2.) Locate the sternocleidomastoid mm. and the superior parts of the trapezius mm. Palpate the inferior border of the mandible and the clavicles. Using these muscles and skeletal landmarks, identify the posterior cervical triangle.

Note: The posterior cervical triangle is bounded anteriorly by the posterior border of the SCM, posteriorly by the superior part of the trapezius, and inferiorly by the clavicle.

Photo 1. Posterior cervical triangle

3.)  Identify the two sub-triangles within the posterior cervical triangle.

Note: The posterior cervical triangle may be subdivided into two triangles. Both triangles are bordered anteromedially by the sternocleidomastoid m., posterolaterally by the upper fibers of the trapezius m., and these triangles are divided from one another by the inferior belly of the omohyoid m.  They are the:

• occipital triangle: the superior-most triangle; typically contains the nerve point of the neck (and its branches) and the accessory n. (CN XI).

• subclavian (omohyoid) triangle: the inferior-most triangle; typically contains the subclavian v. and supraclavicular lymph nodes.

Photo 2. Posterior cervical triangle subdivisions

4.) Reflect the sternocleidomastoid m. from its attachment to the sternum and clavicles. Reflect the SCM superiorly and posteriorly to see deeper structures.

Note: Trapezius and SCM muscles share a common efferent (motor) innervation, the accessory n. (CN XI). The accessory n. is strictly efferent; therefore, afferent supply to these muscles comes from ventral primary rami of cervical nerves (C2-C4). Typically, C2-C3 (sometimes C4) afferently serve the SCM, whereas C3-C4 afferently serve the trapezius. It is likely that C2-C4 also carry some motor fibers.

Photo 3. Accessory n. (CN XI)

Examine the structures of the root of the neck

Note: The root of the neck (RON) is the nexus between the neck, thorax, and upper limbs. The RON is the proximal attachment site for many neck muscles and transmits important neurovasculature (common carotid aa., jugular vv., subclavian aa. & vv., vagus nn., trunks of the brachial plexus).  

Find these structures:

• Thoracic duct

• Arch of aorta

  • Brachiocephalic trunk

    • Right subclavian a.

    • Right common carotid a.

  • Left common carotid a.

  • Left subclavian a. 

• Subclavian a.

  • 1st PART

    • Vertebral a.

    • Thyrocervical trunk

      • Inferior thyroid a.

      • Ascending cervical a.

      • Suprascapular a.

      • Transverse cervical a.

    • Internal thoracic a.

  • 2nd PART

    • Costocervical trunk

      • Deep cervical a.

      • Highest (supreme) intercostal a.

  • 3rd PART

    • Dorsal scapular a.

• Axillary a.

5.) Cut the left brachiocephalic v. at its midpoint. Reflect both ends of the left brachiocephalic vein (and their tributaries) laterally to expose the arch of the aorta and its branches.

Note: Depending on your donor (and manner of how the donor’s head & neck were harvested), you may or may not have a complete arch of the aorta. Nearly all donors will include at least the superior portion of the arch of the aorta, but some may only have the superior portion of the arch.

Note: The arch of the aorta (aortic arch) connects the ascending aorta to the descending aorta. Typically, three branches - brachiocephalic trunk, L. common carotid a., & L. subclavian a. - originate from the arch of the aorta to supply blood to the head, neck, upper limbs, and thorax.

Photo 4. Arch of aorta and branches

At this point in the dissection, we will perform specific dissections on the left versus the right side of the donor. Generally, the dissection plan for the RIGHT SIDE includes locating the three parts of the subclavian artery (and their branches) and the brachial plexus, whereas the plan for the LEFT SIDE includes locating the thoracic duct and the anterior scalene muscle, and preserving the classic anatomical relationships of the anterior scalene muscle.

LEFT SIDE

6.) Gently clean the fascia from the L. brachiocephalic v. at its division into left IJV and subclavian v. In this vicinity, locate the thoracic duct.

Note: The venous angle is the confluence of the internal jugular v. and the subclavian v.

Photo 5. Thoracic duct & left venous angle

7.) Reflect the distal portion of the L. brachiocephalic v. (along with the origins of the IJV and subclavian v.) laterally (beyond the first rib) to expose the vicinity of the proximal portion of the L. subclavian a.

