Thorax
Introduced| English | Meaning |
|---|---|
| mediastinum | The central chest space between the lungs, holding the heart, great vessels and oesophagus. |
| phrenic nerve | The C3–C5 nerve that drives the diaphragm. |
| vagus nerve | Cranial nerve X, the main parasympathetic supply to the heart, lungs and gut. |
| recurrent laryngeal nerve | recurrent laryngeal nerve |
| cardiac tamponade | cardiac tamponade |
The engine room
- The thorax packs the heart, the great vessels, the airways and the oesophagus into one box, with nerves crossing between them.
- Nearly every chest emergency is read from this box.
The mediastinum
- The mediastinum 纵隔 is the space between the two lungs. The superior part lies above the heart; below, the anterior, middle and posterior parts surround it.
- The middle part holds the heart in its pericardium. The posterior part holds the descending aorta and the oesophagus.
Match each structure to its part of the mediastinum.
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The heart defines the middle part; the structures behind it are posterior, and those above it superior.
Two nerves, two routes
- The phrenic nerve 膈神经 (C3–5) passes in front of the lung root and drives the diaphragm.
- The vagus nerve 迷走神经 passes behind the lung root and carries parasympathetic fibres to the organs.
Which nerve passes in front of the lung root?
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The phrenic nerve (C3–5) runs in front of the root to reach the diaphragm; the vagus runs behind it.
Why the routes differ
- In the embryo, the phrenic nerve follows the pericardium and the vagus follows the gut tube. So in the adult the heart's nerve runs in front and the gut's nerve behind.
- A mass in front of the lung root irritates the phrenic nerve: pain at the shoulder tip (C4 skin). On the left, a mass under the aortic arch can catch the recurrent laryngeal nerve 喉返神经 and make the voice hoarse.
A tumour under the aortic arch on the left can make the voice hoarse.
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The left recurrent laryngeal nerve hooks under the aortic arch, so a mass there can press on it and weaken the vocal cord.
Worked example: three anchor points
- The trachea divides at the level of the sternal angle (T4/5). The aortic arch gives off, from right to left: brachiocephalic trunk, left common carotid, left subclavian.
- The apex beat lies in the left fifth intercostal space at the mid-clavicular line. These three facts anchor every chest examination.
Put the branches of the aortic arch in order, from right to left.
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The right side gets one trunk that splits later; the left side gets two separate branches.
In which left intercostal space is the normal apex beat felt, at the mid-clavicular line?
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The fifth intercostal space. A beat further out or lower suggests an enlarged heart.
Worked example: the tamponade
- (Fictional trauma) Low blood pressure, swollen neck veins and quiet heart sounds. The pericardium is a tough, closed bag; 100–200 mL of blood inside it squeezes the heart so it cannot fill.
- This is cardiac tamponade 心脏压塞. The anatomy (a stiff bag) explains the physiology and the procedure: a needle below the xiphoid, aimed at the left shoulder.
Why can 150 mL of blood in the pericardium be deadly?
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A closed, stiff bag means a small volume gives a large pressure, and the thin-walled filling chambers collapse first.
Three common slips
- The heart in the midline. About two-thirds of it lies left of the midline.
- Swapping phrenic and vagus at the lung root. Phrenic in front, vagus behind.
- Forgetting the sympathetic chain. It runs down over the necks of the ribs, at the back of the chest.
Recap
- Middle mediastinum = heart; posterior = aorta and oesophagus.
- Phrenic (C3–5) in front of the lung root, vagus behind.
- Anchors: tracheal split at T4/5, arch branches right to left, apex beat in the left fifth space. A closed pericardium makes small volumes deadly.