Anatomy of the Neck
Neck: Introduction
- Boundaries of the Neck
- Fascial Compartments of the Neck
- Subdivisions of the Posterior Triangle
- Nerve Supply
- Arterial Supply
- Venous Drainage
Boundaries of the Neck
§ The superior boundary of the neck is demarcated by the superior nuchal line of the cranium and the lower margin of the mandible.
§ The inferior boundary of the neck is demarcated by the suprasternal notch, the clavicle and the first rib.
§ Clearly, these boundaries are irregular and difficult to define precisely, because of the continuity of structures running from one region to another (i.e. between thorax and neck and between neck and head).
§ The neck is conveniently thought of as the tissue surrounding the 7 cervical vertebrae.
Fascial Compartments of the Neck
§ For our purposes, there are four layers of deep fascia that separate the neck into compartments.
§ The superficial fascia (tela subcutanea) surrounds the entire neck, but does not contribute to the compartmentalization.
§ One layer is the superficial layer of deep fascia (investing fascia, red), which surrounds the whole neck.
§ The second layer is the prevertebral layer (blue), which surrounds muscles attached to the vertebral column.
§ The axillary sheath continues from the prevertebral layer and surrounds the brachial plexus.
§ The third layer is the pretracheal or visceral layer (green), which surrounds anterior organs.
§ This layer blends with the buccopharyngeal fascia (cyan), which covers the posterior surface of the esophagus.
§ The fourth layer is the carotid sheath (yellow), which is formed by condensation of the superficial, prevertebral and pretracheal layers.
§ The carotid sheath surrounds the carotid artery, the jugular vein and the vagus nerve.
§ Clinical Correlate: Fascial layers are important because they form a barrier to the flow of fluids. Normally, of course, there are no pools of fluid in the neck. However, loose blood (hematoma) may form as a result of traumatic injury, or an exudate (pus) may form as a result of infection.These fluids will tend to flow through specific channels called potential spaces. These channels are generally filled with a loose connective tissue that separates under the pressure of the fluids. The most important potential space in the neck is the retropharyngeal space, also known as the “danger” space. This potential space is anterior to the prevertebral fascia, but posterior to the buccopharyngeal fascia. This space is continuous with the superior mediastinum. Therefore, infections originating in the oral region (e.g. an abscessed tooth) can track into the , potentially causing cardiac tamponade. The suprasternal space is superior to the sternum, and is formed by a split in the investing fascia as it attaches to the manubrium. This space contains the jugular arch. The pretracheal space is anterior to the thyroid cartilage and trachea, but posterior to the pretracheal fascia. This space is continuous with the anterior mediastinum of the thorax.
Subdivisions of the Posterior Triangle
§ The anterior portion of the neck is separated into two large triangles by the sternocleidomastoid muscle.
§ The posterior triangle is formed by the trapezius muscle, sternocleidomastoid muscle (clavicular head) and the clavicle.
§ The posterior triangle is in turn subdivided by the inferior belly of the omohyoid muscle.
§ The larger and more superior of the two resulting triangles is the occipital triangle.
§ Its boundaries are the trapezius muscle, sternocleidomastoid muscle, and the omohyoid muscle.
§ The floor of the occipital triangle is formed by several muscles that are attached to cervical vertebrae: the splenius capitis, levator scapulae, and medial scalene muscles.
§ There may also be a small portion of semispinalis capitis as well.
§ Inferiorly is the supraclavicular (subclavian) triangle.
§ The boundaries of this triangle are inferior belly of omohyoid, sternocleidomastoid (clavicular head), and the clavicle.
§ The floor is formed by the anterior scalene muscle.
§ Clinical Correlate: Torticollis is a condition in which the head is tilted to the affected side and the face is turned away from that side. The condition is induced by persistent contraction or shortening of the sternocleidomastoid muscle. In infants, this can be due to prenatal development of a fibrous tumor or damage to the muscle during birth. In adults, this condition appears to have a neurological basis.
Nerve Supply
§ The nerve supply to the posterior triangle and adjoining structures is from the spinal accessory nerve (CN XI) and branches of the cervical plexus.
§ CN XI innervates the sternocleidomastoid and trapezius muscles. In addition, it forms an important landmark in the posterior triangle.
§ As the nerve courses inferiorly on the surface of the levator scapulae muscle, it separates the occipital triangle into a “carefree” area superiorly (i.e. few arteries and nerves) and a “danger” area inferiorly (i.e. numerous arteries and nerves).
§ Several sensory nerves branching from the cervical plexus appear in the posterior triangle. These are:
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- Lesser occipital (C2)
- Great auricular (C2-C3)
- Transverse cervical (C2-C3)
- Supraclavicular nerves (C3-C4)
§ All of these nerves, with CN XI, emerge together on the posterior border of the sternocleidomastoid.
