Intercostal Spaces and Thoracic Wall

The space between adjacent ribs, numbered according to the rib above it (e.g., the 5th intercostal space lies between ribs 5 and 6).

• Contains three muscle layers (external, internal, innermost intercostals), the intercostal neurovascular bundle, and (in the lower spaces) contributes to the anterolateral abdominal wall's muscular continuity.

• Fibre direction: Run obliquely, downward and forward (from posterosuperior to anteroinferior), direction mnemonic: hands in pockets.
• Extent: From the tubercle of the rib posteriorly to the costochondral junction anteriorly, where the muscle is replaced by the external intercostal membrane continuing to the sternum.
• Action: Elevate the ribs, assisting inspiration (increase thoracic volume).
• Innervation: Corresponding intercostal nerve.

• Fibre direction: Run obliquely, downward and backward (from anterosuperior to posteroinferior), perpendicular to the external layer, deep to it.
• Extent: From the sternum anteriorly to the angle of the rib posteriorly, where the muscle is replaced by the internal intercostal membrane continuing to the vertebral column.
• Action: Traditionally described as depressing the ribs, assisting forced expiration: though the interosseous (lateral) part's precise mechanical role is debated, with some fibres also contributing to stabilisation during inspiration; the standard teaching remains expiratory action for exam purposes.
• Innervation: Corresponding intercostal nerve.

• Fibre direction: Same as internal intercostals (parallel, deep to them).
• Extent: Present only in the middle portion of the intercostal space (do not extend the full length like the external/internal layers); separated from the internal intercostals by the intercostal neurovascular bundle, this is the key anatomical significance of this layer, as the neurovascular bundle runs between the internal and innermost intercostal muscles.
• Action: Assist the internal intercostals (expiration).
• Innervation: Corresponding intercostal nerve.

• Variable, thin muscle slips on the internal surface of the lower rib shafts (posterior thoracic wall), running from the internal surface of one rib to the internal surface of a rib two or three levels below, crossing one or two intercostal spaces.
• Same fibre direction and plane as the innermost intercostals (essentially a posterior extension/continuation of that layer).
• Action: Assist expiration (depress ribs).
• Innervation: Corresponding intercostal nerves.

• Located on the internal surface of the anterior chest wall, deep to the sternum and costal cartilages.
• Origin: Posterior surface of the lower sternum and xiphoid process.
• Insertion: Internal surfaces of costal cartilages 2–6.
• Action: Weak depressor of the costal cartilages/ribs, assisting forced expiration.
• Innervation: Corresponding intercostal nerves.
• Anatomically and functionally continuous with the same fascial plane as the innermost intercostals and subcostales (all in the same deepest muscular layer of the thoracic wall), relevant to understanding the layered plane through which the internal thoracic (mammary) artery runs just superficial to it, close to the sternal margin (at risk during median sternotomy or parasternal procedures).

• Runs within the costal groove, on the internal surface of each rib's inferior border, between the internal and innermost intercostal muscle layers.
• Order from superior to inferior within the groove: Vein, Artery, Nerve (mnemonic VAN).
• Position within the costal groove: Protected along most of the rib's length by the overlying bone, but the neurovascular bundle becomes progressively less protected (more exposed, closer to the inferior rib margin without full bony cover) as it runs anteriorly, and is essentially unprotected/exposed anterior to the mid-axillary line and in the region just below each rib's inferior border generally, the key anatomical fact underlying safe needle/drain placement technique (insert just above the lower rib's superior border, to avoid the neurovascular bundle riding along the upper rib's inferior margin).

• Anterior (ventral) rami of the thoracic spinal nerves T1–T11 (the anterior ramus of T12 becomes the subcostal nerve, running below the 12th rib).
• T1 contributes mainly to the brachial plexus, with only a small intercostal component remaining for the 1st intercostal space.
• Anterior rami: Run within the corresponding intercostal space, giving off branches; lower intercostal nerves (7–11) continue beyond the costal margin into the anterolateral abdominal wall, supplying the abdominal wall muscles and skin (thoraco-abdominal nerves).
• Lateral cutaneous branches: Arise roughly at the mid-axillary line, pierce the intercostal muscles, divide into anterior and posterior branches supplying the skin of the lateral thoracic/abdominal wall.
• Anterior cutaneous branches: Terminal continuation near the sternal border (anteriorly) or midline (for the lower thoraco-abdominal nerves), piercing the muscle layers to supply skin near the midline.
• Provide the characteristic segmental (dermatomal) sensory pattern of the thoracic and upper abdominal wall, a key landmark being the T10 dermatome at the umbilicus, useful for localising spinal cord/nerve root levels clinically.

Posterior intercostal arteries:

• Spaces 1–2: from the superior intercostal artery (branch of the costocervical trunk, itself a branch of the subclavian artery).
• Spaces 3–11 (and the subcostal artery): from the posterior intercostal arteries, direct branches of the descending thoracic aorta.
• Run with the corresponding intercostal nerve and vein in the costal groove.

Anterior intercostal arteries:

• Upper 6 spaces: from the internal thoracic (internal mammary) artery (a branch of the subclavian artery), running just lateral to the sternum, deep to the costal cartilages.
• Lower spaces (7–9): from the musculophrenic artery, a terminal branch of the internal thoracic artery.
• Anastomose with the posterior intercostal arteries within each intercostal space, providing a collateral pathway between the subclavian and aortic arterial systems.

Intercostal veins:

• Generally accompany the arteries; posterior intercostal veins drain into the azygos system (azygos vein on the right, hemiazygos/accessory hemiazygos on the left; see Great Vessels section); the 1st posterior intercostal vein typically drains directly into the brachiocephalic vein, and the 2nd/3rd may form the superior intercostal vein.
• Anterior intercostal veins drain into the internal thoracic veins, which drain into the brachiocephalic veins.

Chest drain insertion:

• Standard site: the "triangle of safety": bounded by the lateral border of pectoralis major (anteriorly), the lateral border of latissimus dorsi (posteriorly), and a horizontal line at the level of the nipple/5th intercostal space (base), with the apex at the axilla.
• Within this triangle, the drain is inserted just above the rib below (i.e., along the superior border of the lower rib of the chosen space) to avoid the neurovascular bundle running in the costal groove of the rib above.
• Avoids injury to the underlying lung, and to the long thoracic nerve and thoracodorsal neurovascular bundle at the triangle's margins.

Thoracentesis:

• Performed for pleural fluid drainage (diagnostic or therapeutic), typically at a level below the fluid meniscus (identified by percussion/ultrasound), usually in the mid-scapular or posterior axillary line.
• Needle inserted just above the superior border of the rib below the chosen intercostal space, for the same reason as chest drain placement, avoiding the neurovascular bundle.

Intercostal nerve block:

• Local anaesthetic infiltration near the intercostal nerve, typically at the angle of the rib posteriorly (where the nerve lies most predictably within the costal groove, before it has given off its lateral cutaneous branch), used for post-thoracotomy or rib fracture pain management.
• Requires awareness of the VAN order and the risk of intravascular injection or pneumothorax if the needle is placed too deep.