Origin: Anterior tubercle of transverse process of C6 or C7
Insertion: First rib - Suprapleural membrane (Sibson's fascia)
Artery: Ascending cervical artery, superficial cervical artery
Action: Elevates the first rib - Supports suprapleural membrane
Extra muscle occasionally present between the scalenus anterior and medius, and having the same action and innervation.