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Evidence-based physiotherapy and rehabilitation resources
Editorial & review policyHuman Anatomy · The trunk
The chest is the only part of the skeleton that has to change shape about twenty thousand times a day. It must be stiff enough to protect the heart and lungs, and mobile enough to pump air without you ever thinking about it. This chapter is about how a cage of bone manages to breathe.
Air is not sucked into the lungs. It is pushed in by the atmosphere. Muscles enlarge the chest, the pressure inside falls, and air flows in until the two pressures match.
So breathing is really a question about volume. The cage grows in three directions at once: deeper from front to back, wider from side to side, and longer as the diaphragm descends. Everything else in this chapter serves those three movements.
| Part | What it is | What to remember |
|---|---|---|
| Twelve thoracic vertebrae | The back wall, with facets on the body and the transverse process | The only vertebrae that carry ribs. |
| Twelve pairs of ribs | Curved flat bones, each continued forwards by a bar of costal cartilage | The cartilage makes the cage springy. A rigid ring could not breathe. |
| Sternum, in three parts | Manubrium, body and xiphoid process | The xiphoid stays cartilaginous for many years. |
The opening at the top is small and rigid, so structures crossing from the neck are crowded there. The opening below is far larger and is closed by the diaphragm. That is the only reason chest and abdomen are separate cavities.
Figure 1 · The thoracic cage and a typical rib
Illustration to be added
Two panels. Panel one: the whole cage from the front, with the twelve thoracic vertebrae behind, all twelve pairs of ribs, the costal cartilages and the sternum in three parts. Shade true ribs one to seven, false ribs eight to ten and floating ribs eleven and twelve in three different tints so the groups read at a glance. Draw a horizontal marker across the sternal angle where the second costal cartilage meets the sternum, carried back to the disc between the fourth and fifth thoracic vertebrae, and outline the costal margin. Panel two: a typical right sixth rib from below and behind, so the whole inner surface shows, with the head and its two facets separated by a crest, the neck, the tubercle and its joint surface, the angle, the shaft and the costal groove inside the lower border. Add a small inset of the first rib from above, short and broad and nearly flat, with the tubercle for scalenus anterior and the grooves for the subclavian vein in front and the artery behind. Name every part listed above on a gold leader line, including the three rib groups, the three parts of the sternum, and each named part of the typical rib and the first rib. Bone warm ivory with navy outlines, cartilage pale blue, white ground, navy line work.
Run a finger down from the notch at the top of the sternum. A few centimetres down you meet a distinct transverse ridge, where the manubrium meets the body.
The second costal cartilage joins the sternum exactly there. The first rib hides under the clavicle, so the second is the first rib you can find with certainty. Every rib count starts at this ridge. Remember that a space takes its number from the rib above it.
| At the level of the sternal angle | Note |
|---|---|
| The second costal cartilage meets the sternum | Where rib counting begins. |
| The trachea divides into the two main bronchi | As far as a suction catheter reaches before it must pick a side. |
| The arch of the aorta begins and ends | Above this the great vessels crowd behind the manubrium. |
| The pulmonary trunk divides, and the ligamentum arteriosum lies here | The remnant of the fetal circulation. |
| The azygos vein arches forwards into the superior vena cava | It crosses the root of the right lung. |
| Upper and lower mediastinum are divided | The mediastinum is the block of tissue between the two lungs, holding the heart and the great vessels. The plane runs back to the disc between the fourth and fifth thoracic vertebrae. |
Ribs are grouped by what happens at the front, because that decides how freely each one moves.
| Group | Which | Front attachment |
|---|---|---|
| True | 1 to 7 | Each reaches the sternum by its own cartilage. |
| False | 8, 9 and 10 | Cartilage joins the one above, forming the costal margin you can feel. |
| Floating | 11 and 12 | None. The tips end in the muscle of the abdominal wall. |
Ribs three to nine share one pattern and are called typical.
| Part | What it is, and why it matters |
|---|---|
| Head | Two facets separated by a crest. It straddles two vertebrae, its own and the one above. |
| Neck | A short flat bar beyond the head, bound to the vertebra by ligaments. |
| Tubercle | A bump with a small joint surface for the transverse process. With the head, it sets the axis the rib turns about. |
| Angle | Where the shaft turns sharply forwards. The rib is mechanically weakest just in front of the angle, a common fracture site when the cage is compressed. |
| Shaft | Thin, flattened and slightly twisted, so it flexes under load. |
| Costal groove | A gutter inside the lower border, sheltering the vein, artery and nerve. |
The first rib is the odd one out: short, broad and nearly flat, with a tubercle for scalenus anterior and grooves for the subclavian vein in front of it and the subclavian artery behind. Those grooves say why the inlet is such a crowded place. The vessels and the lower trunk of the brachial plexus have to cross a small rigid ring, so anything that narrows it presses on them.
