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 placeand note its origins.
Reflect it again and study theunderlying 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 latissimusdorsi 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 major, the surgical neck 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 multipennatestructure, 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 and to prevent dislocation of the shoulder joint under tension.
You have already examined subscapularis and teresminor. 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 inferiormost insertion of muscle onto the posterior aspect of head of the humerus, which belongs to teres minor. The next one up is infraspinatus, and the most proximal is supraspinatus.
Trace supraspinatus from its insertion, then under the arch of the coracoacromial ligament, and back to its origin on 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 deltoid suffers at the beginning of this motion. The supraspinatus also stabilizes the joint while deltoid pulls upward on the humerus.
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 suprascapularartery 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: highest 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:
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 posteriorhumeralcircumflex.
8. FLEXORS OF THE ARM
Anatomically the word “arm” is restricted to the forelimb 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. The origin of the long head of biceps is invisible at this point.
Expose the tendon of origin in the intertubercular groove, noting its synovial sheath. The origin itself 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 again 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 profundabrachii, muscular, and collateralbranches 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 ALL the innervation in this compartment.
Turn the cadaver face down.
Identify the three origins of triceps brachii.
Note that the radial and axillarynerves 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.
Lab 12 - Flexor compartment and palm
Suggested readings from Gray's Anatomy for Students, 2nd ed.
Ch. 7: 724-751
Suggested readings from Langman's Medical Embryology, 11th ed.
Make an incision from the elbow to just below the wrist.
Using your fingers or a blunt probe separate the skin and superficial fascia from the muscles, and reflect the skin to either side
Inspect the skin of the forearm and superficial fascia noting any cutaneous veins and nerves.
2a. NERVES OF THE FOREARM-Ulnar Nerve
Trace the median nerve from the arm to the cubital fossa.
Locate the ulnar nerve passing into the posterior (elevator) compartment of the arm.
Continue tracing the ulnar nerve around the medial epicondyle of the humerus and back into it the ventral-medial portion of the forearm.
2b. NERVES OF THE FOREARM-Radial Nerve
NOTE: The radial nerve -a posterior division nerve- also wanders out of the dorsal compartment, briefly entering the flexor compartment at the elbow to supply the brachioradialis muscle. This muscle is located at the ventral edge of the extensor group originating laterally from the distal humerus.
Probe between the brachialis muscle (a depressor) and brachioradialis muscle (an elevator).
Find the radial nerve as it passes around into the anterior compartment.
Follow the radial nerve past the elbow (under the brachioradialis) until you see it dividing into the deep and superficial branches.
3. SUPERFICIAL AND DEEP FOREARM FLEXORS
NOTE: There are eight muscles in the flexor compartment of the forearm:
Flexor carpi ulnaris
Flexor carpi radialis
Flexor digitorum superficialis
Fexor digitorum profundus
Flexor pollicis longus
These muscles are divisible into two layers: a superficial layer arising from the flexor (or ulnar, or medial) epicondyle of the humerus (I-V); and a deep layer arising from the forearm bones (radius and ulna) and the interosseous membrane (VI-VIII). The tough interosseous membrane gives origin to the deep flexors on its ventral surface and to the deep extensors on its dorsal surface.
Begin cleaning and separating the muscles of the superficial layer that originate from the flexor epicondyle.
Note the fibrous band through which some of the fibers of biceps brachii insert into the deep fascia of the forearm.
Remove the fibrous band.
FUNCTIONAL ANATOMY: The deep fascia of the forearm is thickened into a dense fibrous band at the wrist. This band is the flexor retinaculum. It attaches to the scaphoid and trapezium laterally, and the pisiform and hamate medially creating the carpal tunnel. The tendons of the long flexors pass through this carpal tunnel (which is of obvious clinical relevance), under the retinaculum, preventing them from bowstringing at the wrist when their muscle bellies contract.
NOTE – the flexor retinaculum is deep in the wrist, and it is very short in a mediolateral direction (2-3cm).
Leave the flexor retinaculum intact as you break through the fascia enclosing the forearm flexors.
Identify the four flexors that you now see fanning out from the ulnar epicondyle of the humerus: pronator teres, flexor carpi radialis,palmaris longus (sometimes absent), and flexor carpi ulnaris.
Trace the origin and insertion of the flexor carpi radialis and ulnaris muscles in the cadaver using the descriptions below to guide your identification.
