Posterior triangle of the neck

The posterior triangle (or lateral cervical region) is a region of the neck.

Boundaries
The posterior triangle has the following boundaries:

Apex: Union of the sternocleidomastoid and the trapezius muscles at the superior nuchal line of the occipital bone

Anteriorly: Posterior border of the sternocleidomastoideus

Posteriorly: Anterior border of the trapezius

Inferiorly: Middle one third of the clavicle

Roof: Investing layer of the deep cervical fascia

Floor: (From superior to inferior)

1) M. semispinalis capitis

2) M. splenius capitis

3) M. levator scapulae

4) M. scalenus posterior

5) M. scalenus medius

Divisions
The posterior triangle is crossed, about 2.5 cm above the clavicle, by the inferior belly of the omohyoid muscle, which divides the space into two triangles:
 * an upper or occipital triangle
 * a lower or subclavian triangle (or supraclavicular triangle)

Contents
A) Nerves and plexuses: 
 * Spinal accessory nerve (Cranial Nerve XI)
 * Branches of cervical plexus
 * Roots and trunks of brachial plexus
 * Phrenic nerve (C3,4,5)

B) Vessels: C) Lymph nodes: D) Muscles:
 * Subclavian artery (Third part)
 * Transverse cervical artery
 * Suprascapular artery
 * Terminal part of external jugular vein
 * Occipital
 * Supraclavicular
 * Inferior belly of omohyoid muscle
 * Anterior Scalene
 * Middle Scalene
 * Posterior Scalene
 * Levator Scapulae Muscle
 * Splenius

Clinical significance
The accessory nerve (CN XI) is particularly vulnerable to damage during lymph node biopsy. Damage results in an inability to shrug the shoulders or raise the arm above the head, particularly due to compromised trapezius muscle innervation.

The external jugular vein's superficial location within the posterior triangle also makes it vulnerable to injury.