Lab 10 - Shoulder, Axilla, & Arm
|Suggested readings from
Gray's Anatomy for Students, 2nd ed.
||Ch. 7: 649-724
|Suggested readings from Langman's Medical Embryology, 12th ed.
- Ch. 11: 145-150
Ch. 12: 151-161
Prelab: Osteology and Surface Anatomy of the Arm
Primary Lab Objectives:
- Skin the upper limb to just below the elbow. Look for the cutaneous basilic, cephalic, and median cubital veins. Understand their relevance for blood draw or injection.
- Reexamine the pectoralis major and minor. Follow the tendon of the pectoralis major to the insertion on the humerus. Look between the pectoralis major and pectoralis minor for the lateral and medial pectoral nerves, and the thoracoacromial artery. Review the actions of these muscles.
- Turn the limb over and follow the latissimus dorsi to the insertion on the humerus, then locate the teres major, triceps brachii and subscapularis muscles. Identify the anatomical spaces created by the teress major, humerus, and triceps brachii and what structures pass through those spaces.
- Transect the origins of the deltoid and trace the axillary nerveand the posterior humeral circumflex artery through the quadrangular space. Locate the teres minor, supraspinatus, and infraspinatus muscles. Understand their role in the rotator cuff.
- Reflect the pectoralis major and minor laterally to expose the ventral axilla. Trace the subscapular artery, and note the circumflex scapular branch. Examine the axillary lymph nodes, and open the axillary sheath to expose the axillary vessels. Remove the axillary vein and branches and middle portion of the clavicle to clarify the field. Locate the cords of brachial plexus.
- Identify and trace the musculocutaneous, median, and ulnar nerves into the arm. Locate the axillary, radial, upper and lower subscapular, and thoracodorsal nerves. Identify the biceps brachii, brachialis, and coracobrachialis muscles. Follow the axillary artery into the arm where it is renamed the brachial artery. Trace the brachial artery to where it divides into radial and ulnar branches. Follow the median and ulnar nerves through the arm. Identify the profunda brachii artery and locate triceps brachii. Follow the radial nerve through the arm .
1. CUTANEOUS STRUCTURES
- Remove the skin of the upper limb down to the elbow.
- Note the major cutaneous veins imbedded in the superficial fascia.
- Locate the cephalic vein running between the pectoralis major and deltoid muscles.
- On the medial arm, find the basilic vein.
- Look in the cubital fossa for the median cubital vein.
- Trace its connections with the cephalic and basilic veins.
- Clean the superficial fascia away and attempt to trace the boundaries between the depressor and elevator musculature.
- Note how tough the deep fascia is here, and how it passes between major muscle groups to attach to the limb bones forming the intermuscular septa.
2. PECTORAL GROUP
- Put the reflected pectoralis major muscle back in place and note its origins.
- Reflect it again and study the underlying pectoralis minor muscle.
- Locate pectoralis minor's attachment to the coracoid process.
- Find the pectoralis major crossing the long biceps tendon to insert on the lateral lip of the intertubercular groove.
- Clean the pectoralis major tendon of insertion and note that the deepest fibers of the muscle insert most proximally on the humerus.
Between pectoralis major and minor you will find branches of the lateral and medial pectoral nerves (from the lateral and medial cords of the brachial plexus). Usually, the medial pectoral nerve supplies both pectoral muscles, while the lateral supplies only pectoralis major. You will also find branches of the thoracoacromial artery, a branch of the axillary artery that ramifies between the two pectoral muscles, sending branches to them and to the deltoids.
NOTE – the medial and lateral pectoral nerves are named for the cords of the brachial plexus that they are branching off of, not for their position in the body. Often, you will find the medial pectoral nerve in a more lateral position than the lateral pectoral nerve.
3. LATISSIMUS GROUP
- Follow the latissimus dorsi from its broad origin on the back to its narrow tendon of insertion on the floor of the intertubercular groove of the humerus (deep to the pectoralis major).
- You will find the tendon of teres major crossing latissimus dorsi and inserting on the medial lip of the intertubercular groove of the humerus.
- Locate the subscapularis muscle originating from the ventral surface of the scapula.
- Follow it to its insertion on the lesser tubercle of the humerus proximal to latissimus dorsi and teres major.
