The Axilla - Human Anatomy

4b. The Axilla

FIG. 523– The axillary artery and its branches.

The axilla is a pyramidal space, situated between the upper lateral part of the chest and the medial side of the arm.

Boundaries.—The apex, which is directed upward toward the root of the neck, corresponds to the interval between the outer border of the first rib, the superior border of the scapula, and the posterior surface of the clavicle, and through it the axillary vessels and nerves pass. The base, directed downward, is broad at the chest but narrow and pointed at the arm; it is formed by the integument and a thick layer of fascia, the axillary fascia, extending between the lower border of the Pectoralis major in front, and the lower border of the Latissimus dorsi behind. The anterior wall is formed by the Pectorales major and minor, the former covering the whole of this wall, the latter only its central part. The space between the upper border of the Pectoralis minor and the clavicle is occupied by the coracoclavicular fascia. The posterior wall, which extends somewhat lower than the anterior, is formed by the Subscapularis above, the Teres major and Latissimus dorsi below. On the medial side are the first four ribs with their corresponding Intercostales, and part of the Serratus anterior. On the lateral side, where the anterior and posterior walls converge, the space is narrow, and bounded by the humerus, the Coracobrachialis, and the Biceps brachii.

Contents.—It contains the axillary vessels, and the brachial plexus of nerves, with their branches, some branches of the intercostal nerves, and a large number of lymph glands, together with a quantity of fat and loose areolar tissue. The axillary artery and vein, with the brachial plexus of nerves, extend obliquely along the lateral boundary of the axilla, from its apex to its base, and are placed much nearer to the anterior than to the posterior wall, the vein lying to the thoracic side of the artery and partially concealing it. At the forepart of the axilla, in contact with the Pectorales, are the thoracic branches of the axillary artery, and along the lower margin of the Pectoralis minor the lateral thoracic artery extends to the side of the chest. At the back part, in contact with the lower margin of the Subscapularis, are the subscapular vessels and nerves; winding around the lateral border of this muscle are the scapular circumflex vessels; and, close to the neck of the humerus, the posterior humeral circumflex vessels and the axillary nerve curve backward to the shoulder. Along the medial or thoracic side no vessel of any importance exists, the upper part of the space being crossed merely by a few small branches from the highest thoracic artery. There are some important nerves, however, in this situation, viz., the long thoracic nerve, descending on the surface of the Serratus anterior, to which it is distributed; and the intercostobrachial nerve, perforating the upper and anterior part of this wall, and passing across the axilla to the medial side of the arm.

The position and arrangement of the lymph glands are described on pages 699 and 700.

1. The Axillary Artery—

(A. Axillaris)

The axillary artery (Fig. 523), the continuation of the subclavian, commences at the outer border of the first rib, and ends at the lower border of the tendon of the Teres major, where it takes the name of brachial. Its direction varies with the position of the limb; thus the vessel is nearly straight when the arm is directed at right angles with the trunk, concave upward when the arm is elevated above this, and convex upward and lateralward when the arm lies by the side. At its origin the artery is very deeply situated, but near its termination is superficial, being covered only by the skin and fascia. To facilitate the description of the vessel it is divided into three portions; the first part lies above, the second behind, and the third below the Pectoralis minor.

Relations.—The first portion of the axillary artery is covered anteriorly by the clavicular portion of the Pectoralis major and the coracoclavicular fascia, and is crossed by the lateral anterior thoracic nerve, and the thoracoacromial and cephalic veins; posterior to it are the first intercostal space, the corresponding Intercostalis externus, the first and second digitations of the Serratus anterior, and the long thoracic and medial anterior thoracic nerves, and the medial cord of the brachial plexus; on its lateral side is the brachial plexus, from which it is separated by a little areolar tissue; on its medial, or thoracic side, is the axillary vein which overlaps the artery. It is enclosed, together with the axillary vein and the brachial plexus, in a fibrous sheath—the axillary sheath—continuous above with the deep cervical fascia.

The second portion of the axillary artery is covered, anteriorly, by the Pectorales major and minor; posterior to it are the posterior cord of the brachial plexus, and some areolar tissue which intervenes between it and the Subscapularis; on the medial side is the axillary vein, separated from the artery by the medial cord of the brachial plexus and the medial anterior thoracic nerve; on the lateral side is the lateral cord of the brachial plexus. The brachial plexus thus surrounds the artery on three sides, and separates it from direct contact with the vein and adjacent muscles.

The third portion of the axillary artery extends from the lower border of the Pectoralis minor to the lower border of the tendon of the Teres major. In front, it is covered by the lower part of the Pectoralis major above, but only by the integument and fascia below; behind, it is in relation with the lower part of the Subscapularis, and the tendons of the Latissimus dorsi and Teres major; on its lateral side is the Coracobrachialis, and on its medial or thoracic side, the axillary vein. The nerves of the brachial plexus bear the following relations to this part of the artery: on the lateral side are the lateral head and the trunk of the median, and the musculocutaneous for a short distance; on the medial side the ulnar (between the vein and artery) and medial brachial cutaneous (to the medial side of the vein); in front are the medial head of the median and the medial antibrachial cutaneous, and behind, the radial and axillary, the latter only as far as the lower border of the Subscapularis.

