Forearm and Hand

As with the muscles in the arm, the muscles in the forearm are covered in a heavy connective tissue sheath.  You’ll have to remove most of that sheath to identify the individual forearm muscles, but proximally (near the elbow) that connective tissue sheath also forms part of the origin of many of the muscles, and you don’t want to shred those muscles during your dissection.

The best way to reflect the deep connective tissue sheath around the forearm muscles is to start at the wrist.  There you’ll find the sheath is not as tightly applied to the underlying muscles.  Pull up a fold of the sheath at the wrist and cut it open, and then open it broadly.  Around the wrist the forearm muscles are mostly shiny white tendons.  To begin, grab a tendon and work your way proximally to the muscle belly (the fleshy part of the muscle).  No tools are necessary, so use your hands to separate the muscle from adjacent muscles and from connective tissue.  As you do this with each tendon you’ll separate the forearm muscles from one another, and you’ll be able to start to identify them.

As you separate the muscles of the forearm you’ll come across blood vessels and nerves.  Safeguard those for now, and we’ll identify them as we work our way through the muscles.

Naming conventions.  Many (but not all) of the muscles in the forearm follow this naming convention:  

1. Flexor or extensor - flexors are in the anterior compartment, extensors in the posterior compartment.

2. Carpi or digitorum - muscles that insert in the wrist are carpi, those that insert in the digits are digitorum.

3. Ulnaris or radialis - carpi muscles are either on the lateral (radial, or thumb) side of the forearm, or the medial (ulnar, or pinky) side of the forearm.  

So the flexor carpi ulnaris is a muscle in the anterior compartment that attaches to the wrist on the pinky (medial) side of the forearm.  The extensor carpi radialis is in the posterior compartment (so it’s an extensor), it attaches to the wrist, or the radial (thumb) side.  Further, there are 2 extensor carpi radialis muscles, and one is longer than the other, so theres the extensor carpi radialis longus, and the extensor carpi radialis brevis.  Similarly, there are two flexor digitorum muscles, one is superficial and one is deep, so there’s the flexor digitorum superficialis and the flexor digitorum profundus.  

Other muscles are named according to their attachments (brachioradialis) or their actions (pronator teres).  There’s not a single rule for naming all these muscles, but all the names make sense when you break them down.

Superficial muscles in the anterior compartment:

Start with the most lateral (radial side) muscle of the anterior compartment, the brachioradialis m.  The brachioradialis has the most proximal attachment of all the forearm muscles, attaching proximal to the lateral epicondyle of the humerus (brachio-), and inserting into the distal end of the radius (-radialis).  It’s a good muscle to use as a reference, because it defines the lateral edge of the flexor compartment.  Any muscles posterior to the brachioradialis are extensors.

Moving medially (toward the pinky, or ulnar side), the next muscle you’ll see that has a large distal tendon is the flexor carpi radialis m.  In addition to flexing the wrist, this muscles also contributes to radial deviation at the wrist.  

In the elongate triangular interval between the brachioradialis m. and the flexor carpi radialis m., up toward the elbow, you’ll find another forearm muscle that does not extend into the wrist or digits.  This is the pronator teres m., which originates on the medial aspect of the humerus and ulna, crosses the forearm at an angle to the long axis, and inserts into the radius.  The action of the pronator teres m. is to pronate the forearm, which is the action that rotates the forearm such that the wrist faces posteriorly (assuming anatomical position).  Often the pronator teres m. makes it difficult to supinate the forearm (so that the palm faces forward), which can make it difficult to see the muscles in the anterior forearm.  To remedy this, and to allow a clear view of deeper structures, isolate the pronator teres and then transect the muscle at the midpoint.

Medial to the flexor carpi radialis is a very small muscle belly with a very long tendon that extends into the palm of the hand.  This is the palmaris longus m.  Some individuals do not have a palmaris longus m., and some have the muscle only on one side of the body.  You can check yourself for palmaris longus by flexing the wrist and looking for the broad, flat, superficial tendon just proximal to the wrist.

