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The intercostal spaces, also known as interspaces, are the space between the ribs. There are 11 spaces on each side and they are numbered according to the rib which is the superior border of the space.
The intercostal spaces contain three layers of muscle: the external, internal and innermost layers with the neurovascular bundle found between the internal and innermost layers.
The external intercostal muscle is the outermost muscle of the intercostal space. This muscle has fibers which run in a downwards, forwards and medial direction from the inferior border of the superior rib to insert on the outer lip of the inferior rib.
The internal intercostal muscle is the middle layer of the intercostal space. This muscle has fibers which run in a downwards, backwards and lateral direction from the inferior border of the superior rib to insert on the superior border of the inferior rib.
The deepest layer of muscle in the intercostal space is made up of three muscles which are collectively known as the tranversus thoracis group. This group includes the subcostal muscles posteriorly, the innermost intercostals laterally, and the transversus thoracis / sternocostalis muscles anteriorly.
The neuromuscular bundles are found between the middle and innermost layers, protected by the costal groove of the superior rib of each intercostal space. They are ordered vein, artery, nerve from superior to inferior (mnemonic VAN). The collateral neurovascular bundle runs at the lower edge of the space, just above the inferior rib and the order is reversed, i.e. nerve, artery, vein from superior to inferior.
In the first and second intercostal spaces a large posterior artery arises from the superior intercostal artery (a branch from the costocervical artery). The superior thoracic artery (from the axillary artery) often contributes also.
The arteries in the remaining spaces branch directly from the descending thoracic aorta.
Each intercostal space is supplied segmentally by muscular branches of anterior rami of spinal nerves T1-T11. The branches of anterior rami of spinal nerves are:
- rami communicantes
- lateral cutaneous branch
- muscular branches
- parietal pleura branches
- parietal peritoneal sensory branches (from T7-T11 only)
- collateral branch
- terminal anterior cutaneous branch
The intercostal space is traversed during procedures such as pleural taps, and knowledge of the neurovascular bundle location is important to reduce the risk of hemorrhage.