Anatomy of the Pectoralis Major and Minor

Pectoral Region Overview

• The pectoral region is the soft tissue covering the anterior chest wall, connecting the upper limb to the axial skeleton.
• It comprises various structures including skin, superficial fascia (contains nerves, blood vessels, and breast tissue), pectoral fascia, pectoralis major, pectoralis minor, clavipectoral fascia, subclavius, and serratus anterior.

Pectoralis Major

• Origin: Clavicular head: medial 1/2 of the clavicle. Sternocostal head: upper 6 costal cartilages, front of lateral sternal border, and aponeurosis of external oblique.
• Insertion: Lateral lip of intertubercular groove by a bilaminar tendon. Clavicular head inserts into anterior lamina, sternocostal head into posterior lamina.
• Nerve Supply: Lateral and medial pectoral nerves.
• Action: Flexion, adduction, and medial rotation of the arm. Acting on the ribs, helps with deep inspiration. When the arm is fixed, it lifts the trunk upwards (pull-ups, climbing), and pulls the trunk forward during swimming.
• Clinical Note: The lower border of the pectoralis major forms the anterior axillary fold.

Pectoralis Minor

• Origin: Front of the 2nd, 3rd, 4th, and 5th ribs.
• Insertion: Medial border of the coracoid process.
• Nerve Supply: Medial pectoral nerve.
• Action: Protraction of the scapula, depression of the scapula, assists in deep inspiration

Subclavius

• Origin: Junction of the 1st rib and its costal cartilage.
• Insertion: Subclavian groove (inferior surface of middle 1/3 of clavicle).
• Nerve Supply: Nerve to subclavius.
• Action: Stabilizes the clavicle during movements at the shoulder joint.

Clavipectoral Fascia

• Strong sheet stretched between subclavius (above) and pectoralis minor (below).
• Structures piercing it include: cephalic vein, acromiothoracic artery, lateral pectoral nerve, and lymph vessels.

Serratus Anterior

• Origin: 8 upper ribs (1st and 2nd ribs).
• Insertion: Anterior (costal) surface of medial border of scapula. (1st and 2nd digitations > superior angle, 3rd digitation > medial border, 4th-8th digitations > inferior angle).
• Nerve Supply: Nerve to serratus anterior (long thoracic nerve).
• Action: Main protractor, active in pushing and punching movements, rotates scapula for abduction >90 degrees, raising the hand above the head, assists in deep inspiration.

Breast

• Definition: Modified sweat gland.
• Site: Superficial fascia of the pectoral region in both sexes.
• Shape: Conical or hemispherical, with apex (nipple and areola) and base.
• Lobes: 15-20 radiating from the nipple. Each lobe has acini (grape-like structures) which collect into lactiferous ducts.
• Ducts: 15-20 lactiferous ducts open separately on the nipple and dilate to form lactiferous sinuses before ending.
• Suspensory ligaments: 15-20 fibrous septa (ligaments of Cooper), separating lobes, giving the breast a smooth contour.
• Fat tissue: Fills the space between the lobes, contributing to contour and softness.
• Covering: Nipple is a conical projection; the areola is circular, pigmented; contains sebaceous glands; the color of the areola is pink in women who have not had children and becomes darker after pregnancy.

Blood Supply

• Arterial: Perforating branches of internal mammary (thoracic) artery, superior thoracic, acromiothoracic (pectoral branch), and lateral thoracic. Lateral branches of intercostal arteries.
• Venous: Internal thoracic vein, axillary vein, and posterior intercostal veins.

Lymph Drainage

• 75% of lymph from the breast drains into axillary lymph nodes.
• 20% drain into internal mammary lymph nodes.
• 5% drain into other lymph nodes.
• Subareolar, cutaneous, and submammary plexuses are involved

Clinical Anatomy (Breast Carcinoma)

• Invasion of lactiferous ducts: nipple retraction, blood discharge.
• Invasion of suspensory ligaments: skin dimpling.
• Invasion of subcutaneous lymphatics: peau d'orange.
• Invasion of retromammary space: breast fixation.

Poland Syndrome

• Underdeveloped pectoral muscles; typically present on the right side.

Muscles of the Back

• Trapezius:

  • Origin: Medial 1/3 of superior nuchal line, external occipital protuberance, ligamentum nuchae, spine of C7, and spines of T1-T12.
  • Insertion: Upper fibers: posterior border of lateral 1/3 of clavicle; Middle fibers: medial border of acromion; Lower fibers: upper lip of spine of scapula.
  • Nerve Supply: Spinal accessory (11th cranial nerve).
  • Action: Upper fibers: elevate scapula, lateral flexion of the neck; Middle fibers: retrat scapula; Lower fibers: depress scapula.

• Latissimus Dorsi:

  • Origin: Spines of T6-T12, thoracolumbar fascia, outer lip of the posterior 1/3 of the iliac crest, lower three ribs, and posterior surface of inferior angle of scapula.
  • Insertion: Floor of the intertubercular groove.
  • Nerve Supply: Thoracodorsal nerve.
  • Action: Extension, adduction, and medial rotation of the arm, draws the trunk upward in climbing.

• Levator Scapulae:

  • Origin: Transverse processes of C1-C4.
  • Insertion: Dorsal surface of the medial border of the scapula, from the superior angle to the spine.
  • Nerve Supply: Dorsal scapular nerve.
  • Action: Elevation of the scapula, fixation of the scapula.

• Rhomboideus Minor:

  • Origin: Lower part of ligamentum nuchae, C7, and T1 spines.
  • Insertion: Dorsal surface of the medial border of the scapula opposite the spine.
  • Nerve Supply: Dorsal scapular nerve.
  • Action: Retraction of the scapula.

• Rhomboideus Major:

  • Origin: T2-T5 spines.
  • Insertion: Dorsal surface of the medial border of the scapula from the spine to the inferior angle.
  • Nerve supply: Dorsal scapular nerve.
  • Action: Retraction of the scapula, fixation of the scapula.

Cutaneous Nerve Supply of the Back

• Cutaneous branches of the dorsal rami of T1-T12.

Male Breast Development

• Before puberty and in males, the breast is small due to minimal fat and glandular tissue, and does not extend beyond the areola.
• During puberty, breast size increases due to increased fat more than duct growth.
• In early pregnancy, breast size increases due to increase in the length and branching of ducts and development of alveoli.
• In late pregnancy and lactation, alveoli are distended with milk.