The last of the superficial muscles in the anterior forearm is the flexor carpi ulnaris m., which originates close to the flexor carpi radialis, but inserts on the pinky (ulnar) aspect of the wrist.  In addition to flexion of the wrist, the flexor carpi ulnaris m. also produces ulnar deviation of the wrist.

Blood vessels and nerves:  

You’ll need to mobilize the superficial muscles to see the deeper muscles of the anterior compartment.  If the space seems really tight you should cut the superficial muscles to see the deeper ones, but first find blood vessels and nerves so you can safeguard them.

In the cubital fossa, find the brachial a. and the median n.  If you haven’t yet cut the bicipital aponeurosis you should at this time.  Follow the brachial a. into the forearm and you’ll see that it splits to form the radial a. and the ulnar a.  Usually this split is just distal to the elbow, but sometimes the split is much more proximal in the arm.  

The radial a. runs along the radial aspect of the forearm, on the deep surface of the brachioradialis m.  Soon after the split into radial and ulnar aa. the radial a. gives off a recurrent radial a. that contributes to the anastomosis around the elbow.  Close to the wrist the radial a. emerges from beneath the brachioradialis m., and this is where you take the radial pulse.   We’ll return to the radial a. when we dissect the hand. 

Near the origin of the brachioradialis m., on its deep surface, look for the two branches of the radial n.  The superficial branch of the radial n. runs with the radial a.  The deep branch of the radial n. runs into the posterior compartment of the arm, we’ll pick it back up when we get there.  The superficial branch of the radial n. can be seen in the distal forearm, running out onto the skin of the thumb.

The ulnar a. is usually the larger of the two branches of the brachial.  Close to the split between radial and ulnar aa. look for ulnar recurrent aa., which are part of the anastomosis around the elbow.  

The ulnar a. gives off a large branch that travels deep and lateral, the common interosseous a.  The common interosseous a. then divides to form an anterior interosseous a. and a posterior interosseous a.  These travel anterior and posterior to the interosseous membrane that runs between the radius and ulna.

The main trunk of the ulnar a. runs inferiorly, deep to the superfical muscles, and emerges again at the wrist.  Running with the ulnar a. is the ulnar n.

Intermediate muscles in the anterior compartment:

You may be able to view the intermediate muscles by pulling the superficial muscles out of the way, but you may also transect the more superficial muscles.

In the middle part of the anterior compartment you’ll see the flexor digitorum superficialis m.  This is a broad muscle with 4 tendons that run through the carpal tunnel, though the palm of the hand, and insert into the middle phalanges of digits 2 - 5.  

Also intermediate in position is the flexor pollicis longus m., which originates from the mid-shaft of the radius, passes through the carpal tunnel, and inserts into the distal phalanx of the first digit (thumb, = pollex).  

Deep muscles in the anterior compartment:

Pull the tendons of the flexor digitorum superficialis anteriorly to view the flexor digitorum profundus m., which sits on the interosseous membrane. Again, for a clearer view you may choose to transect the flexor digitorum superficialis m., but first find the median n., which runs between the two flexor digitorum mm.  The median n. runs through the carpal tunnel, along with the tendons of the flexor digitorum superficialis and profundus and the flexor pollicis longus. The tendons of the flexor digitorum profundus pass through the carpal tunnel, through the palm of the hand, and insert into the distal phalanges of digits 2 - 5.

Close to the wrist, and deep to the flexor digitorum mm., look for the pronator quadratus m.  The fascicles of this small muscle run transversely between the ulna and radius, and assist the pronator teres in pronating the forearm.  

When you dissect into the hand you’ll see the tendons of the flexor digitorum and flexor policis mm, and eventually you’ll cut through the transverse carpal ligament to open up the carpal tunnel.  

