Axilla and Brachial Plexus: Laboratory Identifications and Relationships

Axilla

1. Boundaries of Axilla
   • Superior boundary - first rib, clavicle, upper border of scapula
   • Inferior boundary - skin of armpit
   • Anterior border - pectoralis major (pectoralis minor)
   • Posterior border - teres major, latissimus dorsi, subscapularis
   • Medial boundary - thoracic wall (serratus anterior)
   • Lateral boundary - intertubercular sulcus (humerus)
2. Cephalic vein
   • entering the axilla by way of the clavipectoral fascia
3. Axillary sheath
   • Surrounding the brachial plexus and axillary artery (axillary vein?)
   • Invagination of the prevertebral fascia
4. Axillary artery
   • From first rib to inferior border of teres major, divided into 3 parts by anterior relationship of pectoralis minor
5. Six branches of the axillary artery
   1. Superior thoracic artery
      • First and second intercostal spaces
   2. Thoracoacromial artery
      • Branches: acromial, deltoid, pectoral, clavicular (acromial and deltoid branches are key players in shoulder anastomosis)
   3. Lateral thoracic artery
      • Usually off axillary artery (can arise from subscapular or thoracodorsal arteries, runs along thoracic wall about 1-2 inches anterior to long thoracic nerve
   4. Subscapular artery
      • Terminates as the circumflex scapular (scapular anastomosis) and thoracodorsal arteries
   5. Posterior humeral circumflex artery
      • Passes through quadrangular space
      • Can arise from subscapular artery
      • Shoulder anastomosis (ascending branch of deep brachial artery)
   6. Anterior humeral circumflex artery
      • Passes anterior surface of surgical neck of humerus
      • Shoulder anastomosis
      • May arise from common trunk with posterior humeral circumflex artery

Brachial Plexus

1. Cords of the brachial plexus (see the "M" (lonely posterior cord is left out))
   • Lateral cord (3)
     1. Lateral pectoral nerve
     2. Lateral root median nerve
     3. Musculocutaneous nerve
   • Medial cord (3) (This count of 3 does not include the two purely sensory nerves - medial brachial and antebrachial cutaneous nerves)
     1. Medial pectoral nerve
     2. Medial root median nerve
     3. Ulnar nerve
   • Posterior cord (5)
     1. Upper subscapular nerve (subscapularis)
     2. Middle subscapular (thoracodorsal nerve Latissimus dorsi)
     3. Lower subscapular (subscapularis and teres major)
     4. Axillary nerve (deltoid, teres minor)
     5. Radial nerve
2. Axillary nerve
   • Quadrangular space
3. Radial nerve
   • Triangular interval
4. Long thoracic nerve (nerve to serratus anterior (of Bell))
   • Vertical descent on thoracic wall about 1.5" posterior to lateral thoracic artery

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