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:
  • deep cervical lymph nodes
    • digastric node
    • omohyoid node
  • ansa cervicalis (ac)
  • carotid sheath (cs)
    • internal jugular vein and tributaries(ij)
    • common carotid artery and its (cc) bifurcation
      • external carotid artery
        • occipital branch (oc)
      • internal carotid artery
  • vagus nerve (v)



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

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

List of Items Covered in This Lesson


splenius capitis & cervicis
levator scapulae
scalene posterior
scalene medius
scalene anterior
inferior belly omohyoid


mastoid process
occipital bone
superior nuchal line


common carotid
transverse cervical


lesser occipital
great auricular
transverse cervical
spinal accessory
lower trunk of brachial 


external jugular

Lymph nodes

superficial cervical
deep cervical
omohyoid node
digastric node


carotid sheath

Anterior Cervical Triangles   Lateral Pharyngeal Region

Table of Contents for Head & Neck
Practice Practical Exam
Practice Written Exam