posterior triangle of the neck posterior triangle of the neck the posterior triangle (or lateral cervical region) is a region of the neck. posterior triangle of the neckposterior triangleside of neck, showing chief surface markings. (nerves are yellow, arteries are red.)detailsidentifierslatintrigonum cervicale posteriustrigonum colli posteriusregio cervicalis posteriorta98a01.2.02.009ta2239fma57778anatomical terminology 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: an upper or occipital triangle a lower or subclavian triangle (or supraclavicular triangle) contents a) nerves and plexuses: spinal accessory nerve (cranial nerve xi) branches of cervical plexus roots and trunks of brachial plexus phrenic nerve (c3,4,5) b) vessels: subclavian artery (third part) transverse cervical artery suprascapular artery terminal part of external jugular vein c) lymph nodes: occipital supraclavicular d) muscles: inferior belly of omohyoid muscle anterior scalene middle scalene posterior scalene levator scapulae muscle splenius 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.