Anatomy of the neck

Neck is the region connecting the head to the trunk, extending down to the shoulder girdle. It contains the cervical spine and structures that support and move the head, carry air and food, and convey blood and nerves between the head, trunk and upper limbs.

What bones form the framework of the neck?

A cervical vertebra is one of the seven bones forming the cervical spine, the neck portion of the vertebral column. These bones are designated C1–C7. Their vertebral foramina, openings within each vertebra, align to form the vertebral canal containing the spinal cord.

The atlas, or C1, supports the skull and lacks a vertebral body, the weight-bearing anterior portion found in most vertebrae. The axis, or C2, has the dens, an upward-projecting bony process around which the atlas and head rotate. The joints between the skull and atlas chiefly permit nodding, while those between atlas and axis chiefly permit rotation.

The middle cervical vertebrae have relatively small bodies and transverse foramina, openings in their lateral bony projections. The vertebral arteries usually enter these openings at C6 and ascend towards the skull. The seventh cervical vertebra usually has a long, readily palpable spinous process, the backward-projecting part of a vertebra.

Intervertebral discs lie between adjacent vertebral bodies. Each has an outer annulus fibrosus, composed of concentric layers of collagen-rich tissue including fibrocartilage, and an inner nucleus pulposus, a gelatinous core. They allow movement and distribute loads. There is no disc between the atlas and axis.

Where is the hyoid bone, and what attaches to it?

The hyoid bone is a U-shaped bone in the anterior neck between the lower jaw and larynx, the voice-producing part of the airway. It has a central body and paired greater and lesser horns, which are projecting attachment points. Unlike most bones, it does not form a joint with another bone.

Muscles and ligaments suspend the hyoid and connect it to surrounding structures. The suprahyoid muscles, situated above it, link it to the skull and lower jaw and contribute to the floor of the mouth. The infrahyoid muscles, situated below it, connect the hyoid or larynx to the sternum or scapula, or connect the thyroid cartilage of the larynx directly to the hyoid.

Together, these muscles position the hyoid during swallowing and movements of the tongue and jaw. The hyoid also provides attachment for muscles of the tongue and pharynx, the muscular passage behind the nose, mouth and larynx.

Where are the sternocleidomastoid and scalene muscles?

The sternocleidomastoid runs obliquely across each side of the neck. It attaches below to the manubrium, the upper part of the sternum or breastbone, and the medial clavicle, the inner end of the collarbone. Above, it attaches to the mastoid process, a bony projection behind the ear, and the adjacent skull.

Acting on one side, it tilts the neck towards that side and turns the face towards the opposite side. Acting together, the muscles can flex the neck. The accessory nerve, a cranial nerve arising from the central nervous system, supplies their motor innervation, meaning the nerve supply that activates contraction.

The anterior, middle and posterior scalene muscles lie deeper, beside the cervical spine. They extend from transverse processes, the lateral projections of cervical vertebrae, to the upper ribs. They assist lateral bending of the neck and can elevate these ribs during inspiration.

The subclavian artery, which carries blood towards the upper limb, and the roots of the brachial plexus pass between the anterior and middle scalene muscles. The subclavian vein passes anterior to the anterior scalene muscle, rather than through this interval.

What are the borders of the anterior and posterior triangles?

The sternocleidomastoid divides each side of the neck into anterior and posterior triangles. These are surface regions used to describe the positions of deeper structures, rather than enclosed cavities.

The anterior triangle has these boundaries:

• Medially: the anterior midline of the neck.
• Laterally: the anterior border of the sternocleidomastoid.
• Superiorly: the inferior border of the mandible, or lower jaw.

The digastric and omohyoid muscles subdivide it into submental, submandibular, carotid and muscular triangles. The submental region lies beneath the chin; the submandibular region lies beneath the jaw. The carotid triangle contains major vessels, while the muscular triangle overlies the infrahyoid muscles and central neck viscera.

The posterior triangle has these boundaries:

• Anteriorly: the posterior border of the sternocleidomastoid.
• Posteriorly: the anterior border of trapezius, the broad muscle extending across the posterior neck and upper back.
• Inferiorly: the middle part of the clavicle.

Its apex is where the sternocleidomastoid and trapezius approach one another superiorly. The inferior belly of omohyoid divides it into an occipital triangle above and an omoclavicular triangle below. A belly is the fleshy portion of a muscle.

The triangles’ roofs include skin, superficial fascia and the investing layer of deep cervical fascia, connective tissue that surrounds and separates neck structures. The posterior triangle’s floor consists of deeper muscles covered by prevertebral fascia, the fascial layer surrounding the vertebral column and associated muscles. In the adjacent 3D model, rotate and zoom, or select structures and hide, isolate or fade them, to compare different views while studying the anatomy of the neck.

How are the neck viscera arranged?

The viscera are the internal organs of a region. In the neck, the airway lies anterior to the food passage, with the thyroid gland beside and partly in front of the airway.

The larynx connects the pharynx to the trachea, the airway descending into the chest. The pharynx continues inferiorly as the oesophagus, the muscular tube carrying swallowed material towards the stomach. This transition lies approximately at the level of C6, near the inferior border of the cricoid cartilage, the ring-shaped cartilage at the base of the larynx.

The thyroid gland has lateral lobes joined by an isthmus, a connecting bridge of glandular tissue. Its lobes lie beside the lower larynx and upper trachea; the isthmus usually crosses the anterior surface of the upper trachea.

What does the carotid sheath contain?

The carotid sheath is a sleeve of deep cervical fascia extending along each side of the neck. Its principal contents are:

• The common carotid artery inferiorly and the internal carotid artery superiorly.
• The internal jugular vein, which drains blood from the brain and other head and neck structures.
• The vagus nerve, a cranial nerve supplying structures in the neck, chest and abdomen.

Within the sheath, the artery lies medially and the vein laterally. The vagus nerve usually lies posteriorly between them. The common carotid artery divides into internal and external carotid arteries; the external carotid artery does not continue within the sheath.

The sympathetic trunk, a chain of nerve connections involved in involuntary control, lies behind the sheath rather than inside it.

Which roots form the cervical and brachial plexuses?

A nerve plexus is a network formed by interconnecting nerve branches. The cervical plexus arises from the anterior rami of C1–C4 spinal nerves; anterior rami are the forward-directed branches formed after spinal nerves leave the vertebral canal. It supplies skin and muscles of the neck through sensory and motor branches.

The phrenic nerve receives fibres from C3–C5, chiefly C4, and supplies the diaphragm, the main muscle of inspiration. It descends on the anterior surface of the anterior scalene muscle.

The brachial plexus usually arises from the anterior rami of C5–T1. These branches are called its roots and pass between the anterior and middle scalene muscles before forming trunks in the lower neck. The plexus continues towards the armpit and supplies most of the upper limb.