FUNCTIONAL ANATOMY: The pronator teres inserts on the radius and lies on the radial edge of the superficial flexor sheet. It pulls the radius over toward the flexor epicondyle and pronates the forearm. This is the only muscle of the superficial flexor layers that does not cross the wrist.
The flexor carpi radialis is the radial flexor of the wrist. It lies to the ulnar side of the pronator teres muscle. Its tendon inserts on the base of the second and third metacarpal.
The flexor carpi ulnaris is the ulnar flexor of the wrist and lies on the ulnar side of the superficial flexor sheet. It attaches to the base of the fifth metacarpal. It does so indirectly by inserting on the pisiform and pulling on a ligament connecting the pisiform and the base of the fifth metacarpal. The pisiform is a large sesamoid bone embedded in the tendon of the muscle. The flexor carpi ulnaris has a largely aponeurotic secondary origin from the medial edge of the ulna.
Oppose your thumb and ring finger while slightly flexing your wrist.
Look for the tendon of palmaris longus popping up in the center of your wrist.
Examine the wrists of your group members for presence or absence of palmaris longus.
NOTE: The little palmaris longus muscle inserts into the deep fascia of the palm. It runs along between the two marginal muscles that flex the wrist. It is absent in 11% of all people, and is variable in form. The tendon of palmaris longus (if present) is visible in the living body when the wrist is flexed against resistance.
Transect the four superficial flexors that you identified earlier each at different levels (i.e. stagger the level at which each muscle is cut --this will make it easier to tell the cut ends apart).
Reflect the cut superficial flexors.
The muscle exposed is the flexor digitorum superficialis.
Explore its origins from the humerus, ulna, and radius.
NOTE: Most of flexor digitorum superficialis originates from the humerus and ulna, but a thin sheet of its fibers originates from the flexor surface of the radius. Does it belong to the deep flexor layers, to the superficial layer, or as the usual textbook description has it, to a third, “intermediate” layer?
Review the course of the ulnar nerve from the brachial plexus to the elbow.
Extend your dissection of this nerve down to the wrist.
As you follow the ulnar nerve from the plexus to the wrist, note its motor branches.
FUNCTIONAL ANATOMY: Ordinarily, the ulnar nerve innervates only the flexor carpi ulnaris muscle and the ulnar edge of flexor digitorum profundus in the forearm. Its main function is in the hand. The median nerve innervates the rest of the flexors in the forearm.
Trace the median nerve down to the proximal edge of the flexor digitorum superficialis, where you will see it passing between the heads of that muscle.
Transect the humeroulnar and radial heads of flexor digitorum superficialis.
Reflect it distally to expose the flexor digitorum profundus and flexor pollicis longus.
Locate the median nerve and the ulnar artery traversing the space between the deep and superficial digital flexors.
4. ULNAR AND INTEROSSEOUS ARTERIES
NOTE: Near the apex of the cubital fossa the brachial artery divides into the radial and ulnar arteries. Shortly after this division, the ulnar artery gives off the common interosseous artery
Locate the brachial artery in the cubital fossa (between the biceps brachii tendon and the median nerve) and follow it down into the forearm.
Follow the ulnar artery distal to the point at which it crosses under the median nerve.
Locate here the common interosseous branch of the brachial artery.
HINT: You will be able to trace the common interosseous only a very short distance because it splits almost immediately into anterior and posterior interosseous arteries. The anterior interosseous nerve and artery supply the deep flexor sheet. They are branches of the median nerve and common interosseous branch of the ulnar artery, respectively.
Study a diagram of the arterial anastomoses around the elbow.
5. PRONATOR QUADRATUS
Identify the flexor digitorum profundus and flexor pollicis longus muscles.
Transect them at different levels above the wrist
Reflect them to expose the pronator quadratus muscle.
Examine the interosseous membrane where it lies exposed between the pronator quadratus and the deep digital flexor origins.
FUNCTIONAL ANATOMY: The final member of the deep flexor sheet, pronator quadratus is the most used pronator of the forearm. The more superficial pronator teres is brought into play against resistance, as when using a screwdriver.
6. PALMAR SURFACE
Remove the skin of the palm, the thumb, and the second and third digits.
HINT: In the palm, the skin is attached to the deep fascia by fibrous bands, the superficial fascia is reduced to pockets of fat, and the skin is relatively immobile. This makes it quite difficult to skin. Therefore, you should tie the hand to one of the boards provided. You definitely need to use a sharp scalpel for this task.
