Third layer of muscles of back and loin

Serratus dorsalis anterior

• This muscle is thin, quadrilateral, flat muscle situated under the rhomboideus thoracis , serratus thoracis and latissimus dorsi. It consists of aponeurotic and fleshy portions.
• Origin: Lumbo-dorsal fascia .
• Insertion: By three fleshy digitations to the anterior borders and lateral surface of the ribs from the fifth to the seventh.
• Action: Inspiratory muscle.
• Blood supply: 5^th -7^th intercostal arteries.
• Nerve supply: 5^th -7^th dorsal spinal nerves.

Serratus dorsalis posterior

• This is a thin, elongated, quadrilateral muscle but wider than the preceding one. It also consists of aponeurotic and fleshy portions.
• Origin: Lumbo-dorsal fascia.
• Insertion: Posterior borders and lateral surface of the last three ribs.
• Action: Expiratory muscle.
• Blood supply: 10^th -13^th intercostal arteries.
• Nerve supply: 10^th -13^th dorsal spinal nerves.