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AN21.1-11 | Thoracic cage — Part 3

Typical and Atypical Intercostal Nerves (AN21.6)

A typical intercostal nerve (nerves of spaces 3–6) is the ventral ramus of its corresponding thoracic spinal nerve. It travels along the intercostal space with the VAN bundle and has the following branches:

• Muscular branches — supply the three intercostal muscles
• Collateral branch — runs along the upper border of the rib below (unlike the main nerve which runs along the lower border of the rib above)
• Lateral cutaneous branch — pierces the intercostal muscles near the mid-axillary line, divides into anterior and posterior branches to supply skin
• Anterior cutaneous branch — emerges near the sternum to supply the skin of the anterior chest wall

Atypical intercostal nerves:

• 1st intercostal nerve — most of its fibres join the brachial plexus (T1). The remaining small 1st intercostal nerve has no lateral cutaneous branch and no anterior cutaneous branch.

• 2nd intercostal nerve — gives off the intercostobrachial nerve (the lateral cutaneous branch of T2), which crosses the axilla to supply the skin of the medial aspect of the upper arm. This is the nerve that causes referred pain from the heart to the inner arm during a heart attack.

• 7th–11th intercostal nerves — leave the intercostal space anteriorly and continue into the anterior abdominal wall between the internal oblique and transversus abdominis muscles. They are called thoraco-abdominal nerves because they supply both thoracic and abdominal wall.

• Subcostal nerve (T12) — runs below the 12th rib (not between two ribs, so technically not 'intercostal'). It enters the abdominal wall.

Dermatome landmarks: T4 = nipple, T6 = xiphoid, T10 = umbilicus. These are essential for testing sensory levels in spinal injuries.

Costovertebral and Costotransverse Joints (AN21.8)

Each typical rib articulates with the vertebral column at two joints. These joints are crucial because they determine how the ribs move during breathing.

1. Joint of the head of the rib (costovertebral joint)
The head of a typical rib has two facets that articulate with:
• The superior demifacet of its own vertebra
• The inferior demifacet of the vertebra above
• The intervertebral disc between them

A small ligament — the intra-articular ligament — divides this joint into two compartments. It's a synovial joint, but its movement is limited to a small gliding motion.

2. Joint of the tubercle of the rib (costotransverse joint)
The articular facet on the rib's tubercle articulates with the costal facet on the transverse process of the corresponding vertebra. This is also a synovial joint.

The axis of rib movement depends on both joints working together. The rib rotates around an axis passing through these two joints — and the orientation of this axis determines whether the rib swings forward (pump-handle) or outward (bucket-handle). We'll see how in the next section.

Special cases:
• The 1st rib has a single facet (only articulates with T1)
• Ribs 11 and 12 have no costotransverse joint (no tubercle, so no articulation with the transverse process) — they're more mobile but also less stable

Inferior costal facet of vertebra (Fovea costalis inferior vertebrae); Image: Begoña Rodriguez. Brave (kenhub.com). Used for educational purposes.

Mechanics of Respiration — Pump-Handle and Bucket-Handle (AN21.9)

Here's where the thoracic cage comes alive. Breathing is not just the lungs expanding — it's the entire cage changing shape. Two movements increase thoracic volume during inspiration:

1. Pump-handle movement (increases anteroposterior diameter)
Imagine a pump handle — it goes up, and the far end moves forward. This is what the upper ribs (2–6) do. The axis of rotation passes almost transversely (side-to-side) through the costovertebral and costotransverse joints. When the rib elevates, its anterior end and the sternum move forward and upward, increasing the front-to-back dimension of the chest.

Try it: Place one hand on your sternum and breathe in deeply. You'll feel your sternum move forward — that's the pump-handle movement.

2. Bucket-handle movement (increases transverse diameter)
Imagine a bucket handle — it goes up, and the middle moves outward. This is what the lower ribs (7–10) do. The axis of rotation passes anteroposteriorly (front-to-back) through the joints. When the rib elevates, its middle portion (the most lateral point of the rib curve) moves outward, increasing the side-to-side dimension of the chest.

Try it: Place your hands on your lower ribs and breathe in. You'll feel them flare outward — that's the bucket-handle movement.

Both movements happen simultaneously with every breath. The upper ribs primarily pump-handle (more AP expansion), the lower ribs primarily bucket-handle (more transverse expansion), and the middle ribs do a bit of both.

The diaphragm adds the third dimension — it contracts and descends, increasing the vertical dimension of the thorax. Together, the three movements expand the thorax in all three dimensions, creating negative pressure that draws air into the lungs.

Spiral forward: In Respiratory Physiology (PY6), you'll learn how these mechanical changes translate into pressure changes (Boyle's law) and airflow. The anatomy you learn here is the structural basis for everything in respiratory physiology.