Intercostal muscles

Structure

There are three principal layers:

1. External intercostal muscles also known as intercostalis externus aid in quiet and forced inhalation. They originate on ribs 1–11 and have their insertion on ribs 2–12. The external intercostals are responsible for the elevation of the ribs and bending them more open, thus expanding the transverse dimensions of the thoracic cavity. The muscle fibers are directed downwards, forwards and medially in the anterior part.
2. Internal intercostal muscles also known as intercostalis internus aid in forced expiration (quiet expiration is a passive process). They originate on ribs 2–12 and have their insertions on ribs 1–11.Their fibers pass anterior and superior from the upper margin of the rib and costal cartilage to the lower margin of the rib above. The internal intercostals are responsible for the depression of the ribs and bending them inward, thus decreasing the transverse dimensions of the thoracic cavity. The muscle fibers are directed downwards, forwards and laterally forming right angle with external intercostal muscle.
3. Innermost intercostal muscle also known as intercostalis intimus are deep layers of the internal intercostal muscles which are separated from them by a neurovascular bundle. The muscle fibers are directed downwards, forwards and laterally same as intercostalis internus muscle.

Nerve supply

Both the external and internal muscles are innervated by the intercostal nerves(the ventral rami of thoracic spinal nerves), are supplied by the intercostal arteries, and are drained by the intercostal veins. Their fibers run in opposite directions.