Serratus posterior inferior is a thin, irregular quadrilateral muscle deep to latissimus at the junction of the thoracic and lumbar regions.

Proximal Attachment
It arises by a thin aponeurosis, which blends with the lumbar part of the thoracolumbar fascia, from the spinous processes of T11 to L2/L3 and the intervening supraspinous ligaments.

Distal Attachment
It ascends supero-medially ending as four digitations, which attach to the inferior borders and external surfaces of the ninth to twelfth ribs, lateral to their angle.  

Nerve Supply
It is supplied by the ventral rami of T9 to T12.

Action
Serratus posterior inferior pulls the ribs downwards and backwards.