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:
Contents.
A) Nerves and plexuses: 
B) Vessels:
C) Lymph nodes:
D) Muscles:
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.