Middle scalene: origin, insertion, action and innervation

The middle scalene is the largest and longest of the three scalene muscles. It runs from almost the whole cervical spine down to the first rib, lifting that rib during deep breathing and bending the neck to its own side.

Where it is

The middle scalene lies on the side of the neck, behind the anterior scalene and in front of the posterior scalene. Its upper part is hidden under the sternocleidomastoid, but its lower part forms part of the floor of the posterior triangle of the neck, the area between the sternocleidomastoid and the trapezius.

The gap between the anterior and middle scalenes is a key passage. The roots of the brachial plexus and the subclavian artery come out through it on their way to the arm. The middle scalene forms the back wall of that passage.

Two nerves go straight through the muscle instead of around it:

• The dorsal scapular nerve, heading for the rhomboids and the levator scapulae.
• The upper roots (C5 and C6) of the long thoracic nerve, heading for the serratus anterior.

Origin and insertion

• Origin: the posterior tubercles of the transverse processes of the cervical vertebrae from C2 to C7. Some people also have a slip from the atlas.
• Insertion: a broad, rough area on the upper surface of the first rib, behind the groove where the subclavian artery crosses the bone.

Because it starts so high and so far back on the spine, the middle scalene is a long, thick muscle, easily the bulkiest of the group.

What it does

1. Lifts the first rib: with the neck held still, it pulls the first rib upward. Along with the other scalenes, it helps expand the upper chest when you breathe in hard.
2. Bends the neck to the side: with the rib fixed and one side working, it tilts the cervical spine toward the shoulder on the same side.

The middle scalene sits too far back to help much with flexing the neck forward, unlike the anterior scalene.

Innervation and blood supply

The muscle receives small branches from the anterior rami of the cervical spinal nerves, roughly C3 to C8. Since it spans most of the cervical spine, it draws on more nerve roots than the other two scalenes.

Its blood comes from the ascending cervical artery and from branches of the transverse cervical artery, two small vessels of the lower neck. The ascending cervical artery comes from the inferior thyroid artery, and the transverse cervical artery usually comes from the thyrocervical trunk.

How to remember it

• Middle and biggest: think of the middle child who grew the tallest. The middle scalene has the longest origin, C2 to C7.
• Pierced by two nerves: "the middle scalene has two holes". The dorsal scapular nerve and the long thoracic nerve both pass through it.
• Back wall of the gap: anterior scalene in front, middle scalene behind, brachial plexus and subclavian artery in between.
• Posterior tubercles: the anterior scalene uses the front tubercles of the transverse processes; the middle scalene uses the back ones.

Find the transverse processes it grips on the bones of the spine page, then test yourself in the neck quiz.

Frequently asked questions

Which is the largest scalene muscle?

The middle scalene. It has the longest origin, from C2 to C7, and the thickest belly of the three.

What is the action of the middle scalene?

It lifts the first rib during forced breathing and bends the neck to the same side when one side contracts.

Which nerves pierce the middle scalene?

The dorsal scapular nerve and the upper roots of the long thoracic nerve pass through it.

Where does the middle scalene insert?

On the upper surface of the first rib, behind the groove for the subclavian artery.

What nerve supplies the middle scalene?

Branches from the anterior rami of the cervical spinal nerves, roughly C3 to C8.

Sources

• OpenStax, Anatomy and Physiology 2e, 11.3 Axial muscles of the head, neck and back
• OpenStax, Anatomy and Physiology 2e, 7.3 The vertebral column
• Z-Anatomy, the libre 3D atlas of anatomy