Space between adjacent ribs + costal cartilages. 11 intercostal spaces per side, numbered by rib forming upper boundary.

”Typical” Spaces

• 3rd–6th spaces = typical — blood/nerve supply confined entirely to thoracic wall
• 1st + 2nd atypical — 1st nerve small, lacks lateral/anterior cutaneous branches; 2nd gives intercostobrachial nerve (→ medial arm)
• 7th–11th atypical — thoraco-abdominal nerves leave thorax → anterior abdominal wall

Boundaries

Contents

1. Three Intercostal Muscles

• External intercostal (outermost)
• Internal intercostal (middle)
• Innermost intercostal (innermost; deep aspect of ribs)

2. Intercostal Nerve

• Anterior primary ramus T3–T6
• Course: behind sympathetic trunk → between pleura + posterior intercostal membrane → costal groove → neurovascular plane
• Branches: rami communicantes, muscular, collateral, lateral cutaneous, anterior cutaneous

3. Intercostal Arteries

Posterior intercostal artery supplies greater part of space; anastomoses anteriorly at costochondral junction

4. Intercostal Veins

• Accompany arteries; drain posteriorly to azygos/hemiazygos, anteriorly to internal thoracic veins

5. Intercostal Lymphatics

• Posterior intercostal nodes near rib heads

Neurovascular Bundle

• Plane: between internal intercostal + innermost intercostal muscles
• VAN order (above downwards): Vein, Artery, Nerve

Clinical: safest needle insertion = just above upper border of rib below (avoids VAN bundle on lower border of rib above)

Introduction

Intercostal Space

The space between two adjacent ribs (and their costal cartilages) is called an intercostal space. Since there are 12 pairs of ribs, there are 11 intercostal spaces on each side, numbered according to the rib forming their upper boundary (e.g., the 1st intercostal space lies between the 1st and 2nd ribs).

”Typical” Intercostal Space

The 3rd to 6th intercostal spaces are called typical intercostal spaces because the blood supply and nerve supply of these spaces are confined entirely to the thoracic wall — they do not extend beyond the thorax to supply other regions.

• 1st and 2nd spaces are atypical — the 1st intercostal nerve is small and lacks lateral and anterior cutaneous branches; the 2nd gives rise to the intercostobrachial nerve, which leaves the thorax to supply the medial arm.
• 7th to 11th spaces are atypical — their nerves (thoraco-abdominal nerves) leave the thorax to supply the anterior abdominal wall.

The 3rd–6th spaces, being free of such extra-thoracic distribution, are used as the model (“typical”) for describing the general plan of an intercostal space.

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Boundaries

The space is essentially a fascio-muscular interval bridging the gap between consecutive ribs, occupied by three layers of intercostal muscle and the neurovascular bundle.

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Contents

Each typical intercostal space contains the following structures:

1. Three Intercostal Muscles

2. Intercostal Nerve

The anterior primary ramus of the corresponding thoracic spinal nerve (T3–T6 for the typical spaces).

Course: Turns laterally behind the sympathetic trunk → enters the intercostal space between the parietal pleura and the posterior intercostal membrane → enters the costal groove of the corresponding rib → runs laterally and forwards in the neurovascular plane.

Branches:

• Rami communicantes — to the corresponding sympathetic ganglion (white and grey)
• Muscular branches — to intercostal muscles, serratus posterior superior
• Collateral branch — arises near the angle of the rib; runs along the upper border of the rib below
• Lateral cutaneous branch — arises near the angle of the rib; pierces the lateral thoracic wall in the midaxillary line; divides into anterior and posterior branches for the skin of the lateral chest wall
• Anterior cutaneous branch — terminal branch; emerges beside the sternum; divides into medial and lateral branches for the skin of the anterior chest wall

3. Intercostal Arteries

The posterior intercostal artery supplies the greater part of the space and anastomoses anteriorly with the anterior intercostal artery at the level of the costochondral junction.

4. Intercostal Veins

Accompany the corresponding arteries; drain posteriorly into the azygos/hemiazygos system and anteriorly into the internal thoracic veins.

5. Intercostal Lymph Vessels and Lymph Nodes

Drain the thoracic wall; posterior intercostal nodes lie near the heads of the ribs.

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Plane and Arrangement of the Neurovascular Bundle

The intercostal nerve and vessels (the neurovascular bundle) lie in the plane between the internal intercostal and innermost intercostal muscles.

Their order from above downwards in the costal groove:

Mnemonic: VAN (Vein, Artery, Nerve — from above downwards)

This arrangement is clinically significant: a needle inserted into the intercostal space is safest when passed just above the upper border of the rib below, avoiding the neurovascular bundle that runs along the lower border of the rib above.

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