serratus posterior inferior (muscle)

ser·ra·tus pos·te·ri·or in·fe·ri·or (mus·cle)

[TA]
lower intermediate muscle of back; origin, with latissimus dorsi, from spinous processes of two lower thoracic and two upper lumbar vertebrae; insertion, into lower borders of last four ribs; action, draws lower ribs backward and downward; nerve supply, ninth to twelfth intercostal.