The scalene muscles are a group of three pairs of muscles, namely the scalenus anterior, scalenus medius, and scalenus posterior. These muscles are located in the lateral neck. They are innervated by the fourth, fifth, and sixth cervical spinal nerves (C4-C6). Scalenus anterior inserts onto the scalene tubercle on the inner border of the first rib, and to a ridge on the upper surface of the rib, anterior to the groove for the subclavian artery. Scalenus anterior is supplied by the inferior thyroid artery, a branch of the thyrocervical trunk. Scalenus medius originates from the transverse process of the axis and the front of the posterior tubercles of the transverse processes of the lower five cervical vertebrae and it inserts onto the upper surface of the first rib, between the tubercle and the groove for the subclavian artery. Scalenus posterior inserts onto the outer surface of the second rib, behind the tubercle for serratus anterior. Scalenus medius and posterior are supplied by the ascending cervical artery. The action of the anterior and middle scalene muscles is to elevate the first rib and laterally flex (bend) the neck to the same side; the action of the posterior scalene is to elevate the second rib and tilt the neck to the same side. They also act as accessory muscles of inspiration, along with the sternocleidomastoids. The scalene muscles have an important relationship to other structures in the neck. The brachial plexus and subclavian artery pass between the anterior and middle scalenes. The subclavian vein and phrenic nerve pass anteriorly to the anterior scalene as the muscle crosses over the first rib. The phrenic nerve is oriented vertically as it passes in front of the anterior scalene, while the subclavian vein is oriented horizontally as it passes in front of the anterior scalene muscle. The passing of the brachial plexus and the subclavian artery through the space of the anterior and middle scalene muscles constitute the scalene hiatus (the term "scalene fissure" is also used). The region in which this lies is referred to as the scaleotracheal fossa. It is bound by the clavicle inferior anteriorly, the trachea medially, posteriorly by the trapezius, and anteriorly by the platysma muscle.