You will also be able to examine the triceps brachii running from the humerus and scapula to the olecranon process of the dorsal elbow. An important opening called the quadrangular space is formed by the inferior margin of the teres minor, the shaft of the humerus, the superior margin of teres major and the lateral margin of the long head of the triceps. The axillary nerve and posterior humeral circumflex artery pass through this space. Just below the quadrangular space sits the triangular interval formed by the inferior margin of the teres major, the humerus, and the long head of triceps. The radial nerve and deep brachial artery pass through this interval. Nerves and vessels can be injured in these spaces.
4. DELTOID GROUP
The motor nerve for the deltoid group is the axillary nerve; one of the two terminal branches of the posterior cord of the brachial plexus.
- Place your thumb on your clavicle, wrapping the palm of your hand around the ball of your shoulder, and put your fingers on the spine of your scapula -- you have delineated the origins of the deltoid muscle.
- Carefully transect the deltoid along its origins on the cadaver so that you can trace the axillary nerve.
- Underneath the deltoid muscle, note the bursa that lubricates the motion of the deltoid over the humerus and supraspinatus muscle.
- Follow the axillary nerve around the back of the neck of the humerus as it passes through the quadrangular space.
- Locate the teres minor muscle stretching from the scapula to the greater tubercle of the humerus. Like the deltoid, the teres minor is innervated by the axillary nerve.
- Find the posterior humeral circumflex artery accompanying the axillary nerve. It is the major artery to the deltoid.
- Try to find its anastomosis with the anterior humeral circumflex artery. Both anterior and posterior humeral circumflex arteries are branches of the axillary artery.
The deltoid is the most powerful abductor of the humerus. It works against a great mechanical disadvantage; you might compare it to attempting to move a door by pulling on a doorknob affixed near the hinges. Its power is augmented by its multipennate structure, which permits the maximum number of fibers to pull on a given insertion.
5. ROTATOR CUFF
Three short muscles - supraspinatus, infraspinatus and teres minor - originate from the dorsal surface of the scapula and run laterally to insert on the greater tubercle of the humerus. Teres minor is a member of the deltoid group, and receives its innervation via the axillary nerve. Supraspinatus and infraspinatus are innervated by the suprascapular nerve. The fourth rotator cuff muscle, subscapularis, runs from the ventral surface of the scapula to the lesser tubercle of the humerus. Subscapularis is innervated by the upper and lower subscapular nerves. These four muscles form a muscular cuff around the head of the humerus. They act to rotate the humerus in various directions, stabilize the joint and prevent dislocation of the shoulder joint under tension.
You have already examined subscapularis and teres minor. When you reflected the deltoid, you exposed the insertions of supraspinatus, infraspinatus and teres minor, on the posterior surface of the greater tuberosity.
- Underneath the deltoid, locate the most inferior insertion of muscle onto the posterior aspect of greater tubercle of the humerus, which belongs to teres minor. The next one up is infraspinatus, and the most superior is supraspinatus.
- Trace supraspinatus from its insertion, then under the coracoacromial arch , and back to its origin in the supraspinatus fossa of the scapula.
- Verify that it is directly in contact with the loose fibrous capsule of the joint, and partly fuses with it.
While the deltoid is a very powerful abductor of the humerus, the supraspinatus is needed to overcome the initial mechanical disadvantage that the deltoid suffers at the beginning of this motion.
- Follow infraspinatus back to its origin on the scapula. It adjoins teres minor, which it assists in rotating the humerus laterally
Two parallel vessels, the transverse cervical and suprascapular arteries, supply the dorsal aspect of the scapula.
- Locate the origins of the suprascapular and transverse cervical arteries from the subclavian artery.
- Follow them as they run back over the shoulder.
- Find the branches of the transverse cervical artery to the rhomboids and levator scapulae muscles.
- Trace the suprascapular artery to the supraspinatus and infraspinatus muscles.
The transverse cervical, suprascapular, and subscapular arteries anastomose freely around the scapula. This chain of anastomoses can supply blood to the limb if the axillary artery is occluded distal to their origins.
6. THE AXILLA
The fascia in the armpit, or axilla, is thick and extensive, and contains several important groups of lymph nodes clustered around the deep and superficial veins. Vessels and nerves of the forelimb pierce the cervical body wall layers as they cross the first rib, and draw out a fascial prolongation known as the axillary sheath. The lateral thoracic branch of the axillary artery runs down the rib cage near the origins of these lateral cutaneous nerves; its branches help to supply the breast.
- Clear away the superficial fascia, superficial veins and lymph nodes, and cutaneous nerves, exposing the axillary sheath.
