The internal intercostal muscles are the middle of the intercostal muscle groups within each intercostal space.
Within each space they arise from the inferior border of the rib that lies superiorly. The muscle fibres pass inferiorly and posteriorly to insert into the superior border of the rib that lies immediately inferiorly.
Each internal intercostal muscle is muscular from the angle of each respective rib posteriorly to the sternum anteriorly.
The internal intercostals are innervated by collateral branches of the first to eleventh intercostal nerves.
They act to:
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