The subcostales are thin, flat muscles on the inner surface of the lower thoracic cage, running from the angle of one rib to skip one or two ribs and insert on the inner surface of a lower rib. They are variable in number and best developed in the lower thoracic region. They form the innermost layer of the thoracic wall alongside the transversus thoracis anteriorly and the innermost intercostal muscles laterally.
| Origin | Inner surfaces of the lower ribs near their angles |
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| Insertion | Inner surfaces of the second or third ribs below the rib of origin |
| Nerve Supply | Anterior rami of intercostal nerves (T7-T12) |
| Blood Supply | Posterior intercostal arteries |
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They are thought to depress the ribs and assist forced expiration, acting alongside the innermost intercostals.
The subcostales are rarely considered individually in clinical practice but form part of the innermost thoracic wall layer that must be traversed during thoracotomy. Their innervation by the lower intercostal nerves means that thoracotomy pain affects these muscles along with the external and internal intercostals. Understanding the three-layer structure of the thoracic wall (external, internal, innermost layers) is essential for proper intercostal nerve block technique, which targets the neurovascular bundle running in the groove between the internal and innermost intercostal muscle layers.
Not directly palpable due to their internal position on the deep surface of the thoracic cage.
Multiple rib fractures disrupt the continuity of all thoracic wall muscle layers including the subcostales, contributing to paradoxical chest wall movement in flail chest and reducing the mechanical efficiency of the respiratory pump.
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