Scalenus medius muscle

At a glance

English

Scalenus medius muscle

Latin (TA2)

Musculus scalenus medius

System

Muscle

Region

Neck

Side

Paired, left and right

Inserts on

The vertebra c7

About the scalenus medius muscle

Scalenus medius sits in the lateral neck behind scalenus anterior and in front of scalenus posterior. Of the scalenes it is the largest and the longest. The group takes its name from a Greek word meaning uneven, because the muscles differ in length, and older books called them the lateral vertebral muscles.

Its fibres spring from the cervical transverse processes, on their posterior tubercles, usually at every level from the axis to C7; the atlas may add a slip. The muscle slopes downwards and slightly outwards beside the vertebral column and spreads into a wide attachment on the top of the first rib, just behind the groove where the subclavian artery crosses the bone.

Its most important relation is the gap in front of it. The roots and trunks of the plexus for the upper limb, with the subclavian artery, emerge between scalenus anterior and scalenus medius, through the space called the scalene hiatus or interscalene triangle. The dorsal scapular nerve and the upper roots of the long thoracic nerve run through the substance of scalenus medius itself on their way to the back and the chest wall.

Acting from above, the muscle raises the first rib, so it helps to enlarge the upper chest in deep or laboured breathing, along with sternocleidomastoid. Acting from below with the rib fixed, it tilts the cervical spine towards its own side. A tight or bulky scalenus medius, or an extra cervical rib, can narrow the hiatus and squeeze the nerves and artery, one form of thoracic outlet syndrome. The same gap is the target of the interscalene nerve block used for shoulder and arm surgery.

Attachments, action and supply

Origin

Cervical transverse processes, C2 to C7, at their posterior tubercles; sometimes a slip from the atlas.

Insertion

Top of the first rib, behind the groove for the subclavian artery.

Action

Raises the first rib in forced breathing; flexes the neck laterally towards its own side when the rib is fixed.

Nerve supply

Ventral rami of the cervical spinal nerves, roughly C3 to C8.

Blood supply

Ascending cervical branch of the inferior thyroid artery and branches of the transverse cervical artery.