Fig. The right scalenus medius and left scalenus posterior and splenius cervicis muscles, posterior view
The middle and largest of the three scalene muscles. It arises from the transverse processes of the first and second cervical vertebrae, and from the posterior tubercles of the third to seventh cervical vertebrae. The muscle fibres run downwards and laterally to attach to a rough impression on the upper suiface of the first rib behind the groove for the subclavian artery.
By the anterior primary rami of C3-C8 inclusive.
Working singly, scalenus medius produces strong lateral flexion of the neck to the same side. If its upper attachment is fixed, then this strong muscle is very effective in steadying or elevating the first rib during respiration.