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.
On raising the arm directly out from the body the armpit and axillary folds become visible. The rounded edge of the anterior axillary fold is formed by the pectoralis major muscle. It follows the fifth rib and its upper end merges with the lower edge of the deltoid muscle.
If firm pressure is made along the inner or lower edge of the outer extremity of the anterior axillary fold the upper end of the biceps muscle can be felt, and lying along with it, to its inner side, is the swell formed by the coracobrachialis muscle.
Along the inner edge of the coracobrachialis muscle lies the axillary artery with its vein to the inner side. This is a little anterior to the middle of the axilla. The artery can be felt pulsating along the inner edge of the coracobrachialis and can be compressed by pressure made in an outward and backward direction against the humerus. The line of the axillary artery is from the middle of the clavicle down along the inner edge of the coracobrachialis muscle, which will be anterior to the middle of the axilla.
The posterior fold of the axilla is formed by the latissimus dorsi and teres major muscles. By deep pressure in the axilla, posterior to the vessels, the arm being abducted, the rounded head of the humerus can be felt.
When the arm is brought more to the side the tissues of the axilla relax and any enlarged lymph-nodes present may be recognized. When normal they cannot be felt.
Winding around the surgical neck of the humerus from behind forward under the deltoid muscle about at the junction of its upper and middle thirds is the posterior circumflex artery and circumflex nerve. Hence a blow at this point may injure the nerve and cause paralysis of the deltoid muscle. The line of fracture of the surgical neck of the humerus would also lie at this point.
Fig. 242. - Surface anatomy of the axilla.