The structure indicated is the internal intercostal muscle.

The intercostal muscles are a set of three flat muscles which are located in each intercostal space. The three intercostal muscles, from superficial to deep are:

• External intercostal
• Internal intercostal
• Innermost intercostal

Origin: Lateral edge of costal groove of rib directly above

Insertion: Superiorly on rib directly below

Innervation: Intercostal nerves (T1 – T11)

Action: Forced expiration, movement of ribs inferiorly

The muscle fibres of the internal intercostal muscles pass obliquely in the opposite direction to the external intercostal muscles, whose fibres pass anteroinferiorly (when the thoracic wall is viewed in the lateral position).

Learn more about the muscles of the thoracic wall in this anatomy tutorial.