Lab 21 - Neck & Carotid Sheath
|Suggested readings from
Gray's Anatomy for Students, 2nd ed.
- Ch. 8, 947-984
|Suggested readings from Langman's Medical Embryology:
- 11th ed - Ch. 16 (Head & Neck): pp. 265-287
12th ed - Ch. 17 (Head & Neck): pp. 260-282
Primary Lab Objectives and Goals:
Locate the surface anatomical landmarks on the neck.
Remove the skin from the neck.
Reflect the trapezius muscle, transect and reflect the sternocleidomastoid muscle. Identify the omohyoid and sternohyoid muscles. Underneath these locate the sternothyroid and thyrohyoid muscles.
Identify the ansa cervicalis and examine its branches. Follow its superior limb up to the hypoglossal nerve.
Open the carotid sheath, and identify the vagus nerve, internal jugular vein, and common carotid artery. Locate the recurrent laryngeal branches of the vagus nerve.
Find the cervical sympathetic trunk and ganglia.
Locate the termination of the thoracic duct. Examine the thyroid gland and its blood supply. Attempt to find the parathyroid glands.
Examine the scalene muscles and the structures that run across and pass between them. Pay special attention to the structures that pass between the anterior and middle scalene muscles, which are important landmarks.
Locate the four main branches of the subclavian artery, the internal thoracic, vertebral, costocervical, and thyrocervical branches.
Trace the suprascapular nerve.
1. SURFACE ANATOMY EXERCISE: NECK
- On the cadaver review the boundaries of the "triangles" of the neck:
- The posterior triangle formed by the sternocleidomastoid and trapezius muscles and the clavicle.
- The anterior triangle, formed by the sternocleidomastoid muscle, the mandible, and the midline of the neck.
- The anterior triangle is further subdivided into:
- The submandibular triangle: the mandible, and the anterior and posterior bellies of the digastric muscle.
- The submental triangle: the anterior belly of the digastric muscle, the hyoid bone, and the midline.
- The muscular triangle: the omohyoid and sternocleidomastoid muscles, and the midline.
- The carotid triangle: the sternocleidomastoid and omohyoid muscles, and the posterior belly of the digastric muscle.
- Palpate and mark the location of the hyoid bone, and the thyroid and cricoid cartilages of the larynx, and the trachea.
- Mark with a dotted line the location of the thyroid gland.
- Note the potential locations for creating an emergency airway.
- Locate the points of access to the internal jugular vein: 1) between the two insertions of the sternocleidomastoid muscle (SCM) just above the clavicle, and 2) between the medial edge of the SCM muscle and the thyroid cartilage.
- Locate the point of access to the common carotid artery: between the medial edge of the SCM muscle and the thyroid cartilage.
NOTE: The sternocleidomastoid, trapezius and platysma muscles cover the cervical body wall. Innervation is carried to the platysma (2nd arch) by cervical branches of the facial nerve (CN Vll), and to the sternocleidomastoid and trapezius muscles by the spinal root of the accessory nerve (CN XI) .
2. EXPOSING SUBCUTANEOUS STRUCTURES
NOTE: When removing the skin of the neck, be extremely careful to preserve the cutaneous nerves and veins. They are easily lost!
- Reflect the skin of the ventral surface of the neck, together with the attached platysma muscle, all the way up to the lower border of the lower jaw.
- Note the anterior edge of the trapezius muscle.
- Trace the borders of the posterior triangle: the posterior edge of the sternocleidomastoid muscle, the anterior edge of the trapezius muscle, and the clavicle.
- Break (carefully!) through the fascia of the posterior triangle
- Clear the sternocleidomastoid muscle free from the underlying fascia.
- Locate the branches of the cervical plexus
- Identify the accessory nerve (CN XI).
NOTE: you can find the accessory nerve running obliquely back from sternocleidomastoid to trapezius.
NOTE: The deeper layer of fascia in the neck surrounds the "rectus cervicis" or the infrahyoid muscles (sternohyoid, sternothyroid, thyrohyoid, and omohyoid muscles) in front, and covers the epaxial muscles in back. It may be thought of as representing the surface of the cervical body wall. Across the "fascial carpet" of the posterior triangle streams cutaneous branches of the cervical plexus - sensory nerves equivalent to the lateral cutaneous branches of a typical spinal nerve. Note that these nerves may pierce the overlying platysma, sternocleidomastoid and trapezius muscles without innervating them.
3. THE TRAPEZIUS AND STERNOCLEIDOMASTOID MUSCLES
- Reflect the trapezius muscle from the clavicle and reflect it carefully
- Trace some branches of the accessory nerve (CN IX) into the belly of the trapezius muscle itself..
- Identify the transverse cervical artery and its accompanying vein.
NOTE: The transverse cervical artery is a branch of the thyrocervical trunk of the subclavian artery. Sometimes it may arise as a cervicodorsal trunk (which gives rise to the dorsal scapular and transverse cervical arteries). It supplies the trapezius muscle. If you find the vessel on trapezius, you can trace it back over the scalene muscles to its origin.
