Shoulder and axilla

Last updated: June 28, 2023

Summarytoggle arrow icon

The upper limb bones and shoulder girdle form the superior appendicular skeleton. Four joints (glenohumeral, sternoclavicular, acromioclavicular, and scapulothoracic) and two bones (the clavicle and scapula) form the shoulder girdle. The glenohumeral joint (i.e., shoulder joint) is one of the most flexible joints in the body and relies on a complex system of muscles, including the rotator cuff, to keep it in shape and enable mobility. Branches of the subclavian and axillary arteries supply the entire upper extremity. Veins of the upper extremity drain into the axillary vein, and lymphatics of the upper extremity drain into the axillary lymph node groups. The branches of the brachial plexus supply the motor and sensory innervation of the upper extremity. The axilla is a three-dimensional pyramid-shaped area at the junction of the arm and thorax, inferior to the shoulder joint, through which important vessels and nerves pass from the trunk to the arm.

For more information, see “Neurovasculature of the upper limbs,” “Upper arm and elbow,” and “Forearm, wrist, and hand.”

Shoulder girdletoggle arrow icon

Bones of the shoulder girdletoggle arrow icon

Clavicle [1][2]

Scapula [1][2]

Joints of the shoulder girdletoggle arrow icon

Sternoclavicular joint [1][2]

Acromioclavicular joint (AC joint) [1][2]

Scapulothoracic joint [1][2]

Shoulder joint (glenohumeral joint) [1][2]

The head of the humerus is approximately four times larger than the glenoid fossa. The relative size of the humerus allows for a greater range of motion but also increases joint instability.

Muscles of the shoulder girdletoggle arrow icon

Overview [2]

The muscles of the proximal upper limb can be distinguished according to their attachments:

  • Scapulohumeral muscles
  • Axioappendicular muscles

The movements of the scapula and glenohumeral joint are also supported by the muscles of the arm, such as the triceps and biceps brachii muscles (see “Upper arm and elbow”).

Scapulohumeral (intrinsic shoulder) muscles [1][2]

Scapulohumeral muscles





Rotator cuff Supraspinatus muscle
Infraspinatus muscle
Teres minor muscle

Subscapularis muscle

  • Upper and lower subscapular nerves
Deltoid muscle
Teres major muscle

roTISSerie:” Teres minor, Infraspinatus, Supraspinatus, and Subscapularis are the muscles of the rotator cuff.

Anterior axioappendicular muscles [1][2]

SALT: The Serratus Anterior muscle is innervated by the Long Thoracic nerve.

Posterior axioappendicular (extrinsic shoulder) muscles [1][2]

See “Superficial back muscles.”

The axillatoggle arrow icon

Overview [1][2]

The anterior axillary fold is formed from the pectoralis major muscle. The posterior axillary fold is formed from the latissimus dorsi and teres major muscles.

Axillary spaces [1][2]

Medial triangular space (upper triangular space)

Triangular interval (lower triangular space)

Quadrangular space

A simple trick for remembering the structures at the borders of the axillary spaces is to simulate them with the middle and index fingers of both hands. Form a peace sign with both hands and superimpose them at ∼ 90°. Three spaces are created. These represent the axillary spaces.

Vascular supply and lymphatic drainagetoggle arrow icon

Arterial supply [1][2]

Axillary artery [1][2]

Branches of the axillary artery
Origin Course Supplies
Superior thoracic artery
  • 1st part
Thoracoacromial artery (thoracoacromial trunk)
  • 2nd part (first branch)
  • Runs posterior to the pectoralis minor muscle before piercing the clavipectoral fascia
  • Divides into four branches:
    • Pectoral branch
    • Deltoid branch
    • Acromial branch
    • Clavicular branch
Lateral thoracic artery
  • 2nd part (second branch)
Anterior humeral circumflex artery
  • 3rd part
Posterior humeral circumflex artery
Subscapular artery

Scapular anastomosis [2]

Overview of scapular arteries
Artery Origin Location
Dorsal scapular artery
Suprascapular artery
Circumflex scapular artery
Subscapular artery
Intercostal arteries

Venous drainage [1][2]

Axillary vein

Lymphatic drainage [1][2]

Brachial plexustoggle arrow icon

  • Description: a network of nerves derived from the C5–T1 spinal nerves that give rise to the peripheral nerves of the upper limb and shoulder
  • Roots
  • Trunks
    • The roots form three trunks that are located in the neck.
      • C5 and C6 form the upper trunk of the brachial plexus: The suprascapular nerve (innervates the infraspinatus and supraspinatus muscles) and nerve to subclavius (innervates the subclavius muscle) originate here.
      • C7 forms the middle trunk of the brachial plexus.
      • C8 and T1 form the lower trunk of the brachial plexus.
  • Divisions: All three trunks separate into an anterior and posterior division that enters into the axillary fossa.
    • All three posterior divisions innervate the extensor compartment of the arm and forearm (i.e., the posterior compartment).
    • All three anterior divisions innervate the flexor compartment of the arm and forearm (i.e., the anterior compartment).
  • Cords
    • Only the cords and proximal portions of their branches lie in the axilla.
    • There are three cords, which are named according to their position in relation to the axillary artery.
      • The anterior divisions of upper and middle trunks form the lateral cord of the brachial plexus.
      • The anterior division of the lower trunks forms the medial cord of the brachial plexus.
      • The posterior divisions of all the trunks form the posterior cord of the brachial plexus.
  • Functional significance
The brachial plexus
Branches of the brachial plexus Spinal roots Motor innervation Sensory innervation
Lateral cord Musculocutaneous nerve
  • C5–C7
Lateral pectoral nerve
  • C5–C7
  • None
Lateral root of median nerve
  • C5–C7
  • Radial ⅔ of the palm
  • Palmar aspect of the radial 3 ½ fingers (i.e., the thumb, index finger, middle finger, and radial side of the ring finger)
  • Dorsal aspect of the distal phalanges of the radial 3 ½ fingers
  • Autonomous sensory zone: tip of the index finger
Medial cord Medial root of median nerve
  • C8–T1
Medial pectoral nerve
  • None
Medial cutaneous nerve of arm
  • None
  • Inferomedial aspect of the arm

Medial cutaneous nerve of forearm

  • None
Ulnar nerve
  • Ulnar ⅓ of the palm
  • Palmar and dorsal aspects of the ulnar 1 ½ fingers (i.e., the little finger and the ulnar side of the ring finger)
  • Autonomous sensory zone: tip of the little finger
Posterior cord Upper subscapular nerve
  • C5
  • None
Lower subscapular nerve
  • C6–C8
  • None
Thoracodorsal nerve
  • C6
  • None
Radial nerve
  • C5–T1

Axillary nerve

  • C5-6

Branches from the medial and lateral cords form the median nerve.

Rugby Teams Drink Cool Beer!” (Roots, Trunks, Divisions, Cords, Branches is the order of the brachial plexus divisions)


Referencestoggle arrow icon

  1. Standring S. Gray's Anatomy: The Anatomical Basis of Clinical Practice. Elsevier Health Sciences ; 2016
  2. Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. Lippincott Williams & Wilkins ; 2013

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