PROCEDURE - Posterior Triangle

1. Turn the donor over to the supine position, taking care that skin flaps of the back do not get excessively twisted.

You may find it helpful in this lab to place a wood block posterior to the shoulders such that the donor head and shoulders are lifted to better expose the neck.

2. Cut through the skin circumferentially just distal to the deltoid muscle and begin reflection of skin superiorly.
    (Figure644) (Netter405)

3. Continue reflecting the flap of shoulder skin over the shoulder girdle working towards the root of the neck till the reflection meets the region of skin previously exposed over the thorax and neck.
    (Figure601) (Netter405) (Photo6096) (Photo6096a)

This skin removal should match up with the skin removed from the anterior thoracic region over pectoralis major and the skin flap of the lateral neck.

At this point, the skin should be fully reflected away from the shoulder and only connected via skin attachments at the lateral side of the skull/neck.

4. As you remove the skin you may observe a network of veins running under the dermis (i.e. in the hypodermis).
    (Netter405)

These are the superficial venous system of the limb.

Limbs have a primary artery deep to musculature paralleled by a deep vein. In the upper limb this is the brachial artery and vein.

Limbs also have a superficial venous system in the superficial fascia under the dermis of the skin that does not have acompanying arteries. This network of superficial veins drains into several larger superficial veins that rejoin the deep venous return system near the junction with the body wall.

In the upper limb, the major veins are the cephalic and basilic veins (linked by the median cubital vein which is a common venipuncture target for drawing blood).

There are numerous small perforating veins linking the deep and superficial venous systems, so blockage or ligation of a superficial vein has limited impact on venous return. 

5. Use your fingers to trace the boundaries of the posterior triangle of the neck.
    (Figure604) (Netter036) (Photo6096a)

Run your fingers inferiorly along the posterior border of the sternocleidomastoid muscle, laterally along the clavicle, and then superiorly following the anterior border of the trapezius muscle. Together, these form the boundaries of the posterior triangle of the neck.

The posterior cervical triangle derives its name from the position posterior to the sternocleidomastoid muscle...even though anatomically it is 'lateral' on the neck.

The point halfway from the mastoid to the clavicle along the posterior edge of the sternocleidomastoid muscle is referred to as Erb's point. Deep to this landmark is the upper trunk of the brachial plexus. Thus, penetrating damage at this point can result in a upper brachial plexus injury called "Erb's palsy" (also known as "waiters-tip palsy" due to the resulting posture of the arm/hand following injury).

6. Reflect the previously transected sternocleidomastoid muscle superiorly, exposing the carotid artery and internal jugular vein.
    (Photo6097)

7. Work a finger under the anterior edge of the deep cervical fascia along the edge of the clavicle where the sternocleidomastoid muscle was connected.
    (Photo6098)

Everything attached to the clavicle is part of the deep cervical fascia along with the muscle attachments of the sternocleidomastoid and trapezius muscles.

Working your finger between the tissue and the carotid artery should gain access below the tough deep cervical fascia.

The deep cervical fascia is a continuous sleeve of strong fascia that encircles the neck and encases both the trapezius muscle, the sternocleidomastoid muscle and accessory nerve (CN XI). (Netter033)

8. Cut through the deep cervical fascia and trapezius attachments to the clavicle working laterally to approximately the acromion.
    (Figure601a) (Netter032)
    (Photo6099) (Photo6099a) (Photo6099b)

Anything attached to the clavicle can be transected. The brachial plexus and axillary artery do not attach to the clavicle and thus will not be damaged.

The structures connected to the clavicle are the sternocleidomastoid (transected previously) and trapezius, along with the sheet of deep cervical fascia linking these muscles and the clavicle.

9. Using a finger, rub back and forth across the fascia which will remove fat from the floor of the posterior triangle.
    (Figure605) (Netter036)
    (Photo6100) (Photo6101)

This fascia covers the brachial plexus, but do not dissect into it at this time. We will expose the brachial plexus by following a terminal branch from the arm into the plexus in the steps below.

PROCEDURE - Axilla & Brachial Plexus

10. Send a member of your team to get two of the disposable chuck pads from the supplies stations and place them on the floor beside the donor arms.

The pads should be positioned such that when the donor arm is abducted the chuck pads will collect any drips throughout the rest of the lab. At the end of the lab the used pads are discarded in regular trash.

11. Grasp the donor's upper limb and  abduct the limb to approximately 45 degrees away from the body, rotating the limb as best possible so that the palm faces anteriorly (i.e. close to the anatomic position).
    (Photo6102)

Embalming can make the tissue very stiff and one of your dissection team may need to hold the arm in position for the steps below.

12. Reflect the pectoralis major and pectoralis minor muscles superolaterally.
    (Photo6103)

Any attachments of the pectoralis major to the clavicle can be severed as needed.