Cut the palmar digital tendons above the wrist to mobilize the digits.
As you skin, note the cutaneous nerves and vessels running along each side of the flexor surface of every digit.
Note also the small cutaneous muscle, palmaris brevis, which overlies the ball of the little finger.
7. PALMAR APONEUROSIS
On either side of the long flexor tendons, notice that the palmar aponeurosis fuses with the fascias enclosing the thenar muscles and hypothenar muscles.
Make a slit in the palmar aponeurosis near the base of the fingers between the central two long flexor tendons and push them apart.
FUNCTIONAL ANATOMY: Nine intrinsic muscles of the hand (8 interossei muscles and the adductor pollicis muscle) lie between the deep fascia and the dorsal surface of the hand. These short intrinsic muscles are principally concerned with abduction and adduction of the digits.
The heads of the four medial (ulnar-side) metacarpals are bound together by specialized fibers of the deep fascia, forming a fibrous band stretching across the palm. These deep ligaments do not bind the thumb. If it were bound, the thumb would not be divergent and opposable. The dorsal fibers of these ligamentous tubes condense into palmar ligaments over each of the joints. In between these ligamentous tubes, the ventral fibers condense into fibrous digital sheaths, which attach to the bone and hold down the long tendons.
On a skeleton, look at the ventral sides of several phalanges and identify ridges on the radial and ulnar edges left by the fibrous sheaths.
Remove the palmar aponeurosis, but leave the flexor retinaculum intact.
Look at the superficial palmar arch of arteries.
Trace its medial end back to the ulnar artery at the wrist.
Examine its digital branches.
Remove the superficial arch. The long flexor tendons are now exposed, wrapped in the synovial sheaths that lubricate their movement inside the fibrous flexor sheaths.
FUNCTIONAL ANATOMY: The arterial supply to the hand comes principally from the radial and ulnar arteries. These two arteries enter the palm and form two palmar arches. The superficial palmar arch is formed chiefly by the ulnar artery and lies just under the palmar aponeurosis. The radial artery primarily supplies the deep palmar arch.
8. PALM MUSCLES
Cut open the fibrous flexor sheaths and dissect out the insertions of the long flexor tendons of the thumb and one other finger.
Pull on these tendons and experiment with their functions.
Verify that there is no superficial flexor tendon to the thumb (the proximal interphalangeal joint flexed by this tendon is not found in the thumb).
Trace the flexor digitorum profundus and superficialis tendons to their respective insertions on the distal and middle phalanges.
Examine how the superficial tendon splits at its insertion to allow the deep tendon to pass to the distal phalanx.
Reflect the deep tendon distally, so that you can examine the insertion of flexor digitorum superficialis.
FUNCTIONAL ANATOMY: From each of the digital flexor profundus tendons, a lumbrical muscle originates. The lumbrical muscles are associated with the long flexor tendons, while the 8 interosseous muscles belong to the deep intrinsic musculature of the hand. This allows for the tendons of the lumbricals and interossei to insert on opposite sides of the deep ligaments. Blood supply to the deep intrinsic musculature of the hand is via the deep palmar arch, which is supplied mainly by the radial artery.
Trace one of the lumbrical muscles around the radial side of its digit.
Follow it to its insertion into the extensor hood and expansion.
Correlate these attachments with a textbook description of its action.
Examine the thenar and hypothenar muscles in your cadaver.
Separate abductor pollicis brevis, flexor pollicis brevis, and opponens pollicis in the thenar group
Separate abductor digiti minimi, fexor digiti minimi brevis, and opponens digiti minimi in the hypothenar group.
NOTE: Adductor pollicis (one of the deep intrinsic muscles of the hand) and flexor pollicis brevis attach to the sesamoids adjoining the first metacarpal head. The tendon of flexor pollicis longus lies between the two muscles.
FUNCTIONAL ANATOMY: Primitively, there were three layers of deep intrinsic hand muscles:
The adductor pollicis, which adducts the thumb.
The four palmar interosseous muscles, which adduct the digits.
The four dorsal interosseous muscles, which abduct the digits. The 3rd digit needs two abductors, one for each side. The dissection of the interosseus muscles is described below.
9. DEEP PALM
Complete the removal of the superficial palmar arch.
Sever the flexor retinaculum.
