2.) 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 2. Anterior scalene m.

3.) Locate the phrenic n. on the anterior scalene m.

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 3. Phrenic n.

RIGHT SIDE: Identify subclavian a. and branches

Find these structures:

• • Subclavian a.

    • 1st part

      • Vertebral a.

      • Thyrocervical trunk

        • Inferior thyroid a.

        • Ascending cervical a.

        • Transverse cervical a.

        • Suprascapular a.

      • Internal thoracic a.

    • 2nd part

      • Costocervical trunk

    • 3rd part

      • Dorsal scapular a.

  • Axillary a.

4.) 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.

5.) 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 4. 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)

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

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

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

Photo 5. Subclavian a., 1st part

Photo 6. 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 7. 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 8. Subclavian a., 3rd part

Find these structures:

• • Brachial plexus

    • Roots (C5-T1)

      • Dorsal scapular n. (C5)

      • Long thoracic n. (C5,6,7)

    • Trunks

      • Superior trunk

        • Suprascapular n. (C5,6)

      • Middle trunk

      • Inferior trunk 

  • Middle scalene m.

  • Posterior scalene m.

6.) Identify the roots (rami) and trunks of the brachial plexus.

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 sternocleidomastoid mm.). 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 9. Brachial plexus

Note: The dorsal scapular n. branches from VPR of C5, typically piercing the middle scalene m., to innervate the rhomboid mm. and portions of levator scapulae m.

Note: The long thoracic n. forms from branches of VPRs of C5-C7; it innervates the serratus anterior m. and is unique in its course superficial to the muscle.

Photo 10. Dorsal scapular and long thoracic nn.

Note: The suprascapular n. is typically a branch of the superior trunk of the brachial plexus but can branch more proximally; it efferently innervates the supra- and infraspinatus mm.

7.) Clean the connective tissues from the brachial plexus and middle and posterior scalene mm.

Photo 11. Suprascapular n.

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 12. Middle & posterior scalene mm.

LEFT SIDE: Identify the divisions, cords, and terminal branches of the brachial plexus

Find these structures:

• • Brachial plexus

    • Divisions

    • Cords

      • Medial cord

        • Ulnar n. (C7, C8, T1)

        • Contribution to median n.

      • Lateral cord

        • Musculocutaneous n. (C5, C6, C7)

        • Contribution to median n.

      • Median n. (C6, C7, C8, T1)

      • Posterior cord

        • Axillary n. (C5, C6)

        • Radial n. (C5, C6, C7, C8, T1)

8.) Identify the divisions of the brachial plexus.

Note: Each trunk of the brachial plexus divides into anterior and posterior divisions. The superior trunk typically divides just above the clavicle, while the middle and inferior trunks divide deep to the clavicle. Posterior divisions tend to be larger than their anterior counterparts for the superior and middle trunks. The posterior division of the inferior trunk is typically smaller (and sometimes absent) than the anterior division.

Photo 13. Divisions of brachial plexus

10.) Find the bifurcation of the radial and axillary nn. Clean and identify the radial n.

Note: The radial n. (C5-T1) is the largest (in diameter) nerve of the upper limb, and is the terminal branch of the posterior cord. The radial n. efferently innervates muscles of the posterior compartments of the arm and forearm, and afferently innervates the skin of the posterior arm, forearm, and hand.

Photo 15. Radial n. course

11.) Clean the axillary n.

Note: The axillary n. (C5-C6) branches from the posterior cord. It efferently innervates the deltoid and teres minor mm., and afferently innervates the skin of the lateral shoulder.

Photo 16. Axillary n. course

12.) Clean and identify the median n.

Note: The median n. is formed by the coalescence of the lateral contribution/root (C6-C7) from the lateral cord and the medial contribution/root (C8-T1) from the medial cord. The median n. efferently innervates muscles in the anterior (flexor) compartment of the forearm, as well as muscles in the thenar compartment of the hand. This nerve also afferently serves the skin of the lateral half of the palm and lateral 3.5 digits and nail beds.

Photo 17. Median n. course

13.) Clean and identify the ulnar n.

Note: The distal continuation of the medial cord is the ulnar n. (C7-T1). The ulnar n. efferently innervates portions of the anterior (flexor) compartment of the forearm and the majority of intrinsic hand muscles. This nerves afferently innervates the skin of the medial hand and wrist, as well as the medial 1.5 digits.

Photo 18. Ulnar n. course

14.) Clean and identify the musculocutaneous n.

Note: The musculocutaneous n. (C5-C7) is a terminal branch of the lateral cord, and efferently serves the anterior (flexor) compartment of the arm. It also provides afferent innervation for the skin of the lateral forearm.

Photo 19. Musculocutaneous n. course

Photo 20. Brachial plexus