Overview
The scalene muscles are three paired muscles (anterior, middle and posterior), located in the deep lateral aspect of the neck and form part of the floor of the posterior triangle of the neck. They act as accessory muscles of respiration, and perform flexion at the neck.
Muscles
The deep fascia or prevertebral fascia envelop the scalene muscles
Anterior Scalene
The anterior scalene muscle lies on the lateral aspect of the neck, deep to the prominent sternocleidomastoid muscle. The muscle is thickeer at the level of the cricoid cartilage (at level of C6) and thinner at its attachments.
- Origin: Anterior tubercles of the transverse processes of C3-C6
- Insertion: Scalene tubercle, on the inner border of the first rib.
- Function: Elevation of the first rib. Ipsilateral contraction causes ipsilateral lateral flexion of the neck, and bilateral contraction causes anterior flexion of the neck.
- Innervation: Anterior rami of C5-C6.
Middle Scalene
The middle scalene is the largest and longest of the three scalene muscles. It has several long, thin muscles bellies arising from the cervical spine, which converge into one large belly that inserts into the first rib.
- Origin: Posterior tubercles of the transverse processes of C2-C7
- Insertion: Scalene tubercle of the first rib.
- Function: Elevation of the first rib. Ipsilateral contraction causes ipsilateral lateral flexion of the neck.
- Innervation: Anterior rami of C3-C8.
Posterior Scalene
The posterior scalene is the smallest and deepest of the scalene muscles.
- Origin: Posterior tubercles of the transverse processes of C5-C7
- Insertion: Outer surface of the second rib.
- Function: Elevation of the second rib, and ipsilateral lateral flexion of the neck.
- Innervation: Anterior rami of C6-C8.
Arterial Supply
- Inferior thyroid artery (branch of the thyrocervical trunk of the first part (pre-scalene) of the subclavian artery) – supplies the anterior scalene muscle
- Ascending (superficial) cervical artery (terminal branch of the thyrocervical trunk of the first part (pre-scalene) of the subclavian artery
Venous Drainage
- Brachiocephalic trunk
Lymphatic Drainage
Lymph from the scalene muscles are always located in the posterior triangle of the neck. The drained lymph arrives in the jugular lymphatic trunks, which are found both right and left. Further, the right lymph will flow into the right lymphatic duct, while the lymph collected in the left portion will end in the thoracic duct.
Spaces
Scalene hiatus (scalene fissure) – space formed between the anterior and middle scalene muscles through which the brachial plexus and subclavian artery pass. The region may be referred to as the scaleotracheal fossa.
- Boundaries
- Anterior – platysma muscle
- Posterior – trapezius
- Inferior – clavicle
Relations
- Brachial plexus and subclavian artery pass between the anterior and middle scalene muscles
- The subclavian vein and phrenic nerve pass anteriorly to the anterior scalene – the subclavian vein courses horizontally across it, while the phrenic nerve runs vertically down the muscle.
- The subclavian artery is located posterior to the anterior scalene.
Anterior
- Omohyoid
Development
The scalene muscles derive from the myoblasts of the hypaxial portion of the cervical myotomes, together with the prevertebral muscles and the geniohyoid muscle in the seventh week of gestation of the embryo. The muscular formation pattern is controlled by the connective tissue in which myoblasts migrate. In the cervical area of the connective tissue differs from the somitic mesoderm.
Anatomical Variants
- Sibson’s muscle (scalenus minimus) – a fourth muscle sometimes present behind the lower portion of the anterior scalene
- Anterior scalene muscle originating from C2 and/or not involving C6.
- Anterior scalene muscle inserting additionally on second or third rib.
- Anterior scalene muscle could pass behind the subclavian artery, either homolaterally or bilaterally (right and left). Furthermore, it could be found behind the first thoracic nerve. In some cases in the literature, the anterior scalene muscle is absent, both on one side and on both sides.
- The middle scalene muscle could involve in its insertion the transverse processes of the atlas cervical vertebra, leaning in its final path on the second thoracic rib. In some scientific articles, an accessory middle scalene muscle is identified, which builds a bridge with the anterior scalene muscle. This anatomical variant could cause a symptomatic picture that refers to the thoracic outlet syndrome.
- The posterior scalene muscle could involve the cervical vertebrae from C3 to C7; some of its fibers can create a single muscle structure with the middle scalene or the first intercostal muscle. Its final insertion could touch the third thoracic rib. In some subjects, this muscle may be absent. The middle scalene could be perforated by branches of the brachial plexus, such as C7 and C8, which will terminate on the posterior scalene muscle. Some authors have found that posterior scalene muscle could have a double layer, with a ventral area and a dorsal portion.