Middle scalene m

Arises from the transverse processes of the lower six cervical vertebrae and attaches to the first rib.  The roots of the brachial plexus and the subclavian a. and v. pass anterior to the middle scalene m.  When activated unilaterally its action is rotation of the neck.  When activated bilaterally the muscle may elevate the first rib, and so is considered one of the accessory muscles of inspiration.  Innervation is by cervical spinal nn.