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Thoracic wall and internal anatomy

Thoracic Wall Muscles

  • Lie in the same three morphological layers as the abdo wall but divided by presence of ribs
  • Innervated segmentally by anterior rami
  • Outer layer = External Intercostal muscles
  • Middle layer = Internal Intercostal muscles
  • Inner layer = Subcostals + Innermost Intercostals + Transversus Thoracis
  • Neurovascular plane = Between Inner and Middle layers

Outer Layer

  • Posterior Serratus muscles
    • Two small muscles that migrated posteriorly and lie on surface of Erector Spinae mass
    • Supplied by anterior rami
    • Each arises from 4 spinous processes – 2 in the thorax and 2 beyond it
    • Each insert into 4 ribs
  1. Serratus Posterior Superior
    • Origin: Spinous processes of lowest 2 cervical and upper 2 thoracic vertebrae
    • Insertion: Just lateral to angles of Ribs 2-5
  • Dorsal scapular nerve and vessels run down its posterior surface – between it and the Rhomboids
  • Elevates upper ribs (inspiration)
  1. Serratus Posterior Inferior
    • Origin: Spinous processes of lowest 2 thoracic and upper 2 lumbar vertebrae
    • Insertion: Just lateral to angles of Ribs 9-12
    • Depresses lower ribs (expiration)

  • External Intercostal muscles
    • Fibres pass obliquely downwards and forwards from the sharp lower border of the rib above to the smooth upper border of the rib below
    • Each muscle extends from the superior costotransverse ligament at the back of the intercostal space as far forwards as the costochondral junction
    • Here it is replaced by the Anterior Intercostal Membrane – extends to side of the sternum
    • Thus, between the bony ribs is muscle; between the costal cartilages is membrane

Middle Layer

  • Internal Intercostal muscles
    • Fibres run downwards and backwards from the costal groove to the upper border of the rib below
    • Each muscle, unlike the External Intercostals, extends as far forwards as the side of the sternum
    • Replaced posteriorly by the Posterior Intercostal Membrane – which extends from the angle of the rib to the superior costotransverse ligament at the posterior limit of the space
    • Muscles are NOT purely inspiratory

Inner Layer

  • Innermost Intercostals line the rib cage at the side; Subcostals at the back; Transversus Thoracis at the front
    • They cross more than 1 intercostal space
  • Subcostal Muscles
    • Attached to inner surface of ribs
    • Separated from posterior border of the Innermost Intercostals by a space
      • Intercostal nerves and vessels are in contact with parietal pleural across this space
  • Innermost Intercostal muscles
    • Attached to the inner surfaces of ribs on lateral part of thoracic wall
  • Transversus Thoracis
    • Origin: Posterior surface of the lower part of the sternum
    • Insertion: Digitations diverge on each side to the 2-6th costal cartilages

Intercostal Spaces

  • Vein, Artery and Nerve lie in that order from above downwards under cover of the downward projection of the lower border of the rib
  • Thus, needle or trocar for pleural drainage is inserted just above the rib that forms the lower boundary of the space
  • Collateral branches of nerve and vessels that run along the upper border of the ribs are small and can be ignored

Thoracic vascular anatomy

  • See notes

Intercostal Nerves

  • Mixed spinal nerve
    • ⇒ Emerges from Intervertebral Foramen and gives off its posterior ramus (NB: Thus, it is the anterior rami that becomes the remainder of the intercostal nerve)
    • ⇒ Passes around neurovascular plane (between Internal Intercostals and Transversus Thoracis group)
    • ⇒ Gives off a Collateral Branch and a Lateral Cutaneous Branch before it reaches costal angle
  • Anterior rami
    • Lateral Cutaneous Branch
      • Pierces intercostal muscles and overlying muscles of body wall along the Midaxillary line and divides into Anterior and Posterior Branches to supply skin over the space
      • Lateral Cutaneous Branch of the 2nd Intercostal Nerve: Crosses axilla to the medial side of the arm as the Intercostobrachial Nerve
        • NB: Anterior rami have some fibres of the first 3 thoracic nerves that goes to the upper limb
    • Continuation of Anterior Rami
      • Muscles of thoracic wall innervated segmentally by the anterior rami
      • Intercostal nerve ends as an Anterior Cutaneous Nerve
    • Lower 5 Intercostal Nerves (T8 – T12) (and their Collateral Branches) slope downwards behind costal margin into the neurovascular plane of the abdominal wall which they supply
      • NB: These nerves do NOT supply the parietal and visceral peritoneum
    • Anterior ramus of T1 ascends across the neck of the 1st Rib to enter the Brachial Plexus
      • Gives off small 1st Intercostal nerve before entering Brachial Plexus
    • Subcostal Nerve (12th Thoracic)
      • Although arises in thorax, quickly leaves by passing behind Lateral Arcuate Ligament of the Diaphragm into the abdomen, below the Subcostal Artery and Vein
  • Posterior Rami of the trunk:
    • Supplies all muscles of the Erector Spinae and Transversospinalis groups that lie deep to the thoracolumbar fascia
    • Supplies skin overlying

Lymph Drainage

  • Lymph vessels of intercostal spaces follow arteries
  • From the front: Vessels pass to Anterior Intercostal (Parasternal) Nodes that lie along the Internal Thoracic Artery
  • From the back: Drain to Posterior Intercostal Nodes

Sternal Angle of Louis Anatomy

  • At level of 2nd Costal Cartilages
  • Level of lower border of T4
  • Above this level up to the Thoracic Inlet = Superior Mediastinum
  • Below this level = Inferior Mediastinum – divided into 3 parts by the Fibrous Pericardium – Anterior, Middle and Posterior
  • Landmarks at this level:
    • Bifurcation of the Trachea
    • Concavity of the Arch of the Aorta
    • Just above the bifurcation of the Pulmonary Trunk
    • Azygos Vein enters the Superior Vena Cava
    • Thoracic Duct reaches the Left Side of the Oesophagus
    • Ligamentum Arteriosum with the Left Recurrent Laryngeal Nerve recurving below it
    • Tracheobronchial Lymph Nodes
    • Superficial and Deep Parts of the Cardiac Plexus

Thoracotomy Surgical Anatomy

Thoracotomy incision: postero-laterally extending from medial to the scapula posteriorly and curving around the lower edge of the scapula. Muscle layers divided/dissected during this approach.

Graph View

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