Reflect the long flexor tendons along with the lumbricals. The deep palmar arch is now exposed.
Using a blunt probe, establish continuity with the radial artery in the anatomical snuffbox. The radial artery is the main contributor to the deep arch.
Note that the radial artery passes into the deep arch through the two heads of the first dorsal interosseous muscle.
Study a diagram of the arterial supply to the hand and understand the contributions of the deep arch.
Identify the adductor pollicis and the palmar and dorsal interossei muscles.
You should also be able to find the deep branch of the ulnar nerve.
Cut the deep ligament joining the second and third metacarpals.
Trace the second palmar and dorsal interossei to their respective insertions. Each dorsal interosseous arises by two heads, one from each of the adjoining metacarpals.
Trace the median nerve beneath the flexor retinaculum.
Follow the median nerve into the palm, and notice that it breaks up into cutaneous branches to the lateral three and a half digits.
Trace its motor branches to the first and second lumbricals (digital branch) and to the thenar muscles (recurrent branch).
Locate the ulnar nerve in the forearm and trace it to the proximal edge of the flexor retinaculum. Here it divides into a superficial (cutaneous) branch and a deep branch.
Follow the deep branch of the ulnar nerve into and through the hypothenar muscles (it innervates them as it goes by).
Locate the deep branch as it emerges in the deep layer of the palm.
Follow it across the palm and beneath the long flexor tendons (this portion innervates the remaining two lumbricals and all the deeper muscles of the palm).
Lab 13 - Extensor compartment & hand
Suggested readings from Gray's Anatomy for Students, 2nd ed.
Ch. 7: 751-774, 786-791
Suggested readings from Langman's Medical Embryology, 11th ed.
FUNCTIONAL ANATOMY: There are 11 muscles in the extensor compartment of the forearm:
Extensor carpi radialis longus (ECR longus)
Extensor carpi radialis brevis (ECR brevis)
Extensor carpi ulnaris (ECU)
Extensor digiti minimi
Abductor pollicis longus
Extensor pollicis longus
Extensor pollicis brevis
The distinction of these extensor muscles (forearm elevators) as to "superficial" (1-6) and "deep" (7-11) is much less clear and precise than it was in the flexor compartment and some consider there to be a single extensor compartment with digital and thumb muscles. The "deep" group comprises all the extensors of the thumb, plus a separate extensor for the index finger and a supinator (the only one that originates on the humerus).
The superficial group originates exclusively from the humerus and includes extensors to the four fingers, as well as the wrist extensors. The superficial group is further divisible into lateral and medial groups. Between these two groups, the deep layer sends tendons of insertion out toward the thumb. The radial nerve supplies the extensor compartments of both the arm and forearm. Recall that in the previous lab, you located the radial nerve at the elbow and found its deep and superficial branches.
1. SUPERFICIAL LATERAL EXTENSORS OF THE FOREARM
Skin the dorsal surface of the forearm and hand.
Break through the deep fascia and separate the superficial layer, beginning with the lateral group.
From the lateral edge of the humerus, follow the brachioradialis and the two radial extensors of the wrist (extensor carpi radialis longus and brevis) running along the radial edge of the flexor compartment.
Compare their attachments with those of the single radial flexor of the wrist (flexor carpi radialis).
2. SUPERFICIAL AND DEEP BRANCHES OF THE RADIAL NERVE
Lift brachioradialis up at the elbow and again expose the radial nerve splitting into superficial and deepbranches.
Follow the deep branch until it disappears into a muscle. This is the supinator muscle, which will be considered later along with the deep extensors.
Follow the superficial branch of the radial nerve to the wrist.
Atlas Images (note, the first image is as viewed from the flexor surface):
3. SUPERFICIAL MEDIAL EXTENSORS OF THE FOREARM
Separate the superficial medial extensor group.
Locate the most medial of the three muscles in this group: the ulnar extensor of the wrist, extensor carpi ulnaris.
Verify that its attachment mirrors that of the ulnar flexor.
Identify the most lateral muscle in this group, extensor digitorum.
In between these two muscles is a separate slip known as the extensor digiti minimi.
4. EXTENSOR RETINACULUM
Identify the extensor retinaculum.
