Proximal Attachment
Scalenus posterior is the smallest and deepest scalene muscle, it arises from the posterior tubercles of the transverse processes of C4 to C6.

Distal Attachment
It passes infero-laterally to attach by a thin tendon to the outer surface of the second rib behind the tubercle and attachment of serratus anterior.  It may blend with scalenus medius.

Nerve Supply
It is supplied by the anterior rami of C5 to C7.

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