The intermediate layer of the back muscles includes two paired muscles: serratus posterior superior and serratus posterior inferior. 

These muscles extend from the spine to the ribs and help move the rib cage during breathing.

Together, the serratus posterior superior and inferior form the intermediate layer of the extrinsic back muscles. They help with respiration:

• The superior muscle lifts the ribs during inhalation.

• The inferior muscle pulls the ribs downward during exhalation.

Because they connect the spine (vertebrae) to the ribs, these muscles are sometimes called spinocostal muscles.

Serratus Posterior Superior

The serratus posterior superior is a thin, rectangular muscle located deep to the rhomboids in the upper back. It elevates ribs 2–5, helping with inhalation.

• Origin: From the lower part of the nuchal ligament and the spinous processes of vertebrae C7–T3.

• Insertion: Attaches to the upper borders of ribs 2–5.

• Innervation: Supplied by the intercostal nerves (T2–T5).

Serratus Posterior Inferior

The serratus posterior inferior is a broader, stronger muscle found beneath the latissimus dorsi in the lower back. It depresses ribs 9–12, aiding exhalation.

• Origin: From the spinous processes of vertebrae T11–L2 (sometimes up to L3).

• Insertion: Attaches to the lower borders of ribs 9–12.

• Innervation: Supplied by the intercostal nerves (T9–T12) and the subcostal nerve.

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