Ribs one, ten, eleven and twelve carry a single facet on the head, so each of them meets only its own vertebra. Ribs eleven and twelve have no tubercle joint either, as the next table shows. A head on one vertebra and no second fixing leaves them free at the back rather than hinged on a set axis, which is why their tips end in muscle and travel with the abdominal wall.
| Joint | Between | Note |
|---|---|---|
| Costovertebral | Head, and the bodies of two neighbouring vertebrae | Synovial. One of the two points that set the axis. Ribs one, ten, eleven and twelve have a single facet and meet only their own vertebra. |
| Costotransverse | Tubercle, and the transverse process of its own vertebra | Synovial, ribs one to ten. Ribs eleven and twelve have none, which is part of why they are so mobile. |
| Costochondral | Bony rib, and its own costal cartilage | Cartilaginous and immobile. A common site of chest wall pain. |
| Sternocostal | Cartilages one to seven, and the sternum | The first is fixed. The rest are small synovial joints. |
| Interchondral | Neighbouring cartilages at the costal margin | They let the margin flare outwards. |
A rib is fixed to the spine at two points, and a line through them is the axis it must turn about. It has no choice in the matter.
In the upper ribs that line runs almost side to side. Turn a rib about it and the front end has to rise and travel forwards, carrying the sternum with it. The chest deepens from front to back, like a water pump handle swinging up and away.
In the lower ribs the transverse processes are shorter and project less to the side, so the tubercle sits almost directly behind the head and the axis lies closer to a front-to-back line. The lower joint surfaces are flat as well, so they glide rather than spin. Now the front end barely rises. Instead the middle of the shaft swings out and up, and the chest widens, like a bucket handle.
Both happen together on every breath in, weighted differently at the top and bottom of the cage. And neither is a large movement in one rib. It is small, multiplied by twelve pairs.
Figure 2 · Why the upper and lower ribs move differently
There are eleven spaces on each side. Each holds three thin sheets of muscle, with the neurovascular bundle running between the middle sheet and the deepest one.
| Layer | Fibre direction | Job |
|---|---|---|
| External, outermost | Down and forwards, like a hand in a trouser pocket | Lifts the rib below on breathing in. Replaced by a membrane at the front. |
| Internal | Down and backwards, crossing the outer layer | Mainly used in forced breathing out. Replaced by a membrane at the back. |
| Innermost, deepest | Much like the internal layer | Incomplete. It forms the deep wall of the tunnel the bundle runs in. |
The bundle keeps a fixed order from above downwards: vein, artery, nerve. It lies in the costal groove under the rib above, sheltered by bone. A smaller collateral branch runs along the upper border of the rib below.
The practical consequence
If the bundle hugs the lower border of the rib above, anything entering the chest must pass close to the upper border of the rib below. That one sentence explains where every chest drain and every aspirating needle goes. Because of the collateral branch, aim for the middle of the space rather than scraping the bone.
The nerves are the anterior rami of the upper eleven thoracic spinal nerves. An anterior ramus is simply the forward branch each spinal nerve gives off once it has left the cord. The twelfth runs below the last rib as the subcostal nerve. The lower ones carry on into the abdominal wall, which is why chest pain can be felt in the abdomen.
Figure 3 · Inside an intercostal space
The diaphragm is a domed sheet of muscle separating chest from abdomen, and it does most of the work of breathing. Its fibres start round the lower edge of the cage, run upwards and inwards, and end in a flat tendon at the top of the dome. Shortening pulls that tendon down, and the chest gets longer.
| Origin | From, and why it matters |
|---|---|
| Sternal | Two small slips from the back of the xiphoid process. The smallest part. |
| Costal | The inner surfaces of the lower six costal cartilages and their ribs, interlocking with transversus abdominis, so chest wall and abdominal wall are continuous. |
| Lumbar | Two crura from the upper lumbar vertebrae, the right longer than the left, plus two arched thickenings of fascia. The medial arcuate ligament arches over psoas major and the lateral over quadratus lumborum, so both pass beneath uncompressed. |
All of it meets at the central tendon, a trefoil sheet — three lobed, like a clover leaf — with no bony attachment anywhere. The pericardium is fused to its upper surface, so the heart sits on the diaphragm and moves with every breath.
