Proximal Attachment
Scalenus medius is the largest and longest scalene muscle, it arises from the transverse processes of C1 and C2 and the anterior aspect of the posterior tubercles of the transverse processes of C3 to C7.

Distal Attachment
It inserts onto the upper surface of the first rib between the tubercle and groove for the subclavian artery.

Sternocleidomastoid lies antero-laterally, it is separated anteriorly from scalenus anterior by the subclavian artery and roots of the brachial plexus and levator scapulae and scalenus posterior lie postero-laterally.

Scalenus medius is pierced by the upper two roots of the nerve to serratus anterior and the dorsal scapular nerve, which appear on its lateral surface.

Nerve Supply
It is supplied by the anterior rami of C3 to C8.

Action
When fixed distally scalenus medius laterally flexes the neck; when fixed proximally it helps raise the first rib.