Structure
Each space has three muscle layers, with the outer layer running at right angles to the two inner layers.
| Layer | Fibre direction | Extent |
|---|---|---|
| External intercostal | Downwards and forwards | From the tubercle of the rib to the costochondral junction; replaced in front by the external intercostal membrane |
| Internal intercostal | Downwards and backwards | From the sternum to the angle of the rib; replaced behind by the internal intercostal membrane |
| Innermost intercostal | Same as internal | Incomplete, mainly in the lateral part of the space |
Three muscles lie in the innermost plane:
- Innermost intercostals, laterally.
- Subcostales, posteriorly near the angles of the ribs, crossing one or two spaces.
- Transversus thoracis, anteriorly, from the back of the sternum to the costal cartilages, with the internal thoracic vessels in front of it.
Deep to all three are the endothoracic fascia and the parietal pleura.
The neurovascular bundle
The intercostal vein, artery and nerve run together in the costal groove on the inner lower border of each rib, between the internal and innermost layers. Near the vertebral column, where the innermost layer is absent, the bundle lies directly on the endothoracic fascia and pleura.
Arteries
Each space has one posterior and usually two small anterior intercostal arteries, which anastomose.
- Posterior intercostal arteries: those of the first two spaces come from the supreme intercostal artery, a branch of the costocervical trunk; the rest come from the thoracic aorta. Right-sided arteries cross the vertebral bodies behind the oesophagus, thoracic duct and azygos vein.
- Anterior intercostal arteries: from the internal thoracic artery in the upper six spaces and from its musculophrenic branch in the next three. The lowest spaces have none.
Veins
Posterior intercostal veins drain into the azygos vein on the right and the hemiazygos or accessory hemiazygos vein on the left. The upper spaces drain through the superior intercostal veins: the right joins the azygos, the left joins the left brachiocephalic vein.
Nerves
The intercostal nerves are the anterior rami of T1–T11; T12 is the subcostal nerve. Each gives a collateral branch, running along the upper border of the rib below, a lateral cutaneous branch midway round, and ends as an anterior cutaneous branch. Most of T1 joins the brachial plexus. The lateral cutaneous branch of T2 forms the intercostobrachial nerve to the axilla and medial arm. T7–T11 continue into the abdominal wall. Each nerve is linked to the sympathetic trunk by rami communicantes.
Relations
At the back of the thorax, the spaces lie beside the structures on the vertebral bodies. The sympathetic trunk runs down over the heads of the ribs, lateral to the vertebral bodies, giving the splanchnic nerves. Medial to it, the descending thoracic aorta lies to the left of the vertebral bodies and the azygos vein to the right, with the thoracic duct between them behind the oesophagus.
In front, the internal thoracic vessels run down about 1 cm lateral to the sternal border, behind the costal cartilages and in front of transversus thoracis.
Clinical relevance
Every procedure through the chest wall is planned around the bundle under the upper rib.
- Chest drain: inserted in the triangle of safety, bounded by the anterior border of latissimus dorsi, the lateral border of pectoralis major, a horizontal line at the level of the nipple (about the fifth intercostal space) and an apex below the axilla. Blunt dissection runs over the upper border of the rib below to avoid the main bundle.
- Intercostal nerve block: the needle is walked off the lower border of the rib, near the angle, to reach the nerve in the costal groove.
- Herpes zoster reactivates in a dorsal root ganglion and produces a dermatomal band of rash and pain around the chest.
- Coarctation of the aorta: enlarged posterior intercostal arteries carrying collateral flow erode the lower borders of the ribs, seen as rib notching.
On the specimen
This station views the posterior chest wall from inside with the pleura stripped, so the bundle can be followed back to the vertebral column.
- Intercostal vein, artery and nerve: in the costal groove the vein is highest, the artery in the middle and the nerve lowest. The nerve is white and solid, the artery pink with a lumen, the vein dark and flattened.
- Thoracic aorta: the large thick-walled vessel to the left of the vertebral bodies, giving posterior intercostal arteries.
- Azygos vein: the dark vein on the right of the vertebral bodies receiving the right posterior intercostal veins.
- Thoracic duct: a thin, pale, beaded vessel between aorta and azygos vein, easily mistaken for a nerve.
- Sympathetic trunk: a ganglionated cord lying on the heads of the ribs, lateral to the vessels.
- Subcostalis: thin muscle sheets near the angles of the ribs crossing more than one space.
- Rib: the bony bar; the bundle lies under its lower border.