The Axilla: Anatomy and Walls

• The Axilla is also known as the axillary fossa or the armpit.
• The axilla is the passageway for vessels and nerves from the trunk to the upper limb.
• The axilla is roughly pyramidal in shape, with an apex, a floor, and four walls.

Walls of the Axilla

• The anterior wall consists of the pectoralis major, pectoralis minor muscles, and pectoral and clavipectoral fasciae.
• The posterior wall consists of the subscapularis, teres major, and latissimus dorsi muscles.
• The medial wall consists of the 1st to 4th ribs (and associated intercostal muscles) and the serratus anterior muscle.
• The lateral wall consists of the intertubercular groove of the humerus.
• The apex is anteriorly bound by the clavicle, posteriorly by the superior margin of the scapula, and medially by the first rib.
• Its base is directed inferiorly and formed by the facia and skin.

Contents of the Axilla

• The axillary artery and its branches are contained in the axilla.
• The axillary vein and its branches are found in the axilla.
• Axillary lymph nodes are located in the axilla.
• The brachial plexus passes through the axilla.
• The axillary artery, axillary vein, and infraclavicular part of the brachial plexus are enclosed within the axillary sheath.
• The axillary sheath is a continuation of the prevertebral layer of the deep cervical fascia.

Axillary Artery

• The axillary artery starts at the lateral border of the first rib as a continuation of the subclavian artery.
• It terminates at the inferior border of the teres major muscle and continues as the brachial artery.
• It is divided into three parts according to its relation with the pectoralis minor muscle.
• The first part has the superior thoracic artery branch
• The second part has the thoracoacromial and lateral thoracic artery branches.
• The third part has the subscapular, anterior circumflex humeral, and posterior circumflex humeral artery branches.
• In profuse bleeding of the upper limb, the axillary artery may need to be compressed.
• The artery can be easily compressed against the humerus.

Axillary Vein

• The axillary vein lies along the medial side of the axillary artery.
• It is the continuation of the basilic vein.
• The axillary vein begins at the inferior border of the teres major muscle.
• The axillary vein ends at the lateral border of the 1st rib, becoming the subclavian vein.

Axillary Lymph Nodes

• Lymph from the upper limb, shoulder, scapular and pectoral region (including the mammary gland), and upper part of the anterior abdominal wall drain into them.
• There are about 15 to 20 nodes arranged into five groups.
• The groups are pectoral (anterior), lateral, posterior (or subscapular), central, and apical.
• The apical group of axillary lymph nodes receives the lymph from all other groups.
• The efferent vessels of the apical group unite together to form the subclavian lymphatic trunk.
• On the right side, it unites with either the jugular trunk or the bronchomediastinal trunk to form the short right lymphatic duct, which opens into the right venous angle.
• On the left side, the subclavian lymphatic trunk joins the thoracic duct, opening into the left venous angle.
• Enlargement of any lymph node is called lymphadenopathy and may be due to inflammation or cancer spread to the nodes.
• Infections of the upper limb and spread of breast cancer are common causes of axillary lymphadenopathy.
• Excision of the axillary lymph node may be necessary to surgically treat breast cancer.
• Pathologic examination of the lymph nodes determines the staging and treatment of the cancer.

Brachial Plexus

• This network of nerves supplies motor, sensory, and autonomic innervation to the upper limb.
• With one exception, all muscles of the upper limb are supplied by branches of the brachial plexus.
• The exception is the trapezius muscle, which is supplied by the accessory nerve (CN XI).
• The brachial plexus starts in the neck and extends to the axilla, passing through the apex.
• It is formed by the ventral rami of the spinal nerves C5 - T1 (5th cervical to 1st thoracic spinal cord segments).
• Spinal nerves arise from the spinal cord.
• There are 31 pairs of spinal nerves arising from each side of the spinal cord: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.
• A spinal nerve is formed by the union of the sensory (dorsal) and motor (ventral) roots arising from a segment of the spinal cord.
• Autonomic fibers also contribute to the formation of spinal nerves.
• A spinal nerve carries GSA, GSE, GVA, and GVE type axons.
• Once a spinal nerve is formed, it gives off a ventral and dorsal ramus.
• Dorsal rami extend to innervate the skin and muscles of the back.
• Ventral rami form plexuses except for the thoracic region, where thoracic spinal nerves continue as the intercostal nerves.
• The plexuses formed are cervical, brachial, lumbar, and sacral plexuses.
• The various parts are named according to parts of a tree, starting from the roots.
• The roots are formed of the ventral rami of spinal nerves C5, C6, C7, C8, and T1.
• The roots of C5 and C6 unite to form the superior trunk.
• The root of C7 forms the middle trunk.
• The roots of C8 and T1 unite to form the inferior trunk.
• Each of the three trunks divides into an anterior and a posterior division.
• Posterior divisions of all trunks unite to form the posterior cord.
• Anterior divisions of the superior and middle trunks form the lateral cord.
• The anterior division of the inferior trunk forms the medial cord.
• Each cord gives off two terminal branches.
• The posterior cord divides into the axillary and radial nerves.
• The lateral cord divides into the musculocutaneous nerve and lateral branch of the median nerve.
• The medial cord divides into the ulnar nerve and medial branch of the median nerve.
• The brachial plexus is also divided into two parts according to its relation with the clavicle.
• The supraclavicular part lies in the neck, and its branches arise from the roots and trunks.
• The infraclavicular part lies in the axilla, and its branches arise from the cords.