- Open the sheath and identify the axillary artery and vein.
- Sever the axillay vein below the subclavius; reflect it together with its branches, and discard it.
Once the subclavian artery passes the first rib, it is renamed the axillary artery. The axillary artery gives rise to the following branches: superior thoracic, thoracoacromial, lateral thoracic, anterior and posterior humeral circumflex, and subscapular.
- Saw through the clavicle just medial to the trapezius insertion.
- Remove the middle piece together with subclavius, thus exposing the trunks of the brachial plexus.
- Identify the three cords of the plexus, and confirm that all three posterior divisions join to form the posterior cord.
- Trace the following branches toward the muscle groups that they supply:
- musculocutaneous nerve
- ulnar nerve
- median nerve.
- Identify the axillary nerve, the radial nerve, the nerves to the latissimus group (upper subscapular, thoracodorsal, and lower subscapular nerves), and the subscapular artery.
The subscapular artery, a branch of the axillary artery, supplies the latissimus muscle group.
7. SUBSCAPULAR ARTERY
- Trace the muscular branches of the subscapular artery into the three muscles of the latissimus group.
- Note its circumflex scapular branch which hooks around the lower edge of the scapula to help supply the dorsal surface of the shoulder blade.
- Note the point at which the subclavian artery becomes the axillary, and the axillary becomes the brachial.
- Locate other branches of the axillary artery, including the thoracoacromial, lateral thoracic, and anterior and posterior humeral circumflex.
8. FLEXORS OF THE ARM
Anatomically the word “arm” is restricted to the upper limb between the shoulder and elbow. There are only four muscles in the arm: one in the extensor compartment, the triceps brachii, and three in the flexor compartment, the biceps brachii, coracobrachialis, and brachialis.
- Separate the three flexors, biceps brachii, coracobrachialis, and brachialis and compare their origins.
- Expose the tendon of the long head of the biceps in the intertubercular groove, noting its synovial sheath. The origin itself from the supraglenoid tubercle will be seen when the shoulder joint is dissected.
- The insertions of both biceps brachii and brachialis cannot be seen until some dissection has been done in the forearm.
- Locate the musculocutaneous nerve where it originates from the brachial plexus and follow it distally into the coracobrachialis muscle.
- Pull up the biceps muscle to reveal some of the muscular branches of the musculocutaneous nerve.
- If possible, locate its emerging cutaneous portion (the lateral cutaneous nerve of the forearm) above the elbow on the lateral surface of the arm.
The musculocutaneous nerve innervates biceps brachii, coracobrachialis, and brachialis and ends in a cutaneous nerve - hence its name.
- Follow the brachial artery into the flexor compartment of the arm.
- Locate the profunda brachii and collateral branches of this artery.
NOTE - Sometimes the brachial artery divides into radial and ulnar arteries within the flexor compartment of the arm. More commonly, this division occurs in the forearm.
The profunda brachii artery passes back to supply the posterior (extensor) compartment of the arm and joins the radial nerve to run along the spiral groove of the humerus. The profunda brachii artery ends in a couple of radial collateral arteries.
9. EXTENSOR OF THE ARM
The extensor compartment of the arm contains only one major muscle, triceps brachii, along with a small slip of the triceps muscle called anconeus. Do not worry about anconeus in your dissection. The RADIAL nerve provides the innervation to ALL the muscles in this compartment.
- Turn the cadaver face down.
- Identify the three origins of triceps brachii.
- Note that the radial and axillary nerves enter the extensor compartment around opposite edges of teres major.
- Trace the radial nerve (motor to triceps and all the distal extensors in the forelimb) and profunda brachii artery along the spiral groove between lateral and medial heads of triceps.
- Transect and reflect the lateral head as you follow the underlying structures.
The superficial veins of the arm are large and prominent thus they are commonly used to draw blood or inject a solution. The median cubital vein in the cubital fossa is the most commonly used. Due to the close positions of the brachial artery and medial nerve care most be taken when accessing this vein.
2. Axillary nerve injuries
Due to the proximity of the axillary nerve to the surgical neck and head of the humerus it can be damaged in both fractures of the surgical neck and inferior dislocations of the glenohumeral joint. Injury to the axillary nerve impairs the deltoid and teres minor muscles and sensory innervation to the upper lateral portion of the arm. To test deltoid function the arm is abducted against resistance starting from a position of ~ 15 degrees of abduction.
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