Floor and Roof of Posterior Triangle of Neck

Roof

The roof is formed from superficial to deep by:

1. Skin
2. Superficial fascia (containing platysma in its lower part, cutaneous nerves, external jugular vein)
3. Investing layer of deep cervical fascia — the true roof

Platysma is absent over the upper part of the triangle and covers only the lower part (over the subclavian triangle).

Floor

The floor is formed by the prevertebral layer of deep cervical fascia covering the following muscles (from above downward):

Muscle	Origin	Nerve Supply
Splenius capitis	Spinous processes C3–T6	Dorsal rami of middle cervical nerves
Levator scapulae	Transverse processes C1–C4	C3, C4 (dorsal scapular nerve also contributes)
Scalenus posterior	Posterior tubercles C5–C7	Ventral rami C6–C8
Scalenus medius	Posterior tubercles C2–C7	Ventral rami C3–C8
Scalenus anterior	Anterior tubercles C3–C6	Ventral rami C4–C6 (forms medial boundary of subclavian triangle)

Scalenus anterior is technically not on the floor of the main posterior triangle but forms the medial wall of the subclavian triangle and is visible at the base of the posterior triangle.

Fascial Layers — Clinical Point

• The investing fascia (roof) splits to enclose SCM anteriorly and trapezius posteriorly
• The prevertebral fascia (floor) is thick and resists spread of infection into the posterior triangle — infections track along it to the axilla

Posterior Triangle of Neck