Subcostal muscles

Origin and insertion

Subcostal muscles originate from the inner surface of one rib near its posterior angle. Their fibers descend inferomedially in the same fashion as those of the innermost and internal intercostals. Each subcostal muscle inserts to the inner surface of the second or third rib below its origin. In this way, subcostal muscles cross over two or three intercostal spaces, unlike the intercostal muscles that fill in only one intercostal space.

Subcostal muscles are the most developed in the lower part of the thoracic cage and they sometimes fuse with the innermost intercostals in their origin. Subcostals found in the upper parts of the chest are rudimentary and sometimes even do not exist.

Relations

Subcostal muscles are found laterally to the thoracic vertebrae. Lateral borders of subcostal muscles are in contact with the innermost intercostal muscles and may sometimes fuse with them. Their posterior surfaces lie over the inner surface of the ribs and the internal intercostal membranes. The anterior surfaces of these muscles are in relation with the endothoracic fascia and the parietal pleura.

Innervation

Subcostal muscles are innervated by the intercostal nerves (anterior rami of thoracic spinal nerves).

Blood supply

These muscles are supplied by the arteries of the trunk wall namely the muscular branches of the posterior intercostal and musculophrenic arteries. They are drained by the posterior intercostal veins that empty into the superior vena cava via the azygos and the brachiocephalic veins.

Function

Subcostal muscles belong to the accessory respiratory musculature, as their function is to depress the ribs during forced exhalation. This action enables them to pull the ribs towards the central axis of thorax, which then compresses the lungs pushing the air out of them.

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