FUNCTIONAL ANATOMY The deep fascia over the wrist is specialized to form an extensor retinaculum. Under this retinaculum passes the extensor tendons on their way to the digits. One tendon passes to each of the four fingers (excluding the thumb). As you just saw, there is an additional tendon to the little finger, with its own independent belly, the extensor digiti minimi. However, tendinous bands unite these five tendons so they cannot work entirely independently.
5. EXTRINSIC MUSCLES OF THE THUMB and the ANATOMICAL SNUFFBOX
FUNCTIONAL ANATOMY: The deep layer of extensors includes muscles that originate from the radius, ulna, and the interosseous membrane – like the deep flexor layer on the other side of the forearm. Three of these are thumb muscles: an abductor pollicis longus, extensor pollicis brevis, and extensor pollicis longus, which insert in that order on the bases of the three bones of the thumb.
Locate the abductor pollicis longus, extensor pollicis brevis, and extensor pollicis longus. These muscles are found in the mid-forearm between the lateral and medial groups of superficial extensors.
Follow these muscles down to their insertions on the thumb.
FUNCTIONAL ANATOMY: Extensor pollicis longus passes through a different compartment of the extensor retinaculum producing a hollow called the “anatomical snuffbox” between its tendon and those of the other two thumb elevator muscles. The radial artery appears in this hollow between the tendons.
Locate your “anatomical snuffbox” by extending and abducting your thumb.
With your forefinger, feel for the radial pulse here.
Now trace the radial artery through the cadaver's anatomical snuffbox.
Lift the medial group of superficial extensors up and locate the extensor indicis and supinator.
The extensor indicis gives the index finger added independence from the medial three fingers. Included in the deep layer of the forearm extensors is the supinator muscle.
6. POSTERIOR INTEROSSEOUS ARTERY
Find the origin of the posterior interosseous artery and trace it back to where it entered the extensor compartment (i.e., around the superior free edge of the interosseous membrane).
Look for the branches of the posterior interosseous vessels and nerve piercing the supinator to supply the extensor compartment.
In the flexor compartment of the forearm, we saw that the flexor digitorum superficialis combined elements of the superficial and deep layers. The same is true of the supinator in the extensor compartment. Part of this muscle is a superficial bundle coming off the humerus (the equivalent of pronator teres) and part is a deep sheet stretching between ulna and radius (vaguely resembling pronator quadratus). The two origins of the supinator are fused at their insertion.
Between the superficial and deep parts of the supinator, the radial nerve passes back into the extensor compartment of the forearm. It sends motor fibers to the lateral group of superficial extensors before it pierces the supinator, and then to the other extensors after it reaches the posterior compartment (deep branch or posterior interosseous). The superficial branch of the radial nerve running down the forearm under the brachioradialis is entirely a cutaneous sensory nerve to the hand.
FUNCTIONAL ANATOMY: The medial elements of the superficial layer of the elevators receive some arterial supply from branches of the radial artery near the epicondyle, which you saw previously accompanying the deep radial branch through the supinator. Additionally, arterial supply for the extensor compartment comes from the posterior interosseous branch of the ulnar artery.
7. EXTENSOR RETINACULUM REVISTED and DORSUM OF THE HAND
The extensor retinaculum overlies the extensor tendons over the back of the wrist. The deeper fibers of the retinaculum are bound to grooves on the radius, thus producing multiple fibrous tunnels lined with tubular synovial sheaths (overlying each tendon). If the extensor retinaculum stretched tightly across between radius and ulna, it would greatly restrict pronation.
Consult a skeleton and note the grooves in the back of the radius, which correspond to various extensor tendons.
Demonstrate on an articulated skeleton that a tight extensor retinaculum would restrict pronation.
Confirm that the extensor retinaculum does not attach to the ulna.
Sever the extensor retinaculum and trace the extensor tendons onto the back of the hand.
8. DORSAL INTEROSEOUS MUSCLES
Dissect the dorsum of the hand to expose the dorsal interosseus muscles.
Note the course of the radial artery. It traverses through the anatomical snuffbox, into the dorsum of the hand, where it then pierces through the first dorsal interosseus muscle, to anatomose with the deep palmar arch.
Dissect the extensor expansion on the back of the third digit.
Students with orthopedic ambitions may attempt to distinguish the retinacular ligament (part of the lateral band). This ligament ensures that when the proximal interphalangeal joint is extended (by extensor digitorum), the distal joint extends, and it also produces flexion in the proximal joint when the distal joint is flexed (by flexor digitorum profundus).