Scalene muscles

The scalene muscles (from Greek σκαληνός, or skalenos, meaning uneven as the pairs are all of differing length ) are a group of three pairs of muscles in the lateral neck, namely the scalenus anterior, scalenus medius, and scalenus posterior. They are innervated by the fourth, fifth, and sixth cervical spinal nerves (C4-C6).

A fourth muscle, the scalenus minimus (Sibson's muscle), is sometimes present behind the lower portion of the scalenus anterior.

Origin and insertion
They originate from the transverse processes from the cervical vertebrae of C2 to C7 and insert onto the first and second ribs. Thus they are called the lateral vertebral muscles.

Function
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.

Relations
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.

Clinical relevance
Since the nerves of the brachial plexus pass through the space between the anterior and middle scalene muscles, that area is sometimes targeted with the administration of regional anesthesia by physicians. The nerve block, called an interscalene block, may be performed prior to arm or shoulder surgery.