8.) Locate and carefully clean the anterior scalene m. Be sure to not remove the phrenic n. on the anterior scalene m.

Note: The (anterior, middle, posterior) scalene muscles may either act to weakly flex the neck, or serve as accessory muscles of respiration (by slightly elevating the first two ribs, thus expanding the volume of the thoracic cavity). The anterior and middle scalene mm. attach to the first rib, and the posterior scalene m. attaches to the second rib.

Note: The anterior scalene m. is an important anatomical landmark for understanding the neurovasculature of the root of the neck. There are four classic anatomical relationships to understand:

1. the anterior scalene m. is sandwiched between the subclavian v. (anterior) and subclavian a. (posterior),

2. the anterior scalene m. establishes the borders for the divisions of the subclavian a.,

3. the anterior scalene m. is immediately posterior to (and closely associated with) the phrenic n. as the phrenic n. descends the neck and the root of the neck, and

4. the anterior scalene m. is separated from the middle scalene m. by the roots (ventral primary rami of spinal nn.) and trunks of the brachial plexus and subclavian a.

Photo 6. Anterior scalene m.

9.) Follow the phrenic n. superiorly to its roots in the cervical plexus.

Note: The phrenic n. (C3,4,5) descends from the cervical plexus through the root of the neck (just anterior to, and upon the anterior scalene m.) before entering the thorax between the subclavian a. & v. The phrenic n. is efferent (motor) and afferent (sensory) to the diaphragm and afferent (sensory) to the pericardium and diaphragmatic pleura.  

Photo 7. Phrenic n.

RIGHT SIDE

10.) Laterally reflect the R. subclavian v. from the right venous angle to beyond the lateral edge of the 1st rib, and clean the connective tissue to reveal the anterior scalene m., the R. subclavian a., and the 1st rib.

Note: You may be able to find the right lymphatic duct (which typically returns lymph to the vicinity of the right venous angle) at this point.

11.) Detach the anterior scalene m. from the first rib, and gently reflect the muscle superiorly. Maintain the neurovasculature (e.g. thyrocervical trunk and phrenic n.) that are associated with the anterior scalene m.

Note: The subclavian a. is conceptually divided into three parts, with respect to the vessel’s relationship to the anterior scalene m. The first part of the subclavian a. is found medial to the anterior scalene, the second part of the subclavian a. is posterior to the anterior scalene, and the third part is lateral to the anterior scalene.

Photo 8. Right subclavian a. (showing all three parts)

Note: The first part of the subclavian a. typically hosts three major branches:

• vertebral a. (supplies the brain & spinal cord)

• thyrocervical trunk,

  • inferior thyroid a. (supplies the thyroid and parathyroid glands)

  • ascending cervical a. (supplies deep muscles of the neck)

  • suprascapular a. (supplies supra- and infraspinatus mm.)

  • transverse cervical a. (supplies trapezius m. & often gives rise to the dorsal scapular a.)

• internal thoracic a. (supplies anterior thoracic & abdominal walls & the diaphragm)

Photo 9. Subclavian a., 1st part

Photo 10. Subclavian a., thyrocervical trunk

Note: The second part of the subclavian a. typically hosts one major branch, the:

• costocervical trunk

  • deep cervical a. (supplies deep neck muscles)

  • highest (supreme) intercostal a. (supplies 1st & 2nd intercostal spaces)

Photo 11. Subclavian a., 2nd part

Note: The third part of the subclavian a. typically hosts one major branch, the:

• dorsal scapular a. (supplies rhomboid mm. and levator scapulae m.). The dorsal scapular a. may also be a branch of the transverse cervical a.

Note: Beyond the lateral border of the first rib, the subclavian a. transitions into the axillary a.

Photo 12. Subclavian a., 3rd part

Find these structures:

• Brachial plexus

  • Roots (VPR of C5,6,7,8,T1)

  • Trunks (Superior, Middle, Inferior)

11.) Identify the roots (rami) and trunks of the brachial plexus. Clean the connective tissues from the brachial plexus and middle and posterior scalene mm.

