Sternomastoid Region and Posterior Triangle of Neck

The anterior and posterior cervical triangles are interconnected through the substernomastoid space. This space includes the following structures:

Boundaries of Posterior Triangle anterior--sternomastoid posterior--trapezius inferior--middle third of clavicle
Roof of Posterior Triangle skin superficial fascia platysma inferiorly
Structures Superficial to Sternomastoid external jugular vein (blue) superficial cervical lymph nodes (green) lesser occipital nerve (lc) great auricular nerve (ga) transverse cervical nerve (tc) supraclavicular nerves (sc) spinal accessory nerve (sa)
Muscles of the Floor of the Posterior Triangle splenius capitis (sc) levator scapulae (ls) scalenus posterior (sp) scalenus medius (sm) scalenus anterior (sa) inferior belly of omohyoid (io)
Structures Near the Root of the Neck brachial plexus (bp) subclavian artery (sca) subclavian vein (sv) Note: the brachial plexus and subclavian artery pass between the scalenus anterior (sa) and medius (sm) muscles. The subclavian vein passes anterior to the scalenus anterior muscle.
Other Structures in Posterior Triangle spinal accessory (sa) brachial plexus (bp) transverse cervical artery (tc) suprascapular artery (ss)

Table of Muscles

Muscle	Origin	Insertion	Action	Nerve Supply
sternomastoid	sternal head--manubrium of sternum; clavicular head--medial 1/3rd of clavicle	mastoid process, superior nuchal line	rotates, extends the head; flexes the head	spinal accessory
trapezius	superior nuchal line; nuchal ligament; spinous processes of C7 and all of thoracic	superior part--lateral 1/3rd of clavicle; middle part--acomion process and spine of scapula; inferior part--spine of scapula (middle part)	elevates shoulder; rotates scapula laterally to help raise shoulder; retracts scapula	spinal accessory; C2-C3
splenius capitis	spinous processes of upper thoracic vertebrae	mastoid process and superior nuchal line	extends and rotates head	dorsal rami of thoracic nerves
levator scapulae	transverse processes of C1-C4	upper medial border of scapula	raises scapula	dorsal scapular
scalene posterior	transverse processes of C5-C7	upper surface of 2nd rib	raises 2nd rib; bends neck to the side	ventral rami of C7-C8
scalene medius	transverse processes of C1-C6	upper surface of 1st and 2nd ribs	raises 1st rib; bends neck to side	ventral rami of C3-C8
scalene anterior	transverse processes of C3-C6	scalene tubercle of 1st rib	raises 1st rib; bends to side	ventral rami of C6-C8