The posterior triangle of the neck is bounded by the following Except
Posterior Triangle
The posterior triangle is a space on the side of the neck situated behind the
sternocleidomastoid muscle.

Boundaries
Anterior: Posterior border of sternocleidomastoid

Posterior :Anterior border of trapezius

Inferior or base: Middle one- third of clavicle

Apex lies on the superior nuchal line where the trapezius And sternocleidomastoid meet

Roof : Investing layer

Floor: Formed by a prevertebral layer of deep cervical fascia covering
the following muscle: a. splenius capitisb. Levator scapulae; c.
Scalenus medius and occasionally scalenus posterior.
Trapezius
Splenius cervicis
Sternocleidomastoid
Clavicle