Scalenus Anterior Muscle: Anatomy

• The root of the neck is located above the apex of the lung; understanding the former requires understanding the relations of the latter.
• The scalenus anterior muscle serves as a key landmark in the root of the neck.

Scalenus Anterior

• Flat muscle with origin from the anterior tubercles of the C3-6 cervical vertebrae.
• The lowest and largest tubercle of origin is on C6, and referred to as the carotid tubercle.
• The muscle slopes forward and laterally, attaching via a narrow tendon to the scalene tubercle and adjacent ridge on the first rib's inner border and upper surface.
• It is part of the prevertebral flexor muscles of the neck, covered by a prevertebral fascia prolongation.
• Nerve supply: Branches from the anterior rami of C4, 5, and 6 nerves.
• Functionally significant as a landmark; assists in neck flexion and rotation and stabilizes the first rib.
• Electromyographic activity is present even during quiet respiration..

Anterior Relations of Scalenus Anterior

• The phrenic nerve runs vertically down across the muscle, adhered to it by prevertebral fascia.
• The phrenic nerve leaves the muscle's medial border low down and crosses the subclavian artery and its internal thoracic branch behind the subclavian vein.
• It passes medially to the lung apex and in front of the vagus nerve, to enter the superior mediastinum.
• The ascending cervical artery, a branch of the inferior thyroid artery or the thyrocervical trunk, runs up on the prevertebral fascia medial to the phrenic nerve.
• Transverse cervical and suprascapular arteries lie between the scalenus anterior and the carotid sheath.
• The vagus nerve in the carotid sheath passes in front of the subclavian artery, giving off its recurrent laryngeal branch, and runs on the medial surface of the lung apex to enter the superior mediastinum.
• The internal jugular vein is surrounded by the inferior deep cervical lymph nodes, including the jugulomohyoid node.
• The subclavian vein is in a groove on the first rib, lies lower than scalenus anterior insertion, and joins the internal jugular vein at the medial border of scalenus anterior.
• The thoracic duct (left) and right lymph duct enter the angle of confluence of the subclavian and internal jugular veins.
• Catheterization of the right subclavian vein can be performed infraclavicularly for central venous line placement, with common complications including pneumothorax and subclavian artery puncture.
• The intermediate tendon of the omohyoid runs in front of the lower carotid sheath; all enumerated structures are covered by the lower sternocleidomastoid part.

Medial Relations of Scalenus Anterior

• The edge of longus colli runs up to the anterior tubercle on the transverse process of C6 vertebra, forming a pyramidal space with scalenus anterior.
• The subclavian artery and the neck of the first rib form the base of the pyramidal space.
• The carotid tubercle (C6) marks the apex of the pyramidal space, where the common carotid artery can be compressed.
• The common carotid artery lies medial to the internal jugular vein, deep to sternocleidomastoid.
• The space contains the stellate ganglion, vertebral artery and vein(s).
• The inferior thyroid artery arches medially, with its upper convexity in front of the apex of the pyramidal space (C6 level).
• The thoracic duct (or right lymphatic duct) arches over the lung apex and subclavian artery.
• The first part of the subclavian artery arches over the suprapleural membrane.
• The vertebral artery, the first branch of the subclavian, passes up through the foramen of the transverse process of C6.
• The sympathetic 'plexus' on the vertebral artery may be a single nerve running up behind the vessel.
• The middle cervical ganglion is medial and anterior to the vertebral artery, while the inferior cervical or stellate ganglion is medial and posterior to it.
• The ansa subclavia is a connecting loop between middle and inferior cervical ganglia, passing in front of the subclavian artery.
• The thyrocervical trunk arises lateral to the vertebral artery and divides into transverse cervical, suprascapular, and inferior thyroid arteries.
• The internal thoracic artery arises from the lower surface of the subclavian.
• The vertebral vein emerges from the foramen in the transverse process of C6.

Posterior Relations of Scalenus Anterior

• The subclavian artery and anterior rami of the lower cervical and first thoracic nerves separate scalenus anterior from scalenus medius.
• The second part of the subclavian artery, behind scalenus anterior, gives rise to the costocervical trunk.
• The costocervical trunk divides into the superior intercostal artery and the deep cervical artery.

Lateral Relations of Scalenus Anterior

• The trunks of the brachial plexus and the third part of the subclavian artery emerge from the lateral border of scalenus anterior.
• These structures lie behind the prevertebral fascia.
• The subclavian vein lies in front of the prevertebral fascia.
• The dorsal scapular artery usually arises from the third part of the subclavian artery, running laterally in front of scalenus medius.
• The dorsal is frequently replaced by the deep branch of the transverse cervical artery.
• The surface marking of the subclavian artery in the neck arches upwards from the sternoclavicular joint to the middle of the clavicle, about 2 cm above it.
• Surgically, the artery can be exposed by detaching the clavicular head of sternocleidomastoid and scalenus anterior from the first rib.

Scalenus Medius and Scalenus Posterior

• Scalenus medius arises from the posterior tubercles/intertubercular lamellae of all cervical vertebrae and inserts into the first rib between the neck and subclavian groove.
• Scalenus posterior arises from the posterior tubercles of the lower cervical vertebrae and inserts into the second rib.
• Nerve Supply to Scalenus Medius: C3-8.
• Nerve Supply to Scalenus Posterior: C6-8.
• Both muscles primarily act as lateral flexors of the neck and accessory muscles of respiration.

Other Structures in the Root of the Neck

• The suprapleural membrane floors the root of the neck.
• Midline structures include the trachea, oesophagus, thoracic duct, and recurrent laryngeal nerves.
• The recurrent laryngeal nerves run in the groove between the trachea and oesophagus.

Overview of the scalenus anterior muscle, a key landmark in the root of the neck. The flat muscle originates from the anterior tubercles of the C3-6 cervical vertebrae and attaches to the first rib. It assists in neck flexion and rotation and stabilizes the first rib.