§ This is called the nerve point of the neck.
§ The brachial plexus (C5-C8, T1) was studied in greater detail in the Upper Extremity section.
§ It first emerges from the vertebral column in the neck, and is easily identified between the anterior and medial scalenes in the occipital triangle.
§ Branches of the brachial plexus which originate in the neck include:
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- Dorsal scapular nerve (C5) innervates rhomboids
- Long thoracic nerve (C5-C7) innervates serratus anterior
- Nerve to subclavius muscle (C5)
- Suprascapular nerve (C5-C6) innervates supraspinatus and infraspinatus muscles
§ Clinical Correlates: Trauma to nerves in the posterior triangle can result in loss of function. For example, a knife may penetrate the skin during a street fight or a scalpel could slip during surgery. Tumors at the base of the skull may affect function of some cranial nerves, as could a fracture of the base of the skull that affects the jugular foramen. The practical consequence is loss of function. For the spinal accessory nerve (CN XI), this would result in weak actions of sternocleidomastoid and trapezius muscles. The suprascapular nerve, which originates from the brachial plexus, can be damaged by a fracture of the middle third of the clavicle. This would result in weak actions of supraspinatus and infraspinatus muscles.
§ The brachial plexus, particularly the lower trunk, is also susceptible to compression damage. On rare occasions, the transverse process of C7 is enlarged (so-called cervical rib). A fibrous band attached to that process traps the neurovascular bundle as it passes between the clavicle and 1st rib. Chronic compression results in wasting of muscles in the hand. Hypertrophy of the anterior scalene muscle has also been implicated, but that is somewhat controversial.
§ For the sensory nerves of the cervical plexus, traumatic injury would result in anesthesia of the appropriate dermatomes. Reversible anesthesia for pain during surgery is performed by injecting drug along the posterior border of the sternocleidomastoid (cervical plexus block) or superior to the clavicle (brachial plexus block). Remember that the clinical effectiveness of a nerve block depends on a number of factors.
§
Arterial Supply
The major arterial supply to the posterior triangle is the subclavian artery. This will be covered in more detail in the Anterior Triangle lecture. The thyrocervical trunk gives rise to several prominent branches that pass through the posterior triangle. The transverse cervical artery and the suprascapular artery pass through the posterior triangle as they project to the scapular muscles. This drawing illustrates a common variation in which the deep branch of the transverse brachial artery does not arise from the thyrocervical trunk. Rather, it branches directly from the third division of the subclavian artery, and is known as the dorsal scapular artery.
The deep cervical artery supplies splenius capitis et cervicis, which forms part of the floor of the posterior triangle. This artery is, along with the superior intercostal artery, a terminal branch of the costocervical trunk.
One portion of the subclavian artery itself passes through the supraclavicular triangle. The vessel supplying the superior region of the posterior triangle is the occipital artery. This artery is a branch of the external carotid.
Venous Drainage
- Venous drainage from the posterior triangle generally parallels the arterial supply.
- The major named vessel is the external jugular vein.
- This vessel forms behind the mandible and descends on the surface of the sternocleidomastoid muscle into the posterior triangle.
- It joins the subclavian vein at the base of the triangle.
- Other tributaries to the external jugular include:
- Posterior auricular
- Retromandibular and its posterior division
- Transverse cervical (not shown)
- Suprascapular (not shown)
- Anterior jugular (not a tributary in this drawing)
- The appearance of these vessels will vary among cadavers, however.
§ Clinical Correlates: The external jugular vein is a surface landmark of clinical significance. Normally, the vein is visible only a short distance above the clavicle, if at all. When venous pressure rises, however, the vein may be visible along its entire course. This could be an indication of heart failure, obstruction of the superior vena cava, enlarged supraclavicular lymph nodes, or a general increase in intrathoracic pressure.
§ Because of its superficial location, the external jugular vein is easily severed. The lumen may be held open by the superficial layer of deep fascia, allowing air to be sucked in by the negative intrathoracic pressure transmitted through the subclavian vein.
§ The subclavian vein is frequently used to introduce catheters (e.g. central lines, Swan-Ganz catheters) for measurement of cardiac pressure or drug delivery. The needle is inserted inferior to the clavicle and moved medially along the posterior surface until the wall of the vessel is penetrated.
Tags: Anterior Triangle, brachial plexus, buccopharyngeal fascia, cardiac tamponade, deep cervical artery, hyrocervical trunk, infraspinatus, intrathoracic pressure, jugular vein, levator scapulae, mandible, medial scalene, neck anatomy, nuchal line, occipital artery, omohyoid, pericardium, prevertebral fascia, rauma to nerves, Retromandibular, semispinalis, splenius capitis, sternocleidomastoid, sternocleidomastoid muscle, subclavian vein, Thorax, trapezius, upraspinatus, Venous drainage, vertebral column
