Ligature of the third part is performed through an incision along the line of the vessel (at the junction of the middle and anterior thirds of the axilla), which should not extend beyond the lower border of the teres major. The superficial tissues are divided, coraco-brachialis retracted outwards with musculo-cutaneous nerve ; the median nerve is also drawn out, the vein and internal cutaneous nerve drawn in, and the vessel ligatured between the subscapular and circumflex branches. Anastomosis is between the branches of the axillary and those of the thyroid axis. A muscular slip from the latissimus dorsi to the pectoralis major, coraco-brachialis or biceps sometimes exists, crossing the third part of the vessels obliquely, which might be mistaken for the coraco-brachialis. The axillary artery is frequently affected by aneurism, owing to its nearness to the heart, its abrupt curve, and the communication of movements to it from the arm, by which it may be injured. Such aneurisms frequently grow rapidly, and may press on the cords of the brachial plexus and on the axillary vein.

The axillary vein is formed by the junction of the basilic with the venae comitÚs of the brachial artery. This junction frequently occurs at the lower border of the subscapularis muscle, rendering the vein shorter than the artery. Sometimes venae comitÚs may continue close up to the clavicle, many cross branches existing. As the costo-coracoid membrane is adherent to the vein above, maintaining it in a patent condition, it bleeds very freely when cut, and is also liable to suck in air on inspiration. The vein is more often wounded than the artery, but the latter is more liable to injury from traction on the upper limb. The relation of the vein to the first part of the artery varies according to the position of the limb, being inside and a little in front when the limb hangs by the side, and almost entirely in front when the arm is at right angles to the trunk. The glands of the deep axillary group are situated along its antero-internal aspect, and are closely associated with it, rendering their removal in cases of advanced carcinoma difficult, and sometimes necessitating removal of a portion of the vein itself. The lesser internal cutaneous nerve lies along its inner side at the lower part of the space, and it receives the cephalic vein a short distance below the clavicle.

The brachial plexus is derived from the fifth to the eighth cervical and first dorsal nerves, and emerges between the scalenus anticus in front and the scalenus medius and posticus behind. The fifth and sixth cervicals unite, as do the eighth cervical and first dorsal, forming thus three cords ; and these again divide each into two at the level of the entrance of the axilla. The outer cord is formed of the anterior divisions of the fifth, sixth, and seventh cervicals ; the inner of the anterior divisions of the eighth cervical, and nearly all of the first dorsal ; and the posterior of the posterior divisions of all the cervical nerves involved. The outer cord gives off the external anterior thoracic, rnusculo-cutaneous, and outer head of the median ; the inner gives off the inner head of the median, ulnar, internal cutaneous, lesser internal cutaneous, and internal anterior thoracic ; the posterior gives off the circumflex, subscapular, and musculo-spiral. The median is the nerve most frequently damaged by wounds of the axilla, and the musculo-spiral the least. In severe traction the connection with the spinal cord is the part most likely to give way.

The lymphatic glands are classified in four groups :

(1) Anterior pectoral group (5 or 6), lying behind the anterior axillary fold, on the serratus magnus. These drain the pectoral region, outer two-thirds of the mammary gland the antero-lateral chest, and abdominal wall.

(2) Central group (10 to 15), lying under the axillary fascia, and draining the upper limb. (3) Posterior subscapular group (5 or 6), lying along the posterior axillary fold, with the subscapular artery and long subscapular nerve. (4) Deep group (6 to 10), lying along the axillary vessels, and draining the upper limb, and communicating with the glands of the neck and mediastinum, and other axillary glands. The infraclavicular glands, consisting of a few between the pectoralis major and the deltoid, between the pectoralis major and minor, and along the acromio-thoracic artery, drain the outer side of the arm, shoulder, and part of the breast. The axillary glands are regularly affected secondarily in carcinoma of the breast, and occasionally give rise to a tumour much exceeding in size that from which it originated. They may also become enlarged from tubercular and syphilitic affections, and from sepsis, while they are also occasionally the seat of lymphadenoma and lymphosarcoma.

Axillary abscesses may arise from a penetrating wound or from lymphadenitis. When the abscess is superficial to the costo-coracoid membrane, it is generally small, and points either in the infraclavicular fossa or about the anterior axillary fold. An abscess under the pectoralis minor or costo-coracoid membrane generally arises from gland infection, or disease of the shoulder-joint or rib. It cannot pass forwards, owing to the pectoralis minor and costo-coracoid membrane ; backwards, owing to the insertion of the serratus magnus to scapula; inwards, owing to the chest-wall ; outwards, owing to the arm ; downwards, owing to the axillary fascia ; and therefore, after pressing on the axillary fascia, and rendering it convex externally, instead of concave, it tends to pass up into the neck, and possibly to the mediastinum. Sometimes the pus may travel down the arm along the vessels. In opening axillary abscesses the incision should be made midway between the anterior and posterior axillary folds, so as to avoid the long thoracic and subscapular vessels, and at the thoracic rather than the humeral side, so as to avoid the large vessels. The external mammary artery, however, might possibly be wounded by this incision.