Axilla - Anatomy

The axilla is the space in the interval marking the junction of the upper arm and the chest wall. It is a key area because many important neurovascular structures pass through it. It is pyramidal in shape.

The apex of the pyramid is formed by the first rib, the subclavius muscle, and the collar bone, at the side of the lowest part of the neck. The floor of the axilla is tough axillary fascia that connects the thoracic wall, the gleno-humeral joint, and the posterior part of the axilla. The skin of the axilla is supplied by the intercostobrachial nerve.

Boundaries of the Axilla

The anterior boundary of the axilla comprises the following:

• Pectoralis major – the largest muscle in the pectoral region, that lies over the anterior chest wall and is important in moving the arm forward and up
• Clavipectoral fascia – a layer of fascia (tough connective tissue) in the pectoral region
• Clavicle - the only bone that directly links the pectoral girdle and the axial skeleton. It is also the only long bone in the human skeleton that is oriented horizontally.

The posterior boundary consists of:

• Subscapularis –  one of the 4 muscles that make up the rotator cuff
• Teres major - one of the 7 scapulohumeral muscles around the glenohumeral joint that facilitates movement of the shoulder
• Latissimus dorsi - one of the muscles attaching the upper limb and the vertebral column

The medial boundary of the axilla is made up of:

• Serratus anterior
• the upper ribs and their intercostal muscles

The lateral boundary comprises the intertubercular groove on the medial part of the humerus.

The axillary apex is the interval between the superior border of the scapula, the posterior border of the clavicle, and the external border of the first rib.

The pectoralis major and the latissimus dorsi form the major anterior and posterior folds, respectively, of the axilla. If a vertical line were drawn bisecting the distance between the anterior and posterior folds, it would denote the mid-axillary line.

Contents of the Axilla

The main contents of the axilla are the axillary vein and artery, the axillary lymph nodes, with a portion of the brachial plexus and its branches.

• Axillary artery – the major artery which is the continuation of the subclavian artery, and which continues as the brachial artery, the artery of the arm
• Axillary vein – the large blood vessel that drains blood from the lateral thorax, the upper limb and the axilla into the subclavian vein and thence into the right atrium of the heart
• Brachial plexus - a complex network of nerves formed by the lower cervical and upper thoracic ventral nerve roots, which begins in the neck and extends into the axilla. It is responsible for carrying motor and sensory signals with respect to the pectoral girdle and the upper limb.
• Axillary lymph nodes - nodes in the axilla that receive lymph from the arm, the thoracic wall, the upper abdominal wall, and the breast.

The axilla also contains much adipose tissue, which encloses several other structures, as follows:

• The long thoracic nerve - a nerve that arises from the anterior rami of the 5th, 6th, and 7th cervical nerves and supplies the serratus anterior muscle
• The tendons of the heads of the biceps muscle- the biceps muscle is attached to the scapula by 2 tendons – the tendon of the long head, which is attached to the glenoid rim, and the short head tendon which attaches along the scapula
• Pectoralis minor - a muscle in the anterior chest wall that is overlaid by the pectoralis major.
• Coracobrachialis – one of the 3 muscles in the anterior aspect of the arm, that helps to flex the elbow joint. It is located under the biceps brachii.

References

• http://radiopaedia.org/articles/axilla
• https://www.dartmouth.edu/~humananatomy/part_2/chapter_8.html#chpt_8_axilla
• http://pie.med.utoronto.ca/ALND/ALND_content/axillaryAnatomy.html
• https://www.dartmouth.edu/~humananatomy/part_2/chapter_8.html#chpt_8_axillary_artery