subcostals

Subcostals are a group of small muscles of the thoracic wall, located on the inner surface of the posterior lower rib cage. They originate from the inner aspect of a lower rib and insert on the inner surface of the rib two or three levels below. Each side typically contains four to five such slips, though exact numbers vary among individuals.

Anatomy and arrangement: The subcostal muscles run obliquely downward and medially, crossing one or more intercostal spaces. They are considered part of the deeper layer of thoracic wall musculature, lying internal to the internal intercostal muscles and functionally close to them.

Function: The subcostal muscles assist expiration by depressing the ribs, contributing to the reduction of thoracic volume during forced expiration. They may play a minor role in postural stabilization of the thorax.

Innervation and variation: Subcostal nerves (the twelfth thoracic nerve, T12) provide motor innervation to these muscles, with overlap from adjacent intercostal nerves in some individuals. The presence, size, and exact insertion points of subcostals are variable, and they may be absent on some sides or fused with neighboring muscles.

Clinical relevance: Subcostal anatomy is primarily of interest in thoracic surgery, regional anesthesia, and radiologic interpretation of the chest wall. Injury or regional anesthesia targeting the subcostal region can affect the lower thoracic wall or upper abdominal wall.