Note: The brachial plexus consists of contributions from the ventral primary rami (VPR) of cervical spinal nerves (C5,6,7,8,) & a thoracic spinal nerve (T1) that are responsible for innervation of the upper limb and pectoral girdle (excluding trapezius and SCM). The brachial plexus may also include contributions from C4 & T2. The roots (ventral primary rami) and trunks of the brachial plexus may be found between the anterior and middle scalene mm.

Note: The trunks of the brachial plexus are named according to their relative anatomical positions. The superior trunk typically consists of coalescing ventral primary rami (VPR) of C5 & C6. The middle trunk is typically the continuation of the VPR of C7. The inferior trunk typically consists of coalescing VPRs of C8 & T1.

Photo 13. Brachial plexus

Note: The scalene mm. attach transverse processes of cervical vertebrae to the first (anterior and middle scalene) and second (posterior scalene) ribs. The scalene mm. can either laterally flex the neck, or serve as accessory muscles of respiration by elevating the first and second ribs.

Note: The middle scalene m. is typically pierced by the dorsal scapular n. and fibers of the long thoracic n.

Photo 14. Middle & Posterior scalene mm.

Trace the vagus nerves (CN X) from the neck to the thorax, and locate vagus branches - BILATERAL

Find these structures:

• Vagus n. (CN X) 

  • Superior laryngeal n.

    • External br.

    • Internal br. 

  • Recurrent laryngeal n.

12.) Locate the vagus n. in the carotid sheath. Near the hyoid bone, the superior laryngeal n. branches from the vagus n. Follow the superior laryngeal n. to find its external and internal branches.

Note: The vagus n. (CN X) is the major parasympathetic conduit to the thorax and abdomen, and is the major innervation to muscles of the larynx and pharynx, and aspects of the head.

Note: The R. and L. vagus nn. take different routes into the thorax. Both nerves descend the neck within the carotid sheaths, and cross anteriorly over the subclavian aa., deep to the first ribs. The R. vagus n. then sends a major branch (the R. recurrent laryngeal n.) inferiorly and then posteriorly around the R. subclavian a., lateral to the trachea, on a course for the larynx. The L. vagus n., as seen in previous dissections, sends the L. recurrent laryngeal n. inferiorly and then posteriorly around the concavity of the arch of the aorta, lateral to the trachea, also on a course for the larynx.  

Note: The superior laryngeal n. branches from the vagus posteromedially to the internal carotid a. Superior to the larynx, the superior laryngeal n. divides into external and internal branches. The external branch is efferent (motor) supply to the cricothyroid m., whereas the internal branch is the afferent (sensory) to the mucosa of the larynx at and superior to the vocal folds. The internal branch is accompanied by the superior laryngeal a. as it pierces the thyrohyoid membrane.  

Photo 15. Superior laryngeal n.

13.) Follow both vagus nn. inferiorly to the root of the neck. Find both recurrent laryngeal nn.

Note: The vagus nn. (CN X) pass through the root of the neck between the subclavian v. and the subclavian a.  The L. recurrent laryngeal n. branches from the L. vagus n. in the thoracic cavity, then loops posteriorly around the arch of the aorta. The R. recurrent laryngeal n. branches from the R. vagus n. In the root of the neck, then loops posteriorly around the R. subclavian a.

Note: Upon their ascent back into the neck, recurrent laryngeal nn. pass posteriorly to the subclavian aa., then laterally straddle the trachea. The recurrent laryngeal n. is the efferent (motor) supply to all intrinsic laryngeal mm. (except cricothyroid m., which is served by the external br. of the superior laryngeal n.) and is both afferent (sensory) and secretomotor to the mucosa below the vocal folds.

Photo 16. Recurrent laryngeal nn.

Prepare the posterior neck for deep dissection

14.) Reflect the trapezius m. (unless otherwise noted, when reflecting, use blunt dissection techniques to separate the muscle from deeper structures):

a.) Incise along proximal attachments of the trapezius m. (spinous processes of the vertebrae and the ligamentum nuchae). Lift the trapezius m. off of the deep neck muscles.

b.) Reflect the trapezius m. superolaterally.

Photo 17. Procedural: trapezius muscle incisions

Photo 18: Procedural: trapezius m., reflected

Perform a complete cervical laminectomy

15.) On each side of the spinous processes, move all of the superficial back musculature (rhomboid mm.) laterally.