- Trace the transverse cervical artery back over the shoulder along the fascia of the cervical "body wall," until it reaches the edge of the levator scapulae muscle.
- Note that the transverse cervical artery splits into the deep and superficial branches around the levator scapulae muscle (The deep branch may arise separately, from the subclavian artery).
- Transect the sternocleidomastoid muscle and carefully reflect it, noting its attachments, which are implied by its name.
Remember that the sternocleidomastoid muscle separates the posterior and anterior triangles of the neck. With the sternocleidomastoid muscle reflected, the infrahyoid muscles are revealed, which you will dissect next.
- Complete the removal of all of the skin from the neck.
- Identify the sternohyoid muscle running between the sternum and hyoid.
- Identify the inferior belly of the omohyoid muscle lateral to the sternohyoid muscle
- Trace the superior belly of the omohyoid muscle up to the hyoid bone; note that its fascial attachment to the clavicle causes the muscle to bend on its course from the scapula to the hyoid, dividing it into inferior and superior bellies.
- Transect and reflect the sternohyoid muscle.
- Identify the sternothyroid muscle beneath.
- Trace the sternothyroid muscle to the thyroid cartilage of the larynx.
- Distinguish the thyrohyoid muscle running from the thyroid cartilage to the hyoid.
The four infrahyoid muscles (the sternohyoid, sternothyroid, thyrohyoid, and omohyoid muscles), together with the geniohyoid muscle (further up above the hyoid bone) are the cervical equivalent of the rectus abdominis muscle. They are innervated, like all hypaxial muscles, by ventral rami.
4. THE ANSA CERVICALIS
The motor branches to the "rectus cervicis" muscles (infrahyoid muscles) form the cervical plexus, connecting the C.1- C.3 ventral rami. The superior root of the ansa cervicalis, comes from C.1, and travels for a short distance with the hypoglossal nerve (CN XII). The ansa cervicalis surrounds the carotid sheath. The superior root of the ansa cervicalis crosses the facial vein near where it empties into the internal jugular vein. The ansa cervialis lies on top of the carotid sheath, which is a tube of condensed areolar tissue surrounding the common carotid artery, the accompanying internal jugular vein, and the vagus nerve.
- Remove the facial vein if it is still present.
- Identify the hypoglossal nerve (CN XII) just below the inferior border of the mandible.
NOTE: the hypoglossal nerve can be found angling forward along the lower border of the posterior belly of the digastric. It passes behind this muscle and the stylohyoid muscle to get to the tongue.
- Locate the superior limb of the ansa cervicalis splitting off from the hypoglossal nerve, and follow it down.
- Display the fascial carotid sheath around the great vessels by reflecting the previously transected sternocleidomastoid muscle. You may need to transect and reflect the superior belly of the omohyoid muscle as well.
- As you reflect the hypaxial muscles, notice motor fibers entering them from the ansa cervicalis.
5. THE CAROTID SHEATH AND RECURRENT LARYNGEAL NERVE
- Open the carotid sheath.
- Identify the vagus nerve between the internal jugular vein and the carotid artery (either common or internal carotid depending on the level).
- Locate the recurrent laryngeal nerve.
NOTE: the recurrent laryngeal nerves will be medial to the contents of the carotid sheath, running up the trachea on either side.
- Recall that on the RIGHT side, the laryngeal nerves recurs around the subclavian artery, whereas on the LEFT side, it recurs around the ligamentum arteriosum.
NOTE: The vagus runs down the neck in the carotid sheath and then into the thorax. In the neck and upper thorax it has limited proximity to the esophagus and trachea, thus it innervates the upper esophagus and the trachea by its recurrent laryngeal branch.
6. THE CERVICAL GANGLIA AND SYMPATHETIC TRUNK
- Note the recurrent laryngeal nerve running in the groove between the trachea and the esophagus.
- Pull the trachea, esophagus, and the contents of the carotid sheath to one side, thus exposing the prevertebral fascia.
- At the level of C.2-C.3, look for the superior cervical ganglion.
- Between C.2-C.3, locate the rather tenuous cervical part of the sympathetic trunk.
NOTE: the superior cervical ganglion is a large ganglion, an inch or longer. It is deep in the neck.
NOTE: The sympathetic trunk in the neck gives off gray communicating rami, but receives no white communicating rami. Therefore, there is no sympathetic outflow from the cervical segments of the spinal cord. The trunk ends in the superior cervical ganglion at about the level of the second cervical nerve. From this uppermost sympathetic ganglion, post-ganglionic sympathetic fibers follow the internal carotid artery into the braincase, contributing to a carotid plexus on the artery's surface.
7. THE THYROID AND PARATHYROID GLANDS
- Transect the sternothyroid muscle to display the thyroid gland.
- Locate the paired or fused inferior thyroid veins leading from the gland to the brachiocephalic vein(s).
- Trace and then remove the thyroid veins.
- Trace the superior and inferior thyroid arteries back to their parent vessels.
NOTE: the superior thyroid artery is a branch of the external carotid. The inferior thyroid artery is a branch of the thyrocervical trunk.
