Superficial structures of the neck: Posterior triangle

The neck is the anatomical region between the base of the cranium superiorly and the clavicles inferiorly, joining the head to the trunk and limbs, and serving as a major conduit for structures passing between them.

In the neck, there are superficial structures, located in the anterior and posterior triangles, and deep structures, including the cervical viscera and prevertebral muscles.

Now, the neck is divided into the anterior and posterior triangles mainly by the borders of the sternocleidomastoid, or SCM, and trapezius muscles.

Now, the SCM is a broad, strap-like muscle which has a sternal and a clavicular head.

The sternal head is rounded and its inferior portion originates from the manubrium of the sternum, while the clavicular head is thick and its inferior portion originates from the superior surface of the medial third of the clavicle.

The two heads of the SCM are separated inferiorly, forming a superficially visible space called the lesser supraclavicular fossa, which looks like a small triangular depression.

Superiorly, the two heads of SCM join as they go toward the cranium and insert on the mastoid process of the temporal bone and the superior nuchal line of the occipital bone.

The posterior border of this muscle forms the anterior boundary of the posterior triangle.

When the SCM contracts, it produces movement at either the craniovertebral joints, or the cervical intervertebral joints, or at both.

So, unilateral contraction of SCM leads to lateral flexion of the neck to the same side, and also rotates the head so your face is turning superiorly towards the opposite side.

Bilateral contraction of SCM can lead to three different movements: extension of the neck at the atlanto-occipital joint, which makes the chin rise, flexion of the neck which makes the chin approach the manubrium, and extension of the superior cervical vertebrae combined with flexion of the inferior vertebrae which makes the chin thrust forward with head kept level.

Additionally, bilateral contraction of SCM with the head and neck fixed can lead to elevation of the clavicles and manubrium and thus the elevation of the anterior ribs.

This action makes the SCMs act as accessory muscles of respiration as they assist in lifting the sternum anterosuperiorly, which is referred to as the pump-handle movement of the thoracic wall.

Finally, the SCM is innervated by the spinal accessory nerve or cranial nerve 11, as well as fibers from C2 and C3.

And then there’s the trapezius, which is a large triangular muscle that covers the posterolateral aspect of the neck and thorax.

It originates from the medial third of superior nuchal line, external occipital protuberance, nuchal ligament and the spinous processes of the C7 toT12 vertebrae.

It inserts onto the lateral third of the clavicle, acromion and spine of the scapula.

The anterior border of the trapezius forms the posterior boundary of the posterior triangle.

Now, the trapezius also causes many different actions.

Its superior fibers elevate the scapula, its middle fibers retract the scapula, and its inferior fibers depress the scapula.

The superior and inferior fibers can also work together to produce upward rotation of the scapula, causing the glenoid cavity to tilt superiorly.

And finally, when the shoulders are fixed, bilateral contraction of the superior fibers extends the neck, while unilateral contraction causes lateral flexion to the same side.

Just like the SCM, the trapezius is innervated by the spinal accessory nerve and also by spinal nerves C2 and C3.

Now, the posterior triangle, like any respectable triangle, has three sides, called boundaries.

The anterior boundary is formed by the posterior border of the SCM, the posterior boundary is formed by the anterior border of the trapezius and the inferior boundary is formed by the middle third of the clavicle between the trapezius and the SCM.

The posterior triangle has an apex where the SCM and trapezius meet on the superior nuchal line of the occipital bone, a deep boundary - or floor - formed by several prevertebral muscles covered by the prevertebral layer of deep cervical fascia, and a superficial boundary - or roof - formed by the investing layer of deep cervical fascia.

The floor of the posterior triangle is formed primarily by four muscles: the splenius capitis, levator scapulae, middle scalene and posterior scalene.

Ok, so, the splenius capitis originates on the inferior half of the nuchal ligament and the spinous processes of C7 to T4.

It inserts onto the mastoid process of the temporal bone and the lateral third of the superior nuchal line.

Levator scapulae originates from the posterior tubercles of the transverse processes of the C1 to C4 vertebrae and inserts onto the superior part of the medial border of the scapula.

The middle scalene is the largest and longest of the three scalene muscles and it originates from the posterior tubercles of the transverse processes of C3 to C7 and inserts onto the superior surface of the first rib, posterior to the subclavian groove.

The posterior scalene muscle is the smallest and most deeply seated of the scalene muscles.

It originates from the posterior tubercles of the transverse processes of C5 to C7 and inserts onto the external border of the second rib.

There’s also the anterior scalene muscle, which lies deep to the sternocleidomastoid muscle.

This means it’s usually hidden, but sometimes it can be visualized in the inferomedial portion of the posterior triangle.

The anterior scalene originates from the anterior tubercles of the transverse processes of the C3 to C6 vertebrae and inserts onto the scalene tubercle on the first rib.

Let’s take a quick break and try to identify the muscles that form the boundaries and floor of the posterior triangle.