| Opening | Level | Contents, and where exactly |
|---|---|---|
| Caval | T8 | Inferior vena cava, with branches of the right phrenic nerve. In the central tendon, right of the midline. The vein is stuck to the tendon, so the opening widens on breathing in rather than closing. |
| Oesophageal | T10 | Oesophagus, the two vagal trunks and small gastric vessels. One trunk lies in front of the oesophagus and one behind, not one on each side. Set in the muscle of the right crus, whose fibres act as a sling. |
| Aortic | T12 | Aorta, thoracic duct and azygos vein. Behind the diaphragm rather than through it, arched over by the crura, so the aorta is never compressed. |
A way to hold the levels
Count the letters. Vena cava has eight and crosses at T8. Oesophagus has ten and crosses at T10. Aortic hiatus has twelve and crosses at T12. Even numbers, in order, running from the front of the body backwards.
Figure 4 · The diaphragm seen from below
Illustration to be added
One large panel with one inset. Main panel: the diaphragm from below, looking up at the underside of the dome with the abdominal contents removed. A pale trefoil central tendon in the middle, with brick muscle running out to three origins: two small sternal slips from the back of the xiphoid at the front; costal fibres from the inner surfaces of the lower six costal cartilages, interdigitating with transversus abdominis at the sides; and behind, the right and left crura descending onto the upper lumbar vertebral bodies, the right longer than the left. Draw the median arcuate ligament arching between the crura over the aorta, the medial arcuate ligament over psoas major and the lateral arcuate ligament over quadratus lumborum to the twelfth rib. Place the three openings correctly: the caval opening in the central tendon, forward and right of the midline, with the inferior vena cava; the oesophageal opening behind and left, in the muscle of the right crus, with the anterior vagal trunk on the front of the oesophagus and the posterior vagal trunk behind it, not one on each side. The anterior trunk is derived from the left vagus and the posterior from the right, so draw them front and back. Place the aortic opening furthest back in the midline behind the arch of the crura, with the aorta, the thoracic duct and the azygos vein. Inset: a side view of the trunk in section with level markers at the eighth, tenth and twelfth thoracic vertebrae aligned with the three openings, and the phrenic nerve descending on each side. Name the central tendon, the three origins, the three arcuate ligaments and the three openings on gold leader lines. Bone ivory, muscle brick, tendon pale ivory, nerves gold, artery red, vein blue, navy outlines, white ground.
The entire motor supply of the diaphragm is the phrenic nerve, and it comes from the neck. It is formed from the anterior rami of the third, fourth and fifth cervical nerves. The old line is that C3, 4 and 5 keep the diaphragm alive.
A cord injury above the third cervical segment takes out the diaphragm, and the patient cannot breathe unaided. An injury lower in the neck may spare enough supply for it to keep working when everything below is paralysed. The exact level is the difference between a ventilator and independent breathing.
Why the shoulder tip hurts
The phrenic nerve is sensory to the central diaphragm as well as motor. The skin over the point of the shoulder is supplied from the same segments, and the brain cannot tell the two sources apart. So blood, air or inflammation under the central diaphragm is felt at the shoulder tip on that side. The outer rim of the diaphragm is supplied by the lower intercostal nerves instead.
In quiet breathing in, the diaphragm shortens and the central tendon descends by roughly a centimetre and a half. That accounts for two thirds to three quarters of the air moved. The external intercostals stiffen the spaces and lift the ribs.
In quiet breathing out no expiratory muscle contracts. The stretched lung and chest wall simply recoil to their resting size. The diaphragm eases off gradually rather than switching straight to nothing, so it brakes the first part of the breath out, but nothing is pushing. It is the most useful single fact in the chapter, because it tells you what an abnormal breath looks like.
Figure 5 · Which muscles breathe, and when
As demand rises, breathing in gains the scalenes, sternocleidomastoid and pectoralis minor, which pull the upper ribs and sternum up. Bracing the arms on a table or on the knees holds the shoulder still. Pectoralis minor then pulls against the held arm instead of moving it, so its pull lands on the ribs and lifts them. That is why a breathless person leans forward on their hands.
Breathing out gains the abdominal wall. It raises the pressure inside the abdomen and drives the relaxed diaphragm upwards, while the internal intercostals pull the ribs down. This is the engine of a cough, and it is why a painful abdominal wound leads so quickly to retained secretions.
