Anatomy First Year

first year – anatomy – Posterolateral Triangle of the Neck

Posterolateral Triangle of the Neck

The neck region  is anatomically divided into an anterior triangle and a posterolateral triangle. This lab will focus on the posterolateral triangle. Initially, the skin covering both regions of the neck will be reflected, and then the boundaries of the posterolateral triangle will be identified. The fascial roof of the posterolateral triangle will be removed to study the structures found within the triangle. Finally, the fascial floor of the triangle will be removed and the muscles which lie deep to it will be examined. Due to the density of the fascial layers, dissection is often tedious and slow.

Clavicle
first year – anatomy – Posterolateral Triangle of the Neck
 Mastoid process of the temporal bone
first year – anatomy – Posterolateral Triangle of the Neck


Step 1. Reflection of the Platysma Muscle

If the skin of the neck has not been removed, cut and reflect it posteriorly Next identify and reflect the platysma muscle  superiorly towards the mandible. Be careful not to injure the structures which cling to the deep surface of this muscle, e.g., the cervical branch of the facial nerve and cutaneous branches of the cervical plexus.

platysma muscle
first year – anatomy – Posterolateral Triangle of the Neck


Step 2. Boundaries of Posterolateral Triangle

first year – anatomy – Posterolateral Triangle of the Neck

Identify the boundaries of the posterolateral triangle

  • Anteriorly, it is bounded by the posterior edge of the sternocleidomastoid muscle.
  • Posteriorly, it is bounded by the anterior edge of trapezius muscle.
  • Inferiorly it is bounded by the clavicle.


The floor of the triangle is made up of deep investing fascia which covers prevertebal muscles. The roof is formed by skin, platysma muscle, and fascia which spans between the trapezius and sternocleidomastoid muscles.

  sternocleidomastoid muscle.
first year – anatomy – Posterolateral Triangle of the Neck
clavicle
first year – anatomy – Posterolateral Triangle of the Neck
trapezius muscle
first year – anatomy – Posterolateral Triangle of the Neck

Step 3. Cutaneous Branches of the Cervical Plexus

first year – anatomy – Posterolateral Triangle of the Neck

 

Identify Erb’s point (punctum nervosum), which is located on the posterior border of the sternocleidomastoid muscle midway between its attachments to the mastoid process, and the sternum and clavicle. At Erb’s point, cutaneous branches of the cervical plexus  emerge from behind the posterior border of the sternocleidomastoid muscle. The four cutaneous nerves of the cervical plexus are the lesser occipital nervegreat auricular nervetransverse cervical nerve, and supraclavicular nerve.

Along the superior aspect of the posterior edge of the sternocleidomastoid muscle, identify the lesser occipital nerve. It runs superiorly and innervates the skin superior and posterior to the ear.

Erb’s point (punctum nervosum)
first year – anatomy – Posterolateral Triangle of the Neck
  lesser occipital nerve.
first year – anatomy – Posterolateral Triangle of the Neck

Step 4. The Spinal Accessory Nerve (CN XI)

first year – anatomy – Posterolateral Triangle of the Neck

Identify the spinal accessory nerve (CN XI) in the fascia of the posterolateral triangle. It passes on the deep surface of the sternocleidomastoid muscle. It then emerges from a point slightly superior to the middle of the posterior edge of the sternocleidomastoid muscle. It crosses the posterolateral triangle, passing inferiorly and posteriorly in the fascia between the sternocleidomastoid and trapezius muscles. About 2 cm above the clavicle, it passes to the deep surface of the trapezius muscle. It innervates the sternocleidomastoid muscle and the trapezius muscle.

spinal accessory nerve (CN XI)
first year – anatomy – Posterolateral Triangle of the Neck
 

Step 5. Reflection of the Sternocleidomastoid Muscle

first year – anatomy – Posterolateral Triangle of the Neck


To examine the structures at the base of the posterolateral triangle, it is necessary to remove a portion of the clavicle. Using a Striker saw, cut through the clavicle just laterally to the attachment of the sternocleidomastoid muscle  . Remove the cut segment of the clavicle and retract the sternocleidomastoid muscle superiorly .

sternocleidomastoid muscle 
first year – anatomy – Posterolateral Triangle of the Neck

