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.
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.
Step 2. Boundaries of Posterolateral Triangle
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.
Step 3. Cutaneous Branches of the Cervical Plexus
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.
Step 4. The Spinal Accessory Nerve (CN XI)
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.
Step 5. Reflection of the Sternocleidomastoid Muscle
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 .
Step 6. The Inferior Belly of Omohyoid Muscle
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.
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
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.
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:
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.
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 vein, transverse 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.
Step 12. Nerves of the Brachial Plexus in the Posterolateral Triangle
Identify the long thoracic nerve, suprascapular 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.