This section is from the book "Anatomy Of The Arteries Of The Human Body", by John Hatch Power. Also available from Amazon: Anatomy of the Arteries of the Human Body, with the Descriptive Anatomy of the Heart.
The Lymphatic Glands found in the axilla are classed into two sets; a superficial set, which are found along the inferior margins of the axillary folds; and a deeper set, which accompany the great vessels. In the advanced stages of cancer, we find these glands enlarged and hardened, as also those along the outer edge of the sternum, and above the clavicle.
After having raised the integuments from off the anterior wall of this region, we will observe some scattered fibres of the origin of the platysma myoides together with the supra-clavicular branches of the cervical plexus of nerves situated underneath. Having cleanly removed these parts, the great pectoral muscle becomes exposed: it has three sets of origins,—one from the clavicle, the second from the sternum, and the third from the ribs; these are separated by areolar intervals. It is into that interval which separates the clavicular from the sternal origin that some of the continental surgeons propose to make their incision, in order to come down on the axillary artery in its first stage. The outer edge of this muscle is separated from the deltoid by another areolar interval, triangular in form, called the del-toidal groove, the base of which is situated superiorly at the clavicle, the apex inferiorly at the insertion of the pectoralis major and deltoid muscles: this space contains the cephalic vein and the thoracico-humeraria artery. More externally, but not forming a part of the anterior wall of the axilla, we observe a rounded prominence corresponding to the head of the humerus, and covered by the anterior or clavicular division of the deltoid muscle. A little internal to this, and also covered by the deltoid muscle, may be felt the coracoid process, between which and the head of the humerus, Lisfrane proposed to sink the knife, for the purpose of amputating at the shoulder-joint.
On raising the pectoralis major, we bring into view the anterior thoracic nerve, and the thoracica longa artery which was concealed by the lower border of the muscle; also the pectoralis minor, which becomes narrow as it passes upwards and outwards, to be inserted under cover of the deltoid muscle into the coracoid process of the scapula. We may observe that the cephalic vein ascends in front of this muscle, and the axillary vein behind it, and that the former empties itself into the latter opposite to its superior margin. Corresponding to the upper edge of this muscle we also find the acromial axis or artery, which separates it from the subclavian muscle and costo-coracoid ligament or ligamentum bicorne. This ligament arises by rather a narrow origin or cornu from the cartilage of the first rib, and passing outwards becomes attached by a second cornu to the coracoid process : its upper margin is attached to the clavicle, and the inferior, which is lunated, looks downwards and inwards : in front it is covered by the great pectoral muscle, and posteriorly it lies on the subclavius muscle, behind which it sends a delicate production: from its inferior or concave margin an expansion more or less strong-descends over the vessels, and covers the anterior surface of the pectoralis minor muscle. We may now detach the origin of this latter muscle from the thorax, and we will observe, on reflecting it outwards, a small slender nerve, the middle thoracic, entering its posterior surface. The contents of the axilla are now brought fully into view. Externally we observe descending along the humerus, the biceps and coraco-brachialis muscles; more internally the axillary artery, with its accompanying vein and the brachial plexus of nerves. Two nerves cross the axilla, from within outwards, to reach the arm; these are sometimes called the nerves of Wrisberg; they are branches of the second and third intercostal nerves, and pass from them through the corresponding intercostal spaces: the superior is the larger. Lastly, far back, and on the inner wall of the axilla, we observe a long thoracic nerve, descending behind the axillary vessels, on the axillary or external surface of the serratus major anticus muscle: this is the posterior thoracic or external respiratory nerve of Bell. These parts, in addition to the lymphatic glands already noticed, and a considerable quantity of areolar tissue, together with numerous branches of arteries, veins, and nerves, form the contents of the axilla.