13. Holding the limb extended and rotated into the anatomic position, palpate the arm to find the medial side of the biceps muscle.
    (Netter402)

14. Make an incision through the skin along the medial side of the biceps muscle extending distally along the arm to a point approximately 5cm proximal to the cubital fossa (pit of the elbow).
    (Figure608) (Photo6003)

15. Using blunt dissection spread the skin opening the incision widely.
    (Photo6004)

If your incision was shallow you may need to use scissors to cut through the brachial fascia just below the skin.

The brachial fascia forms a dense 'sleeve' wrapping the arm.  The fascia sleeve wrapping the forearm is called the antebrachial fascia.

16. Using your finger, at approximately the halfway point of the arm, blunt dissect posteriorly along the medial side of the biceps muscle.
    (Photo6005)

As you push to the deep side of the biceps muscle your fingers will be blocked by the fascial layer separating the anterior and posterior compartments of the arm.

Don't dissect through this layer yet, as we will examine the posterior arm in detail in a later lab.

17. Blunt dissect into the fascia medial to the groove your fingers made in the previous step where you should encounter a 'ridge' or 'tube' of tissue, the axillary sheath.
    (Netter423) (Photo6005)

The axillary sheath contains the brachial/axillary artery/vein and terminal nerve branches of the brachial plexus traveling into the upper limb.

Note, the subclavian artery changes its name to the axillary artery as it passes the first rib entering the axilla, and changes its name again to the brachial artery as it passes the level of the teres major muscle entering the arm. It is the same vessel, just named based on the regions traversed.

18. Blunt dissect with fingers distally and proximally along the sides of the axillary sheath without opening the sheath yet.
    (Netter423) (Netter189) (Photo6005)

Observe how the sheath forms a conduit passing deep to the clavicle to enter the arm. The sheath is relatively delicate, so your blunt dissection may have already partially exposed some of the nerves.

19. Return to the clavicle and clean the anterior surface of any superficial fascia so you can observe the bone.
    (Photo6006)

20. Sever any remaining attachments of the trapezius muscle, deep cervical fascia or sternocleidomastoid muscle, along the superior and lateral surfaces of the clavicle.
    (Photo6006)

21. Sever any remaining attachments of the pectoralis major muscle attachments along the inferior edge of the clavicle.
    (Photo6006)

22. Using a Stryker saw, make a cut through the medial part of the clavicle at the lateral edge of the clavicular attachment of the sternocleidomastoid.
    (Figure609) (Photo6007)

Note, one clavicle had the medial region removed in a previous lab during Foundations, so the medial cut has already been completed on one side of the donor.

23. Using a Stryker saw, make a cut through the lateral end of the clavicle close to the acromion process of the scapula (leaving just a little stump of clavicle against the acromion).
    (Figure609) (Photo6007)

This step applies to both sides, as the previous Foundations lab left the lateral clavicle intact.

24. Resect the cut middle portion of the clavicle, separating any muscle/fascia attachments against the deep side of the bone. 
    (Figure609) (Photo6007)

As you lift the cut segment of the clavicle, the muscular attachments you are cutting on the deep side are formed by the subclavius muscle.

The subclavius muscle connects the lateral deep side of the clavicle to the first rib, assisting in moving the clavicle. (Photo6002)

25. Return to the axillary sheath at approximately the mid-arm level and tease/cut an opening in the sheath to expose the contents.
    (Netter423) (Photo6008)

Inside will be the brachial artery/vein and terminal branches of the brachial plexus coursing distally towards the arm, forearm, and hand.

These terminal nerve branches are the median nerve, the ulnar nerve, the medial cutaneous nerve of the forearm, and the radial nerve.

Typically, the most anterior nerve is the median nerve, which lies superficial to the brachial artery.

26. Gently separate the nerves/vessels and follow the median nerve proximally (superiorly) along the arm to where it splits, receiving a branch from the lateral and medial cords of the brachial plexus.
    (Netter419) (Netter420) (Photo6009)

It does not matter if you follow the wrong nerve proximally, the goal is to reach the point at which the cords are organizing into their terminal branches as the starting point for examining the brachial plexus.

Before sorting the nerves of the brachial plexus, briefly review their overall organization. The brachial plexus does exhibit variations in some individuals. The dissection below describes the most common pattern.

The brachial plexus consists of five parts that often use the mnemonic "Real Teachers Drink Cold Beer" representing the first letter of each part.

Roots are the five anterior primary rami of the spinal nerves emerging from intervertebral foramina (roots are deep to other structures in the neck and will not be seen easily during this lab).

Trunks are the convergence of several of the roots and are partly deep to the scalenus anterior muscle.
    The upper trunk is formed by C5 and C6.
    The middle trunk is a continuation of C7.
    The lower trunk is formed by C8 and T1.

Divisions are formed when each of the trunks splits in two forming an anterior and posterior division (e.g. anterior division of the upper trunk, posterior division of the upper trunk, etc.). Thus, there are six divisions. Divisions pass deep to the clavicle.

