side of the chest.
From Quain's Anatomy.