The axillary artery, gives off several branches, which supply the neighbouring structures. They consist of the branches furnished to the muscles on the chest (external thoracic) ; a large branch to the shoulder (subscapular) ; and two to the upper part of the arm (anterior circumflex and posterior circumflex). The branches are not constant in their number, Size, or mode of origin.
External thoracic branches
These branches vary much in number; but, after the method of Haller, four are usually described.
The superior thoracic artery (thoracica suprema : prima — Haller) a branch of inconsiderable size, arises just above the border of the pectoralis minor, from the axillary artery itself, or, perhaps, as frequently from its next branch, the acromial thoracic. It inclines forwards and inwards, scapular artery, getting between the pectoral muscles, to which it is distributed; some of its branches anastomose with those of the internal mammary and intercostal arteries in the first and second intercostal spaces.
The accromial-thoracic artery (ar thoracica humeraria: acromialis— Haller),11 arises from the fore part of the axillary artery, being rather a large, and by far the most constant of the thoracic branches. It projects forwards at the upper border of the pectoralis minor, and soon divides into branches, which take opposite directions. One set inclines inwards to the thorax, and the other outwards to the acromion, whence the vessel derives its name. The thoracic branches are two or three in number, and are distributed to the serratus magnus and pectoral muscles, their extreme ramifications communicating with those of the other thoracic branches, as well as with the intercostal branches of the internal mammary artery. The acromial branches incline outwards, and subdivide into a descending and transverse set. The latter proceed towards the acromion, and are distributed partly to the deltoid muscle ; whilst others, upon the upper surface of that process, maintain an anastomosis with branches of the suprascapular and posterior circumdex arteries. A descending branch passes down in the interval between the pectoralis major and deltoid, accompanying the cephalic vein and ramifying in both muscles.
The long thoracic artery (thoracica altera major sive longior, — Haller), is directed downwards and inwards, along the lower border of the pectoralis minor, and is distributed to the mamma, (hence it has been called external mammary,) to the serratus and pectoral muscles, and anastomoses with the external branches of the intercostal arteries. This vessel sometimes arises with the acromial, and occasionally with the subscapular.
The alar thoracic branch (alaris, ultima thoracicarum, — Haller), when it exists, for it appears to be generally wanting, and its place to be supplied by branches from the thoracic and subscapular arteries, is a very small vessel. It is distributed to the lymphatic glands and cellular tissue in the axilla.
The subscapular (scapularis inferior aut infrascapularis, — Haller), is the largest branch given off by the axillary artery. It arises from that vessel, close by the lower border of the subscapular muscle, along which it proceeds downwards and backwards, soon becoming considerably diminished in size, owing to its giving off a large branch to the dorsum of the scapula. The continuation of the vessel passes down towards the inferior angle of the scapula, accompanied by the subscapular nerve, and lying on the muscle of that name, to which it gives branches, as well as to the serratus magnus, teres major, and latissimus dorsi muscles. Its final ramifications anastomose with those of the posterior scapular artery, and with its own dorsal branch.
The dorsal branch (dorsalis scapulce), turns backwards from the scapular artery, about an inch and a half from its origin, and is larger than the continuation of the vessel. Descending along the lower border of the scapula, the dorsal branch passes first through the interval between the subscapulars and latissimus dorsi muscles, and then between the teres major and teres minor, and may be found in the fissure between the last-named muscles, immediately behind the long head of the triceps. It gives several branches to these muscles, one of which descends between the teres major and teres minor towards the lower angle of the scapula. The dorsal artery next turns round the lower border of the scapula, frequently grooved to receive it; and on reaching the dorsum of that bone and ramifies extensively upon it in the infraspinous fossa, beneath the infraspinatus muscle, which it supplies, and ultimately anastomoses with the suprascapular and posterior scapular arteries. From the subscapular artery (its dorsal division) is given a slender branch, which enters the subscapular fossa under the subscapularis muscle, and, after ramifying between that muscle and the bone, anastomoses with other slender branches given to the same surface of the scapula from the suprascapular and the posterior scapular arteries.
The two succeeding branches of the axillary artery belong to the arm, and are called circumflex, from the manner in which they wind round the neck of the humerus. They are distinguished as anterior and posterior, from the course they take respectively around the bone. These branches come off close to the lower border of the axilla, as the axillary artery is about to become the brachial.
The posterior circumflex artery [a. circumflexa humeri post.], is not so large as the subscapular, near which it arises. It takes origin opposite to the lower border of the subscapular muscle, passes backwards immediately after its origin, and winds round the humerus, lying between the bone and the long head of the triceps, having the teres major muscle below, and the teres minor above it; and being accompanied by the circumflex nerve. This artery terminates by ramifying in the deltoid muscle and on the shoulder-joint, and by anastomosing with the anterior circumflex and suprascapular arteries, as well as with the acromial thoracic.
The anterior circumflex [a. circumflexa humeri ant.], much smaller than the preceding, arises somewhat lower down, and from the outer side of the axillary artery. It passes from within outwards and forwards, under the coraco-brachialis and inner head of the biceps muscle, resting on the fore part of the humerus, until it reaches the bicipital groove. There it divides into two branches, or, in some cases, into two sets of branches ; one of these ascends by the long head of the biceps through the groove in which it runs, and is distributed to the head of the bone and the capsule of the joint; the other continues outwards in the original direction of the vessel, anastomoses with the posterior circumflex branch, and with the acromial thoracic, and is lost in the deltoid muscle.
From Quain's anatomy.