Collateral Circulation after Ligature of the Axillary Artery.—If the artery be tied above the origin of the thoracoacromial, the collateral circulation will be carried on by the same branches as after the ligature of the third part of the subclavian; if at a lower point, between the thoracoacromial and the subscapular, the latter vessel, by its free anastomosis with the transverse scapular and transverse cervical branches of the subclavian, will become the chief agent in carrying on the circulation; the lateral thoracic, if it be below the ligature, will materially contribute by its anastomoses with the intercostal and internal mammary arteries. If the point included in the ligature is below the origin of the subscapular artery, it will most probably also be below the origins of the two humeral circumflex arteries. The chief agents in restoring the circulation will then be the subscapular and the two humeral circumflex arteries anastomosing with the a. profunda brachii.

Branches.—The branches of the axillary are:

From first part,	Highest Thoracic.	From second part	Thoracoacromial.
From first part,	Highest Thoracic.	From second part	Lateral Thoracic.
	From third part	Subscapular.
		Posterior Humeral Circumflex.
		Anterior Humeral Circumflex.

1. The highest thoracic artery (a. thoracalis suprema; superior thoracic artery) is a small vessel, which may arise from the thoracoacromial. Running forward and medialward along the upper border of the Pectoralis minor, it passes between it and the Pectoralis major to the side of the chest. It supplies branches to these muscles, and to the parietes of the thorax, and anastomoses with the internal mammary and intercostal arteries.

2. The thoracoacromial artery (a. thoracoacromialis; acromiothoracic artery; thoracic axis) is a short trunk, which arises from the forepart of the axillary artery, its origin being generally overlapped by the upper edge of the Pectoralis minor Projecting forward to the upper border of this muscle, it pierces the coracoclavicular fascia and divides into four branches—pectoral, acromial, clavicular, and deltoid. The pectoral branch descends between the two Pectorales, and is distributed to them and to the mamma, anastomosing with the intercostal branches of the internal mammary and with the lateral thoracic. The acromial branch runs lateralward over the coracoid process and under the Deltoideus, to which it gives branches; it then pierces that muscle and ends on the acromion in an arterial network formed by branches from the transverse scapular, thoracoacromial, and posterior humeral circumflex arteries. The clavicular branch runs upward and medialward to the sternoclavicular joint, supplying this articulation, and the Subclavius. The deltoid (humeral) branch, often arising with the acromial, crosses over the Pectoralis minor and passes in the same groove as the cephalic vein, between the Pectoralis major and Deltoideus, and gives branches to both muscles.

3. The lateral thoracic artery (a. thoracalis lateralis; long thoracic artery; external mammary artery) follows the lower border of the Pectoralis minor to the side of the chest, supplying the Serratus anterior and the Pectoralis, and sending branches across the axilla to the axillary glands and Subscapularis; it anastomoses with the internal mammary, subscapular, and intercostal arteries, and with the pectoral branch of the thoracoacromial. In the female it supplies an external mammary branch which turns round the free edge of the Pectoralis major and supplies the mamma.

4. The subscapular artery (a. subscapularis) the largest branch of the axillary artery, arises at the lower border of the Subscapularis, which it follows to the inferior angle of the scapula, where it anastomoses with the lateral thoracic and intercostal arteries and with the descending branch of the transverse cervical, and ends in the neighboring muscles. About 4 cm. from its origin it gives off a branch, the scapular circumflex artery.

FIG. 524– The scapular and circumflex arteries.

The Scapular Circumflex Artery (a. circumflexa scapulæ; dorsalis scapulæ artery) is generally larger than the continuation of the subscapular. It curves around the axillary border of the scapula, traversing the space between the Subscapularis above, the Teres major below, and the long head of the Triceps laterally (Fig. 524); it enters the infraspinatous fossa under cover of the Teres minor, and anastomoses with the transverse scapular artery and the descending branch of the transverse cervical. In its course it gives off two branches: one (infrascapular) enters the subscapular fossa beneath the Subscapularis, which it supplies, anastomosing with the transverse scapular artery and the descending branch of the transverse cervical; the other is continued along the axillary border of the scapula, between the Teres major and minor, and at the dorsal surface of the inferior angle anastomoses with the descending branch of the transverse cervical. In addition to these, small branches are distributed to the back part of the Deltoideus and the long head of the Triceps brachii, anastomosing with an ascending branch of the a. profunda brachii.

5. The posterior humeral circumflex artery (a. circumflexa humeri posterior; posterior circumflex artery) (Fig. 524) arises from the axillary artery at the lower border of the Subscapularis, and runs backward with the axillary nerve through the quadrangular space bounded by the Subscapularis and Teres minor above, the Teres major below, the long head of the Triceps brachii medially, and the surgical neck of the humerus laterally. It winds around the neck of the humerus and is distributed to the Deltoideus and shoulder-joint, anastomosing with the anterior humeral circumflex and profunda brachii.

6. The anterior humeral circumflex artery (a. circumflexa humeri anterior; anterior circumflex artery) (Fig. 524), considerably smaller than the posterior, arises nearly opposite it, from the lateral side of the axillary artery. It runs horizontally, beneath the Coracobrachialis and short head of the Biceps brachii, in front of the neck of the humerus. On reaching the intertubercular sulcus, it gives off a branch which ascends in the sulcus to supply the head of the humerus and the shoulder-joint. The trunk of the vessel is then continued onward beneath the long head of the Biceps brachii and the Deltoideus, and anastomoses with the posterior humeral circumflex artery.

Peculiarities.—The branches of the axillary artery vary considerably in different subjects. Occasionally the subscapular, humeral circumflex, and profunda arteries arise from a common trunk, and when this occurs the branches of the brachial plexus surround this trunk instead of the main vessel. Sometimes the axillary artery divides into the radial and ulnar arteries, and occasionally it gives origin to the volar interosseous artery of the forearm.