Cords are formed by the rearrangement of the divisions and are named based on their positions relative to the axillary artery. Cords pass deep to the pectoralis minor muscle.
    The posterior cord is formed by the three posterior divisions.
    The lateral cord is formed by the anterior upper and anterior middle divisions.
    The medial cord is a continuation of the anterior lower division.

Branches are nerves departing the plexus for their target structures. The terminal branches are those that continue into the arm marking the end (i.e. 'termination') of the brachial plexus.

Any of the nerves that you followed will be one of the terminal branches arising from a cord. 

Since we are progressing from distal to proximal, below we will follow the reverse order through the brachial plexus of Branches, Cords, Divisions, Trunks, and Roots.

27. In the vicinity of the split observed in the median (or other) nerve you followed, tease apart and remove fascia to expose the nerve branches in that vicinity.
    (Netter419) (Netter420) (Photo6010)

As you remove fascia, try not to damage branches of the axillary artery as we will examine the arteries after we study the nerves. When the area is clear of loose fascia, you should find the following:

The terminal branches of the musculocutaneous, median, and ulnar nerves are often described as making an 'M' shaped pattern as they arise from the cords that surround the axillary artery. Look for this M pattern to help identify these terminal branches, but be aware there can be variations between individuals.

The musculocutaneous nerve is a branch of the lateral cord that projects sharply laterally to pierce the coracobrachialis muscle. This forms the first 'arm' of the M pattern.

The median nerve arises from the convergence of the terminal branches of the lateral cord and medial cord. These form the middle arms of the M pattern.

The ulnar nerve is the last branch arising from the medial cord before the medial and lateral cords join to form the median nerve. This forms the final arm of the M pattern.

28. Follow the median and ulnar nerves proximally to find the medial cord.
    (Netter419) (Netter420) (Photo6011)

Cords are named based on their position relative to the axillary artery. The medial cord is medial to the axillary artery.

Several cutaneous nerves of the upper limb also arise from this cord. Observe the medial cutaneous nerve of the forearm (sometimes called the medial antebrachial cutaneous nerve) arising from the medial cord and traveling within the axillary sheath into the upper limb.

The medial cutaneous nerve of the arm (sometimes called the medial brachial cutaneous nerve), also arises from the medial cord and starts branching rapidly to innervate the cutaneous tissue of the medial arm where we made the skin incision. Thus, it is often severed when opening the skin/fascia.

29. In the region of the 'M' pattern of terminal branches, gently retract the axillary artery to identify the posterior cord.
    (Netter419) (Netter420) (Photo6011)

The posterior cord lies directly posterior to the axillary artery (the name is defined by this position).

The radial nerve is the major terminal branch of the posterior cord. It courses into the arm to pass to the posterior side of the humerus.

The axillary nerve arises from the posterior cord and courses to the quadrangular space. It will look like the branch 'ends' in tissue as it turns posterior to pass through the quadrangular space and emerge on the posterior side (observed in the shoulder girdle dissection).

30. With the lateral, medial, and posterior cords identified, follow them proximally observing that they split to come from the divisions.
    (Netter419) (Netter420) (Photo6012)

Divisions are difficult to accurately demarcate so we will not be individually identifying any of these.

The posterior cord is formed from the three posterior divisions, while the lateral cord is formed from two of the anterior divisions, and the medial cord is formed from the other anterior division.

31. Continue cleaning fascia proximally and observe the divisions condensing into the three trunks, the upper (or superior), middle, and lower (or inferior) trunks.
    (Netter419) (Netter420) (Photo6012)

These trunks emerge from between the scalenus anterior and scalenus medius muscles in the posterior triangle of the neck. You may need to partially retract scalenus anterior to see the trunks.

The anterior scalene muscle has key topographical relationships you may recall from Foundations.

The muscle spans from the  cervical vertebrae to the first rib. The trunks of the brachial plexus and the subclavian artery pass posterior to muscle while the subclavian vein passes anterior to the muscle (important for central line placement). The phrenic nerve descends along the anterior surface of the scalenus anterior muscle.

When performing a brachial nerve block (anesthesia injection to the brachial plexus roots),because the roots of the brachial plexus are close to the phrenic nerve, incorrect placement of the needle when the brachial plexus is anesthetized can sometimes result in a temporary paralysis of the hemi-diaphragm on that side.

The roots lie close to the vertebral column and are not visible without removing the anterior scalene muscle which we will not perform. The C5-6 roots form the upper trunk, C7 the middle trunk, and the C8-T1 roots form the lower trunk.
(Photo6013) 

CHECKLIST

Soft Structures

Sternocleidomastoid muscle

Trapezius

Brachial plexus
    Trunks
        Upper trunk
        Middle trunk
        Lower trunk
    Cords
        Lateral cord
        Posterior cord
        Medial cord
    Branches
        Musculocutaneous nerve
        Median nerve
        Ulnar nerve
        Medial cutaneous nerve of the forearm