- Transect and reflect the internal jugular vein to display the thyrocervical trunk.
- Note that the internal jugular and subclavian veins lie superficial to their companion arteries, as veins generally do.
- Look for the ending of the thoracic duct into the subclavian vein on the left.
- Cut the thyroid gland at the isthmus and reflect it laterally on both sides.
- Try to distinguish the two parathyroid glands on the deep surface of the thyroid gland (dark, pea-sized ovals that can be found by tracing their arterial supplies)
- Remove the thyrohyoid muscle on one side, leaving the cricothyroid muscle intact (which is inferior to the thyrohyoid).
- Note the motor nerve to the thyrohyoid coming from the ansa cervicalis, via the hypoglossal nerve (CN XII).
- Trace the two paired motor nerves to the muscles of the larynx:
- the external branches of the superior laryngeal nerves from above, and
- the recurrent laryngeal nerves from below.
8. SCALENE MUSCLES
- Cut and reflect the inferior belly of the omohyoid muscle.
- Leave intact the omohyoid fascia that attaches this two-bellied muscle to the clavicle.
- Note the ventral rami of C.5- C.8 deep to the omohyoid muscle as they pass under the clavicle to form the brachial plexus.
- Identify the anterior and middle scalene muscles.
- Note the phrenic nerve running down the surface of the scalenus anterior muscle.
- Follow the brachial plexus up the neck until you see it emerging between the anterior and middle scalene muscles, together with the subclavian artery.
NOTE: The three scalene muscles (anterior, middle, and posterior) arise from the transverse processes of all the cervical vertebrae except the atlas, and insert into the first rib. Scalenus posterior is actually just a poorly differentiated bundle of the scalenus medius that makes it to the second rib, and is awarded a separate name. These three remnants of the lateral cervical body wall are innervated by the segmental branches of neighboring ventral rami.
9. THE SUBCLAVIAN ARTERY
- Locate the four branches that leave the subclavian artery before it passes through the scalene "body wall":
- The internal thoracic artery (passing caudally to run on the dorsal surface of the rib cage)
- The vertebral artery (passing cranially to run in the cervical transverse foramina)
- The small costocervical trunk (giving off the first intercostal artery and a deep cervical branch)
- The thyrocervical trunk (giving off the inferior thyroid, transverse cervical, and suprascapular arteries).
- Determine the relationships of the transverse cervical and the suprascapular arteries to the phrenic nerve.
- Determine the relationships of the transverse cervical and the suprascapular arteries to the anterior scalene muscle.
- Demonstrate the venous tributaries (i.e., transverse cervical and suprascapular veins) and course of the subclavian vein here.
- Note especially that the subclavian vein passes in front of the scalenus anterior muscle while the subclavian artery passes behind.
NOTES: The upper part of the neck receives blood from the branches of the external carotid artery. The lower part of the neck is supplied by the branches of the subclavian artery. The subclavian artery leaves the thorax by curving up over the top of the dome of the pleura, and down over the top of the first rib. In so doing, it passes between the anterior and the middle scalene muscles, together with the roots of the brachial plexus.
The deep cervical branch of the costocervical trunk anastomoses with the occipital artery to form an alternative pathway for blood to reach the head.
The veins draining the neck correspond roughly to the arteries; but the internal jugular vein drains most of the territory supplied by both the internal and the external carotid arteries. The smaller external jugular vein receives blood from the scapular region (corresponding to the transverse cervical and suprascapular branches of the subclavian artery: the thyrocervical trunk) and from the superficial veins of the face and scalp.
10. THE SUPRASCAPULAR NERVE
- Again, follow the brachial plexus up the neck until you see it emerging between the anterior and middle scalene muscles, together with the subclavian artery.
- Trace the suprascapular nerve (accompanied by the suprascapular artery on its way to supraspinatus and infraspinatus muscles) as it branches from the plexus and runs back across the neck parallel to the transverse cervical artery.
- Remember that the phrenic nerve arises from the ventral rami of C.3- C.5, and travels caudally to innervate hypaxial muscle (the diaphragm).
NOTE: The suprascapular, transverse cervical, and dorsal scapular arteries originate very close together on the subclavian artery, while their terminal branches anastomose around the scapula.
Blocking of the arteries of the head
Obstruction of the arterial flow to the brain by compression of the common carotid arteries within the carotid sheaths can cause oxygen deprivation to the brain and may eventually lead to death. The compression of the carotids can lead to loss of consciousness within a few seconds due to anoxia of the brain.
Whiplash is also known as “flexion-extension syndrome,” which is caused by the rapid change in motion from hyperextension of the head and the upper part of the neck to hyperflexion in rear-end collisions, or from extreme flexion to maximal extension in head-on collisions. The tissue damage in the neck ranges from:
- muscle strain, or sprain of the cervical ligaments;
- rupture of the ligaments and the intervertebral disks;
- fractures of the vertebrae;
- injury to the cervical spinal cord and emerging cervical nerves;
- and rarely, damage of the vertebral artery with resulting interference with cerebral circulation.
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