Dissector Answers - Posterior Triangle of the Neck

Learning Objectives:

Upon completion of this session, the student will be able to:

1. Identify the boundaries of the posterior cervical triangle and its subdivisions.
2. Identify the scalene muscles and the first rib and relate them to the neurovascular structures at the root of the neck.
3. Identify and list the parts and branches of the subclavian artery and vein, and describe their course in the neck.
4. Identify the neurovascular entities that have different relationships to structures on the right and left sides of the root of the neck.
5. In the root of the neck, locate the vagus and phrenic nerves and describe their relationships to the organs, fascia, vessels, and viscera of the neck.
6. Identify the deep cervical lymph nodes and explain their significance.

Learning Objectives and Explanations:

1. Identify the boundaries of the posterior cervical triangle and its subdivisions. (WB 187, 3-1; N28, TG7-02A, TG7-02B)

Border:	Formed by:
anterior	posterior border of the sternocleidomastoid muscle (SCM)
posterior	anterior border of the trapezius
inferior	middle third of the clavicle between the trapezius and SCM
apex	where the SCM and trapezius meet on the superior nuchal line of the occiptal bone
roof	superficial layer of deep cervical fascia
floor	formed by the levator scapulae, middle scalene, and posterior scalene, which are all covered by the prevertebral layer of deep cervical fascia

The posterior cervical triangle is subdivided into the following triangles by the inferior belly of the omohyoid muscle:

• Occipital triangle, whose contents are:
  • part of external jugular vein
  • posterior branches of cervical plexus of nerves
  • accessory nerve
  • trunks of brachial plexus
  • transverse cervical artery
  • cervical lymph nodes
• Subclavian triangle (aka omoclavicular), whose contents are:
  • third part of subclavian artery
  • part of subclavian vein
  • suprascapular artery
  • supraclavicular lymph nodes

2. Identify the scalene muscles and the first rib and relate them to the neurovascular structures at the root of the neck. (N30, N32, N33, N186, N192, N194, N429, TG7-15A, TG7-15B, TG7-17, TG7-18)

For orientation, it is good to know the origins and insertions of the scalenes relative to the rib:

• The anterior scalene muscle descends inferolaterally from the transverse processes of C3 through C6 to the scalene tubercle on the 1st rib.
• The middle scalene muscle descends inferolaterally from the transverse processes of C2 through C7 to the upper surface of the 1st rib.
• The posterior scalene muscle passes from the transverse process of C5 through C7 to the lateral surface of the 2nd rib.

Nerves and their relations to the scalene muscles and 1st rib:

• The relation of the scalene muscles to nerves mainly occurs between the anterior and middle scalene muscles. In the space between these two muscles, known as the interscalene triangle:
  • C3, C4, and C5 spinal nerves emerge and give off their contributions to the phrenic nerve, which runs inferiorly on the anterior scalene muscle
  • inferior to the three aforementioned spinal nerves, the roots of the brachial plexus (C5 through T1) emerge as well
• Another nerve that has a specific relation with the scalene muscles is the vagus nerve while it is contained in the carotid sheath. Plates N31, N32, TG7-14, TG7-20 show the vagus nerve descending on the origin of the anterior scalene muscle.
• Other relevant nerves are the dorsal scapular nerve and long thoracic nerve, both of which pass through the middle scalene muscle after branching off of the roots of the brachial plexus (C5 for dorsal scapular, C5-7 for long thoracic).

Arteries and veins and their relations to the scalenes and 1st rib:

• The most important relations to notice are those of the subclavian artery and vein. The subclavian arteries are separated from the veins by the anterior scalene muscle, with the artery being posterior to the muscle and the vein anterior to it. Both of these vessels start posterior to the sternoclavicular joint and parallel each other as they pass inferior to the clavicle and superior to the 1st rib to become the axillary artery/vein. Since both vessels rest on the 1st rib, there are grooves that mark their positions on the rib.
• As for the branches/tributaries of the subclavian artery/vein, three branches of the thyrocervical trunk (1st part of subclavian) rest on the anterior scalene muscle as they travel to their destinations. These branches are the transverse cervical artery, suprascapular artery, and ascending cervical artery (actually a branch of inferior thyroid artery, the third branch of the thyrocervical trunk). Veins parallel these arteries and ultimately reach the subclavian vein. (N32, TG7-15A, TG7-15B)