Step 6. The Inferior Belly of Omohyoid Muscle

first year – anatomy – Posterolateral Triangle of the Neck


Identify the inferior belly of the omohyoid muscle. The omohyoid muscle is flattened and strap-like. It consists of a superior belly and an inferior belly. The two bellies of this muscle are connected by an intermuscular tendon, that is held by connective tissue to the anterior surface of the internal jugular vein to prevent the vein from collapsing under negative pressure. The position of the inferior belly of the omohyoid muscle divides the posterior triangle into an upper occipital triangle and a lower subclavian (or supraclavicular) triangle.

inferior belly
first year – anatomy – Posterolateral Triangle of the Neck
 superior belly
first year – anatomy – Posterolateral Triangle of the Neck

Step 7. The External Jugular Vein

Identify the external jugular vein and its tributary, the transverse cervical vein. The external jugular vein drains into the subclavian vein

external jugular vein
first year – anatomy – Posterolateral Triangle of the Neck
 subclavian vein
first year – anatomy – Posterolateral Triangle of the Neck
 transverse cervical vein
first year – anatomy – Posterolateral Triangle of the Neck


Step 8.
 The Transverse Cervical Artery and Suprascapular Artery

Identify the transverse cervical artery [tip of probe] and suprascapular artery as they arise from the thyrocervical trunk and pass anterior to the scalenus anterior muscle.

transverse cervical artery
first year – anatomy – Posterolateral Triangle of the Neck
 suprascapular artery
first year – anatomy – Posterolateral Triangle of the Neck
scalenus anterior muscle. 
first year – anatomy – Posterolateral Triangle of the Neck



Step 9. The Prevertebral Muscles

To identify the prevertebral muscles which form the floor of the posterolateral triangle, remove their investing fascia. These muscles include the:

 
scalenus anterior muscle
first year – anatomy – Posterolateral Triangle of the Neck
 levator scapulae muscle
first year – anatomy – Posterolateral Triangle of the Neck
 splenius capitis muscle
first year – anatomy – Posterolateral Triangle of the Neck
 scalenus medius and posterior muscles
first year – anatomy – Posterolateral Triangle of the Neck
semispinalis capitis muscle
 

Step 10. Trunks of the Brachial Plexus

Between the scalenus anterior muscle and scalenus medius muscle, identify the trunks (upper, middle, and lower) of the brachial plexus.

 scalenus medius muscle
first year – anatomy – Posterolateral Triangle of the Neck
 scalenus anterior muscle
first year – anatomy – Posterolateral Triangle of the Neck
 runks (upper, middle, and lower) of the brachial plexus.
first year – anatomy – Posterolateral Triangle of the Neck
 

Step 11. Structures Anterior to Scalenus Anterior Muscle

Identify the phrenic nerve (C3, C4, C5) as it crosses vertically, along with the ascending cervical artery, on the anterior surface of the scalenus anterior muscle. Anterior to the scalenus anterior muscle also are the subclavian veintransverse cervical artery [superior structure], and suprascapular artery [inferior structure]. The transverse cervical,suprascapular, inferior thyroid, and ascending cervical arteries arise from the thyrocervical trunk [to be dissected in another exercise], which, in turn, is a branch of the subclavian artery.

 
scalenus anterior muscle
first year – anatomy – Posterolateral Triangle of the Neck
 subclavian vein
first year – anatomy – Posterolateral Triangle of the Neck
 transverse cervical artery
first year – anatomy – Posterolateral Triangle of the Neck
  phrenic nerve (C3, C4, C5)
first year – anatomy – Posterolateral Triangle of the Neck
 thyrocervical trunk
first year – anatomy – Posterolateral Triangle of the Neck
 

Step 12. Nerves of the Brachial Plexus in the Posterolateral Triangle

Identify the long thoracic nervesuprascapular nerve, and dorsal scapular nerve. The long thoracic nerve (C5, C6, C7) descends posterior to the brachial plexus and the subclavian vessels to supply the serratus anterior muscle. The suprascapular nerve (C5, C6) arises from the upper trunk of the brachial plexus and runs posteriorly toward the scapular region to supply the supraspinatus and infraspinatus muscles. The dorsal scapular nerve arises from the C5 spinal nerve and pierces the scalenus medius muscle. It passes posteriorly to supply the levator scapulae and rhomboid muscles.

first year – anatomy – Posterolateral Triangle of the Neck