In long-standing obstructive disease the lungs empty poorly and stay over-inflated, so the cage sits near full inspiration and the diaphragm is pushed down and flattened.
A flat diaphragm is a weak diaphragm. Its fibres are already short and its line of pull has changed, so it can draw the lower ribs inwards instead of lifting them outwards.
The work therefore shifts upwards. Neck muscles are recruited at rest, the shoulders rise with each breath, and breathing out becomes an active job. All of it is the anatomy of this chapter under load, and all of it is visible from the end of the bed.
| Problem | Anatomy behind it | What you find |
|---|---|---|
| Rib fracture | A rib usually breaks where the blow lands. Under a squeeze it gives way just in front of the angle, where it is mechanically weakest | Sharp pain on breathing in and coughing, so breaths turn shallow. Secretions collect and part of the lung may close down. |
| Flail chest | Several neighbouring ribs broken in two places each, freeing a segment | That segment is drawn in on breathing in and pushed out on breathing out. The bruised lung beneath usually matters more. |
| Pneumothorax | The pleura is two thin films, one on the lung and one lining the chest wall, normally held together by a little fluid. Air between them uncouples chest wall from lung | Sudden pain and breathlessness, reduced expansion, quiet breath sounds. Under tension the mediastinum is pushed away. |
| Injury from a chest drain | The bundle in the costal groove, and the liver and spleen sitting high under the dome | Avoided by using the safe area in the armpit, above nipple level, and passing the tube close to the upper border of the lower rib. |
| Diaphragmatic paralysis | Phrenic nerve injury from surgery, tumour or a neck injury | On one side, a raised dome and breathlessness lying flat. On both sides, the abdomen draws inwards on breathing in when supine. |
| Hiccup | A sudden involuntary contraction of the diaphragm with the cords snapping shut | Harmless when brief. Persistent hiccup raises the question of something irritating the diaphragm or its nerve. |
| Shoulder-tip pain | Shared segmental supply of the central diaphragm and the skin over the shoulder | A shoulder that moves fully and painlessly while the pain persists. Look below the diaphragm, not at the joint. |
The first rib hides beneath it. Start at the sternal angle, which marks the second costal cartilage.
They happen together on every breath in. Upper and lower ribs simply contribute more of one than the other, because their axes point differently.
The bundle runs on the underside of the rib above. Enter close to the upper border of the rib below, aiming for the middle of the space.
It is driven by elastic recoil. No expiratory muscle contracts, so visible expiratory effort at rest means a patient in difficulty.
It passes behind it, arched over by the crura. That is exactly why the muscle can contract hard without squeezing it.
A diaphragm pushed down by over-inflated lungs is already short and pulls the wrong way. Its owner is working harder for less air.
Ten questions on this chapter. Tap one to see the answer and the reasoning.
Answer: (B) The first rib hides under the clavicle and cannot be counted, so the second is where you start.
Answer: (C) The caval opening lies at T8, far below.
Answer: (C) They reach the sternum only indirectly. Ribs eleven and twelve have no front attachment at all.
Answer: (B) Those are the two points fixing the rib to the spine, so they are the only axis it can turn about.
Answer: (B) Their axis lies closer to a front-to-back line, so the shaft swings out and the chest widens.
Answer: (C) Vein, artery, nerve. The bundle hugs the rib above, so a needle passes close to the rib below.
Answer: (B) T10, in the muscle of the right crus. Count the letters.
Answer: (B) The opening is behind the muscle rather than through it, so contraction cannot squeeze it.
Answer: (B) The third, fourth and fifth cervical segments. An injury above C3 abolishes it altogether.
Answer: (C) No expiratory muscle contracts, which is why expiratory effort at rest always needs explaining.
Everything on this page, in one screen
| Book | What it adds here |
|---|---|
| Anatomy and Human Movement Palastanga, Field and Soames |
The clearest account of rib movement and breathing mechanics. |
| BD Chaurasia's Human Anatomy, Volume 1: Upper Limb and
Thorax B D Chaurasia |
The thorax in regional detail, including the intercostal spaces. |
| Clinical Anatomy by Regions Snell |
The clinical consequences of chest wall injury. |
| Last's Anatomy: Regional and Applied Sinnatamby |
A careful description of the diaphragm and its openings. |
| Gray's Atlas of Anatomy Drake, Vogl and Mitchell |
Keep the thorax plates open alongside. |
Reviewed by the Physiotherapist India Team. · Human Anatomy contents