3. Identify and list the parts and branches of the subclavian artery and vein, and describe their course in the neck. (N33, N70, N74, N238, TG7-15A, TG7-15B)

The subclavian artery arises superiorly from the brachiocephalic trunk (right side) or the aortic arch (left side) and travels between the anterior and middle scalene muscles. It then travels inferiorly between the clavicle and first rib to enter the pectoral region, where it becomes the axillary artery. The subclavian artery is divided into three parts by the anterior scalene muscle (this is similar to the pectoralis minor dividing the axillary artery into three parts):

• The first part of the subclavian artery is medial to the anterior scalene muscle and gives off the following branches:
  • vertebral artery - the cervical part of the vertebral artery ascends just medial to the muscles and passes deeply at its apex to course through the foramina of the transverse processes of C1 through C6. The suboccipital part of the vertebral artery courses in a groove on the posterior arch of the atlas before it enters the cranial cavity through the foramen magnum.
  • internal thoracic artery - arises from the anteroinferior aspect of the subclavian artery and passes inferomedially into the thorax. The cervical part of this artery has no branches.
  • thyrocervical trunk - arises from the anterosuperior aspect of the first part of the subclavian artery, just medial to the anterior scalene muscle, and has three branches:
    • inferior thyroid artery
    • transverse cervical artery - sends branches to muscles in the posterior cervical triangle, the trapezius, and medial scapular muscles
    • suprascapular artery
• The second part of the subclavian artery is posterior to the anterior scalene muscle and only has one branch:
  • costocervical trunk - arises from the posterior aspect of the subclavian artery. It passes posterosuperiorly and divides into:
    • superior intercostal artery - supplies first two intercostal spaces
    • deep cervical artery - supplies posterior deep cervical muscles
• The third part of the subclavian artery is lateral to the anterior scalene muscle and only has one branch:
  • dorsal scapular artery - occasionally arises as a branch of the transverse cervical artery. When it is a branch of the subclavian, it passes laterally through the trunks of the brachial plexus, anterior to the middle scalene muscle, and then runs deep to the levator scapulae to reach the scapula and supply the rhomboid muscles.

The subclavian vein begins at the lateral border of the 1st rib as a continuation of the axillary vein and ends when it unites with the IJV, posterior to the medial end of the clavicle (SC joint). The important part of the subclavian vein is that when it meets the IJV, it forms the brachiocephalic vein. This union is known as the venous angle and is the site where the thoracic duct and right lymphatic duct drain their lymph into the venous circulation. The subclavian vein receives the external jugular vein, anterior jugular vein, and vertebral vein. The external jugular vein receives the suprascapular vein and the transverse cervical vein. One thing to note is that even though the inferior thyroid ARTERY is a branch of the thyrocervical trunk of the subclavian artery, the inferior thyroid VEIN drains into the brachiocephalic vein, separate from the other branches of the thyrocervical trunk, which drain into the subclavian vein.

4. Identify the neurovascular entities that have different relationships to structures on the right and left sides of the root of the neck. (N32,N33,N75,N193,N237,N238,N240,N266, TG4-18, TG4-38, TG4-44, TG7-14, TG7-15)

Vagus nerves - The vagus nerves pass anterior to the first part of the subclavian artery and posterior to the brachiocephalic vein and SC joint to enter the thorax. (N31, N32, TG7-13, TG4-45)

Recurrent laryngeal nerves - branches of the vagus nerves - The right recurrent laryngeal nerve loops inferior to the right subclavian artery at approximately the T1 vertebral level. The nerve then ascends in the tracheoesophageal groove to supply all the intrinsic muscles of the larynx, except the cricothyroid (will need to know this later). To better orient yourself, remember that the vagus nerve descends anterior to the subclavian artery and posterior to the vein. The left recurrent laryngeal nerve loops inferior to the arch of the aorta behind the ligamentum arteriosum at approximately the T4/T5 intervertebral disc level. Recurrent nerve ascends in the tracheoesophageal groove to supply all the intrinsic muscles of the larynx, except the cricothyroid (will need to know this later).

Brachiocephalic trunk - This vessel is only found on the right side of the body as it branches into the right common carotid artery and the right subclavian artery. On the left side, there is no brachiocephalic trunk since the left common carotid artery and left subclavian artery directly branch off of the arch of the aorta.

