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.
This region has the form of a three-sided pyramid. The apex is truncated and directed upwards and inwards, and is hounded posteriorly by the superior margin of the scapula, anteriorly by the clavicle, and internally by the first rib: through this truncated apex the region of the axilla communicates freely with the supra-clavicular region of the neck. The base, directed downwards and outwards, presents the excavation termed the arm-pit ; by abducting the arm, the concavity of the surface may be diminished, but certainly cannot be rendered convex, as some writers represent. The anterior wall is formed by the greater and lesser pectoral muscles; the posteroexternal wall by the sub-scapular, the teres major, and latissimus dorsi muscles; and the internal wall, which is convex externally, is formed by the ribs, intercostal muscles, and serratus major anticus. The anterior and posterior walls are united by a strong fascia, which contributes to form the base of this cavity, and may be exposed by raising the integuments : externally, this fascia is continuous with the aponeurosis covering the inside of the arm; and internally, it is lost on the muscles of the thorax. We usually find the fascia at the base of this region strengthened by firm narrow tendinous bands passing from the anterior to the posterior fold of the axilla; and occasionally there may be observed muscular bands taking the same direction; several authors have described them, particularly Mr. Lucas, in his paper on the " Anomalies of the muscular system."* The student may now abduct the arm, and remove these structures, in order to examine the contents of the axilla. The muscles and the great axillary vessels and nerves descend externally along the humerus, the vein being most superficial: a large artery, the thoracica longa, may be felt descending behind the lower margin of the pectoralis major ; and another, the inferior or sub-scapular, along the lower margin of the sub-scapularis muscle. When the arm is very much abducted, this last-mentioned artery has its direction altered so as to make it nearly parallel with the axillary artery, for which it may possibly be mistaken. From this account it is evident that, if we proceed to extirpate diseased glands from the axilla, we should cut towards the thorax; as in every other direction we encounter important vessels.
* Lancet, September 22, 1838.