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AN23.1-6 | Mediastinum — Part 2

Thoracic Duct (AN23.4)

The thoracic duct is the largest lymphatic vessel in the body — it drains lymph from everything except the right thorax, right arm, right head, and right neck (those drain via the right lymphatic duct).

Origin: Cisterna chyli (at L1–L2), a sac-like dilated beginning at the abdominal aorta.

Course in thorax:
- Enters through the aortic hiatus at T12 — lying between the aorta (left) and azygos vein (right)
- Ascends on the right side in the posterior mediastinum
- At T5, crosses to the left behind the oesophagus
- Continues up into the superior mediastinum on the left side of the oesophagus
- Arches laterally (at C7 level) → drains into the left subclavian vein at its junction with the left internal jugular vein (left venous angle)

Why it matters clinically:
- Chylothorax — injury to the thoracic duct (surgery, trauma) causes milky chyle to fill the left pleural cavity (occasionally right if injury is below T5)
- Chyle has a triglyceride level > 110 mg/dL — this distinguishes it from transudates
- The duct crosses from right to left at T5 — a fact that determines which side a chylothorax appears on depending on the level of injury

Oesophagus in the Thorax (AN23.5)

The oesophagus (25 cm total) runs from the pharynx (C6) to the stomach (T10/cardia).

Thoracic course:
- Enters at the thoracic inlet, lies posterior to the trachea in the superior mediastinum
- In the posterior mediastinum, it lies anterior to the vertebral column
- Deviates: at T4 it passes to the left (posterior to the aortic arch); at T7–T8 it comes back to the midline
- Passes through the oesophageal hiatus in the diaphragm at T10 (with vagal trunks), 2 cm left of midline

Constrictions (important for oesophagoscopy and impacted food/tablets):
1. Pharyngo-oesophageal junction (C6) — 15 cm from incisors
2. Aortic arch crossing (T4) — 22.5 cm
3. Left main bronchus crossing (T5) — 27 cm
4. Oesophageal hiatus (T10) — 40 cm

Blood supply:
- Thoracic oesophagus: oesophageal branches of descending aorta; lower portion also from left gastric artery (portal territory)
- Venous: upper → azygos; lower → portal (left gastric vein) — explains oesophageal varices in portal hypertension

Nerve supply: Autonomic via the oesophageal plexus formed by the two vagus nerves wrapping around the oesophagus as it descends. Left vagus → anterior vagal trunk; right vagus → posterior vagal trunk (at hiatus).

Cross-ref PY: The lower oesophageal sphincter (LOS) tone is regulated by vagal (ACh) and enteric inputs. Reduced LOS tone → GORD. Achalasia = failure of LOS relaxation due to loss of inhibitory enteric neurons.

Vagus, Phrenic, and Sympathetic Chains in the Thorax (AN23.6)

Vagus Nerves (CN X) — parasympathetic to thoracic and abdominal viscera:

Right vagus:
- Gives right recurrent laryngeal nerve looping under the right subclavian artery (at the thoracic inlet)
- Runs posterior to the right lung hilum
- Forms the right side of the oesophageal plexus
- Exits as the posterior vagal trunk (right + some left fibres)

Left vagus:
- Crosses the aortic arch — here gives the left recurrent laryngeal nerve looping under the aortic arch at the ligamentum arteriosum
- Passes posterior to the left lung hilum
- Forms the anterior vagal trunk

Clinical: Left recurrent laryngeal palsy (hoarse voice) from tumours near the aortic arch — an important sign of lung cancer, aortic aneurysm, or lymphoma.

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Phrenic Nerves (C3, C4, C5):

Both phrenic nerves enter the thorax and descend on the lateral surface of the pericardium (anterior to the lung hilum — mnemonic: phrenic = anterior to hilum, vagus = posterior).
- Right phrenic — straighter course, pierces diaphragm through the central tendon (with IVC) at T8
- Left phrenic — crosses left margin of the heart, pierces muscular diaphragm separately

Clinical: Diaphragmatic paralysis from phrenic nerve injury (birth trauma, surgery, apical lung tumour) causes paradoxical movement on fluoroscopy.

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Thoracic Sympathetic Chain:

• 11–12 thoracic ganglia lateral to vertebral bodies, connected by the chain
• Rami communicantes to all intercostal nerves
• Splanchnic nerves (pre-ganglionic) arise from T5–T12:
• Greater splanchnic (T5–T9) → coeliac ganglion
• Lesser splanchnic (T10–T11) → superior mesenteric ganglion
• Least splanchnic (T12) → renal plexus
• Cross-ref PY: Referred visceral pain travels via sympathetic afferents. Cardiac pain is referred to T1–T5 dermatomes (left arm, jaw) — because the heart's afferents travel with sympathetic fibres at these levels.