This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The second portion of the axillary lies beneath the pecto-ralis minor muscle. It is 3 cm. (1 1/4 in. ) long and while never ligated at this point it is nevertheless frequently exposed while clearing out the axilla for malignant growths of the breast. Owing to its being covered by the pectoralis minor and major muscles the artery cannot be compressed at this point in its course.
Its branches are the alar thoracic and long thoracic. The alar thoracic are small branches of little importance supplying the fat and glands of the axilla.
The long thoracic or external mammary is of considerable importance on account of its size and because it is encountered in operations on the breast and axilla. It passes down along the lower (outer) border of the pectoralis minor, giving branches to it and the pectoralis major; some branches go to the axilla and serratus anterior, and others, which may be of considerable size in the female, wind around the lower portion of the pectoralis major or pierce it to supply the mammary gland. Posterior
Pectoralis major to it is the long or posterior thoracic nerve, or external respiratory nerve of Bell, going to supply the serratus anterior muscle (Fig. 272).
Fig. 272. - Dissection of the axilla.
Anteriorly is the pectoralis minor muscle, superficial to which is the pectoralis major and skin. Posteriorly lie the posterior cord of the brachial plexus, the fat of the axilla, and the subscapularis muscle; intertially is the axillary vein, with the inner cord of the brachial plexus separating the two. Externally is the outer cord of the plexus and farther out is the coracoid process.
This is about 7.5 cm. (3 in.) long and runs from the lower border of the pectoralis minor to the lower border of the teres major. Its upper portion is under the pectoralis major but its lower portion is subcutaneous because the teres major, forming the edge of the posterior fold of the axilla, extends lower than the anterior fold. It is here that the axillary artery is most easily reached and most often ligated.
It gives off three branches, the subscapular, the anterior circumflex, and the posterior circumflex.
The subscapular artery is of considerable practical importance; it is the largest branch of the axillary and is given off opposite the lower border of the subscapularis muscle. It follows the lower edge of this muscle down the axillary or outer border of the scapula to its angle, where it anastomoses with the posterior scapular, one of the terminal branches of the transverse cervical from the thyroid axis.
Four centimetres (1 1/2 in.) from its origin the subscapular gives off the dorsalis scapulae, which is as large or larger than the continuation of the artery downward. The position of this artery should be borne in mind in operating. It winds around the outer edge of the scapula between it and the teres minor muscle to supply the muscles posteriorly. The subscapular artery is accompanied by the long subscapular nerve to its inner side. (The first or short subscapular nerve supplies the subscapularis muscle, the second supplies the teres major and the third or long subscapular supplies the latissimus dorsi muscle. )
The posterior axillary chain of lymph-nodes accompanies the subscapular artery, hence it is involved in operations for their removal. The point at which the dorsalis scapulas winds around the axillary border of the bone is at or just above the level of the middle of the deltoid muscle and below the level of the posterior circumflex artery.
The anterior circumflex artery is comparatively insignificant. It winds anteriorly around the surgical neck of the humerus beneath the coracobrachialis muscle and both heads of the biceps and gives off an ascending bicipital branch which ascends in the bicipital groove and a small descending branch to the tendon of the pectoralis major. As pointed out by Walsham, the anterior circumflex artery on account of the closeness with which it hugs the bone may be difficult to secure if wounded in the operation of resection of the humerus.
Fig. 273. - Ligation of the third portion of the axillary artery.
The posterior circumflex artery is much larger than the anterior. It runs around the surgical neck posteriorly, below the teres minor, above the teres major, and between the long head of the triceps and the humerus. It is accompanied by the circumflex (axillary) nerve and they run transversely around beneath the deltoid muscle on a level with the junction of its upper and middle thirds. It is to avoid wounding these two important structures that the operation of resection is done anteriorly instead of posteriorly. Being covered only by the skin of the axilla and the superficial and deep fascias, it can readily be compressed by pressure directed outwardly against the humerus along the inner edge of the coracobrachialis muscle.