Posterior Triangle of the neck: Divisions and boundaries



The posterior triangle of neck is an anatomical area located in the lateral aspect of the neck between the sternocleidomastoid muscle, trapezius muscles and 1/3 of clavicle bone.
Neck is the transitional area between the base of the cranium superiorly and clavicles inferiorly. The neck joins the head to the trunk; it is also the beginning of the spinal column and spinal cord.
To allow clear communication regarding the location of structures, injuries or pathologies, the neck is divided into four regions based on the usually and palpably borders. The neck is divided into two triangles by the sternocleidomastoid muscle.


The triangle of the neck is divided into two by the sternocleidomastoid muscle; which are:
  • Anterior Triangle of the Neck (Anterior cervical region).
  • Posterior Triangle of the neck (lateral cervical region). 


The posterior triangle of neck is an anatomical area located in the lateral aspect of the neck between the sternocleidomastoid muscle, trapezius muscles and 1/3 of clavicle bone.


The posterior triangle of neck is bounded by some structures that separates from other cervical regions (triangles as the case may be). The boundaries (borders) of the posterior triangle of the neck are as follows:
  • Anteriorly: posterior border of sternocleidomastoid muscle.
  • Posteriorly: Anterior border of the trapezius.
  • Inferiorly: middle 1/3 of the clavicle.
Outside the above borders of the triangle of the neck; the triangle is also characterized by
  • A roof
  • A floor
  • A apex
The roof: The roof is formed by the investing layer of the deep cervical fascia alongside the muscles it covers.

The investing layer is the most superficial of the deep cervical fascia; it lies deep to the platysma and surrounds all the structure where it meets the trapezius and sternocleidomastoid muscles. The investing layer can be as a tube with superior, inferior, anterior and posterior attachments.

  • Superior: It attaches to the: external occipital protuberance, the superior nuchal line, mastoid process, External acoustic meatus and the base of the mandible.
  • Anterior: It attach to the hyoid bone.
  • Inferior: It attached to the spine and acromion of the scapula, the clavicle and the manubrium of the sternum.
  • Posterior: It attaches along the ligamentum nuchae.
  • The apex: Lies high up at the back of the skull on the superior nuchal line, where the trapezius and sternocleidomastoid muscles met.
Floor:The floor of the posterior triangle of the neck is formed by the prevertebral layer of deep cervical fascia, covering the following muscles:
  • Splenius Capitis
  • Levator scapulae
  • Scalenus medius
  • Semispinalis capitis may also form the part of the floor.


The subdivision of the posterior triangles of neck is done by the inferior belly of the omohyoid muscle. The inferior belly of omohyoid muscles splits the triangles into the following:

  • Occipital triangle(a larger and superior part).
  • Subclavian/omoclavicular/supraclavicular(a smaller and inferior part).



The posterior triangle of the neck contains many muscles, which make up the borders and the floor of the area.

A significant muscle in the posterior triangle region is the omohyoid muscle. It is split into two bellies by a tendon. The inferior belly crosses the posterior triangle, travelling in an supero-medial direction, and splitting the triangle into two. The muscle then crosses underneath the SCM to enter the anterior triangle of the neck.

A number of vertebral muscles (covered by prevertebral fascia) form the floor of the posterior triangle:

  • Splenius capitis
  • Levator scapulae
  • Anterior, middle and posterior scalenes


The external jugular vein is one of the major veins of the neck region. Formed by the retromandibular and posterior auricular veins, it lies superficially, entering the posterior triangle after crossing the sternocleidomastoid muscle. Within the posterior triangle, the external jugular vein pierces the investing layer of fascia and empties into the subclavian vein.

The subclavian vein is often used as a point of access to the venous system, via a central catheter.

The transverse cervical and suprascapular veins also lie in the posterior triangle

The subclavian, transverse cervical and suprascapular veins are accompanied by their respective arteries in the posterior triangle.

The distal part of the subclavian artery can be located as it emerges between the anterior and middle scalene muscles. As it crosses the first rib, it becomes the axillary artery, which goes onto supply the upper limb.


The accessory nerve (CN XI) exits the cranial cavity, descends down the neck, innervates sternocleidomastoid and enters the posterior triangle. It crosses the posterior triangle in an oblique, inferoposterior direction, within the investing layer of fascia. It lies relatively superficially in the posterior triangle, leaving it vulnerable to injury.

The cervical plexus forms within the muscles of the floor of the posterior triangle. A major branch of this plexus is the phrenic nerve, which arises from the anterior divisions of spinal nerves C3-C5. It descends down the neck, within the prevertebral fascia, to innervate the diaphragm.

Other branches of the cervical plexus innervate the vertebral muscles, and provide cutaneous innervation to parts of the neck and scalp.

The trunks of the brachial plexus also cross the floor of the posterior triangle.




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