The transversus thoracis (sternocostalis) is a thin muscle on the inner surface of the anterior thoracic wall, continuous inferiorly with the transversus abdominis. Its fibres radiate laterally and superiorly from the sternum to the inner surfaces of the costal cartilages of ribs 2 to 6. It lies deep to the internal intercostal muscles and the internal thoracic vessels, which pass between the muscle and the overlying costal cartilages.
| Origin | Inner surface of the lower third of the sternum and xiphoid process |
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| Insertion | Inner surface of the costal cartilages of ribs 2 to 6 |
| Nerve Supply | Anterior rami of intercostal nerves T2-T6 |
| Blood Supply | Internal thoracic artery |
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It depresses the costal cartilages and is thought to assist forced expiration, such as during coughing and speaking.
The transversus thoracis is encountered during internal thoracic artery harvesting for coronary artery bypass grafting and during median sternotomy approaches. The internal thoracic (mammary) vessels run between the transversus thoracis and the costal cartilages, a surgical corridor important for harvest technique. Ossification of the transversus thoracis is occasionally an incidental radiological finding on CT. The muscle is a minor expiratory muscle with a minor functional role compared to the abdominal wall muscles.
Not directly palpable as it lies on the deep (posterior) surface of the sternum and costal cartilages.
The transversus thoracis fibres are divided with the sternum during median sternotomy and must be carefully reapproximated during sternal closure to restore anterior chest wall stability and prevent sternal wound complications.
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