Photo 19. Procedural: superficial back musculature, reflected

16.) Using sharp dissection, perform two mirroring parasagittal incisions as close as possible to the midline. Begin superiorly with two incisions just lateral to the external occipital protuberance, continue lateral to the spinous processes of all vertebrae, and end at the inferior extent of your donor.

Photo 20. Procedural: parasagittal incisions

17.) Pull and reflect the splenius mm., erector spinae mm., and serratus posterior superior mm. laterally 4-5 cm. This deep musculature may be removed if the muscles will not stay in a reflected position.

Photo 21. Deep back musculature

Photo 22. Procedural: posterior cervical laminae

18.) Remove remaining musculature from the laminae with a chisel, which will allow clean and precise cuts for the laminectomy.

Photo 23. Procedural: laminae, prepped for laminectomy

19.) Make two cuts per vertebra. Cuts should be on the lateral-most extent of the lamina before the transition to the transverse processes at an angle. Please refer to the photos and notes below to better understand this procedure.

Notes:

• Be sure to wear the appropriate personal protective equipment (PPE) for any type of bone cutting. This includes protective eyewear. When using an autopsy saw, a combination of protective eyewear and face mask should be worn.

• Be very deliberate with cuts, and refer to articulated skeletal specimens in the laboratory before and during this stage of the dissection.

Photo 24. Procedural: laminae incisions

Photo 25. Procedural: laminae incisions

Observe the meninges and examine the spinal cord

Find these structures:

• Meninges

  • Dura mater

  • Arachnoid mater

  • Pia mater

• Spinal cord

  • Gray matter

  • White matter

  • Cervical enlargement

20.) After completing all cuts, remove the posterior wall of the vertebral column. You may widen the window into the vertebral canal by trimming any remaining laminae with bone shears. Allow enough room to view the spinal cord and meninges in their entirety.

Photo 26. Dura mater, intact

21.) Make a shallow midline incision using scissors through the dura mater. Stay in the midsagittal plane, and avoid incising too laterally to protect rootlets and roots of spinal nn.

Photo 27. Procedural: midsagittal incision

Photo 28. Dura mater, reflected

22.) Observe the arachnoid mater. This may appear as thin wisps between the dura mater and pia mater.

Photo 29. Arachnoid mater

Note: Dorsal rootlets should be evident at each spinal cord level, extending inferolaterally and coalescing into dorsal roots. Please feel free to observe these features, but you are not responsible for finding them.

Photo 30. Dorsal rootlets and roots

23.) Observe the pia mater. It is translucent on the spinal cord, and it is very difficult to separate from the cord.

Photo 31. Pia mater

24.) Observe the various structures and landmarks of the spinal cord. The cervical enlargement of the spinal cord extends from C4-T1 segments, which roughly corresponds to the origin of the brachial plexus (C5-T1).

Photo 32. Cervical enlargements

25.) Transect the spinal cord, and examine the spinal cord in cross section. Try to distinguish between the grey and white matter. This is often difficult to distinguish in embalmed cadavers.

Photo 33. Grey and white matter

Optional Experience (not on laboratory practical)

A.) At one (or more) spinal cord level, expose the dorsal root ganglion, trunk of the spinal n., and dorsal and ventral primary rami of the spinal n. This is completed by following the dorsal root through the dural root sheath.

B.) Additional bone (transverse process of the vertebrae) may need to be removed using bone shears to identify structures located within and lateral to the intervertebral foramen.

C.) At the level of the intervertebral foramen, the dorsal root ganglion will be visible as a bulge on the dorsal root.

D.) Continue to follow the spinal nerve distally to see the nerve bifurcate into the dorsal and ventral primary rami.

Note: Ventral primary rami (VPR) and dorsal primary rami (DPR) are mixed divisions of a spinal nerve. Not visible at this time are the rami communicantes (branches to and from the sympathetic chain). VPR serve anterior and lateral aspects of the trunk and limbs, whereas DPR serve deep muscles of the back and skin of the back. VPR are larger than DPR and typically form named nerves.

Photo 34. Spinal nerve

Photo 35. Spinal nerve