The serratus posterior inferior is one of the two muscles within the intermediate group of extrinsic back muscles, the other being the serratus posterior superior. It is slightly wider than the serratus posterior superior. These two thin muscles lie deep to the superficial extrinsic muscles.
Origin
- Spinous processes T11-L3
Insertion
- Lower border of ribs 9-12 (lateral to their angles)
Action
- Depresses ribs 9-12
Innervation
- Anterior rami of lower thoracic nerves: T9-T12
Blood Supply
- Intercostal arteries