Subclavian arteries - The right subclavian artery arises from the brachiocephalic trunk, posterior to the right SC joint. The first part courses superolaterally, extending between its origin and the medial margin of the anterior scalene muscle. The left subclavian artery arises from the arch of the aorta, ascends through the superior mediastinum and enters the root of the neck posterior to the left SC joint. The subclavian veins both end at their respective SC joints by uniting with the internal jugular vein to form the brachiocephalic veins. The left brachiocephalic vein is longer than the right because it needs to cross the branches of the aortic arch to reach the right brachiocephalic to form SVC.

Common carotid arteries - The right common carotid artery branches off the brachiocephalic trunk and ascends lateral to the trachea. The left common carotid artery branches off of the aortic arch and then ascends lateral to the trachea, thus it is a little longer than the right common carotid artery.

Lymphatic ducts - the right lymphatic duct usually drains to the union of the right subclavian and right internal jugular veins. The thoracic duct wraps around the posterior aspect of the left internal jugular vein before it comes around to drain lateral to the left brachiocephalic vein. (Note: these are the only two lymphatic vessels large enough to be called ducts. Everything else is either a lymph trunk or simply a lymph vessel.)

5. In the root of the neck, locate the vagus and phrenic nerves and describe their relationships to the organs, fascia, vessels, and viscera of the neck. (WB 220; N32, N33, TG7-13 TG7-14)

For a description of the vagus, see Objective 4. The phrenic nerve arises by a large root from C4 and is reinforced by smaller contributions from C3 and C5. It passes out along the lateral border of the anterior scalene muscle and enters the chest along its medial border. It lies behind the prevertebral layer of deep cervical fascia and is crossed by the transverse cervical and suprascapular vessels. At the root of the neck, the phrenic nerve passes between the first portion of the subclavian vein and subclavian artery and in front of the internal thoracic artery and vein.

6. Identify the deep cervical lymph nodes and explain their significance. (N72, N239, N266, TG7-74)

The deep cervical nodes are mostly lateral and posterior to the IJV. The nodes are divided into superior and inferior subgroups at the point where the omohyoid muscle crosses over the IJV. Thus, those deep nodes above this crossing are the superior deep cervical lymph nodes and those below the crossing are the inferior deep cervical lymph nodes. (WB 208) Channels from the inferior deep cervical lymph nodes, also called supraclavicular nodes, join to form the jugular lymphatic trunks, which usually join the thoracic duct on the left side and the right lymphatic duct on the right side (sometimes it will enter the right venous angle directly).

Questions and Answers:

The internal jugular vein originates at the jugular foramen. (N8, TG7-06)

This is occasionally absent. When present, it will lie near the level of the inferior thyroid artery and the cricoid cartilage and the transverse process of C6, just anterior to the vertebral artery. (N130, TG7-15)

Inferior cervical ganglion - In approximately 80% of people, it fuses with the 1st thoracic ganglion to form the large stellate ganglion (a.k.a. cervicothoracic ganglion). It lies anterior to the transverse process of C7and the neck of the 1st rib on each side and posterior to the origin of the vertebral artery.

Middle cervical ganglion - This is occasionally absent. It lies near the inferior thyroid artery at the level of the cricoid cartilage and the transverse process of C6, just anterior to the vertebral artery.

Superior cervical ganglion - This is a huge ganglion that can be confused with the nodose ganglion of the vagus. This is found at the level of C1 and C2. (N130, N208, TG7-15, TG7-95)

Yes, but you may NOT see any white rami communicantes because those are only present from T1 through L2. One white ramus reaches the stellate ganglion, only. (N130, TG7-15, TG7-95)

Both of these arteries should be branches of the thyrocervical trunk which is a branch of the first part of the subclavian artery. Sometimes they arise as a common trunk. Suprascapular vessels travel laterally immediately behind the clavicle, while the transverse cervical arches higher across the posterior triangle. (N33, TG7-15A, TG7-15B)

The thyroid gland as a whole will be anteromedial to the sympathetic trunk as it ascends on the longus colli muscle. The inferior aspect of the thyroid gland should be just medial to the middle cervical ganglion. (N39, N130, TG7-14)

The interscalene triangle is defined as the area between the posterior border of the anterior scalene muscle, anterior border of the middle scalene muscle, and the superior border of the 1st rib. The major structures that are located in this structure are the trunks of the brachial plexus. (N34, TG7-15)

The posterior scalene muscle, since it inserts onto the second rib. Anterior and middle scalene muscles would effectively raise the second rib also, however they insert on the first rib. (N34, N186, TG7-15)