Summary - Muscles of the Anterior Forearm:

Superficial layer (lateral to medial):

• Brachioradialis

• Pronator teres (proximal forearm only)

• Flexor carpi radialis

• Palmaris longus

• Flexor carpi ulnaris

Intermediate layer (lateral to medial):

• Flexor pollicis longus

• Flexor digitorum superficialis x

Deep layer:

• Flexor digitorum profundus x4

Deepest layer:

• Pronator quadratus (distal forearm only)

Just as the posterior compartment of the arm was the extensor compartment (mainly of the elbow, less so of the shoulder), the posterior compartment of the forearm is also an extensor compartment.  And similar to the organization of the anterior compartment of the forearm, the muscles in the posterior compartment of the forearm may insert in the wrist or the digits, and thereby cause extension of the wrist, or of the digits and (indirectly) the wrist.  There are fewer extensor muscles in the forearm, likely because extension of the wrist need not be as powerful as flexion, as flexion is used for grasping.  Conveniently, the extensor compartment of the forearm is innervated by the radial n., as is the extensor compartment of the arm.  Many of the extensor muscles of the forearm originate from the lateral epicondyle of the humerus.

As with the anterior compartment of the forearm, there will be some heavy connective tissue that you’ll need to work through to isolate the muscles and tendons of the posterior compartment of the forearm.  And as with the anterior compartment the best way to go about this part of the dissection is to find the distal tendons of the posterior compartment muscles around the wrist, isolate those tendons, and then follow them proximally to define the muscle bellies.  There is a heavy band of transverse connective tissue on the posterior wrist, the extensor retinaculum.

In the middle of the posterior compartment look for the tendons of the extensor digitorum m.  You can easily see these muscle tendons in the back of your hand by flexing and extending your digits at the metacarpophalagneal joints.  Typically you’ll find 4 tendons, one going to each of the digits 2-5.  There is only one extensor digitorum, so there’s no superficialis or profundus distinction.

Just medial (on the ulnar side) of the extensor digitorum m. look for a small muscle belly that leads to a second tendon that goes to digit 5, the extensor digiti minimi m.  

Continuing medially, look for the extensor carpi ulnaris m., which inserts into the 5th metacarpal and so both extends and medially deviates the wrist.

The most medial muscle of the forearm is the anconeus m., a small triangular muscle that does not extend much beyond the elbow.  The anconeus is said to assist in extending the elbow, but given its size that contribution must be minor.

Now go back to the extensor digitorum m., and re-identify the brachioradialis m.  Sitting between these two muscles are the extensor carpi radialis longus m. and the extensor carpi radialis brevis m.  The longus is the more superficial of the muscles, as it inserts more proximal (higher) on the humerus, just distal to the insertion of the brachioradialis m.  The brevis is just deep to the longus.  Sometimes the two muscles are difficult to differentiate.  If that’s the case find the tendon of the longus and follow it proximally to the muscle belly.

There are several deep muscles in the posterior compartment.  You can probably see them by mobilizing the superficial muscle and moving them around out of the way, but if you need or want to cut any of the superficial muscles, that’s fine.

Just distal to the anconeus, deep to the proximal attachments of the extensor carpi radialis mm., is the supinator m..  You should have seen the supinator in the deep space of the anterior compartment of the elbow, but check out the posterior view as well.

Distal to the supinator m. are 3 muscles that are referred to collectively as the outcropping muscles.  All three muscles insert into and move the thumb, and their names describe their actions (1 abductor and 2 extensors).  The muscle bellies of these three can be hard to separate, especially the abductor pollicis longus m. and the extensor pollicis brevis m., but again isolate their tendons and then work proximally toward the muscle bellies.  There is a space between the extensor pollicis brevis m. and the extensor pollicis longus m., you can palpate that space on your own wrist.

The last of the deep muscles of the posterior compartment of the forearm is the extensor indicis m.  Just like the pinky (5th digit), the pointer finger (second digit) has an additional muscle (besides the extensor digitorum) that extends it. 

Arteries and nerves:

Most of the arteries and nerves in the forearm are in the anterior compartment, and have already been seen; the median and ulnar nn., and the radial and ulnar aa.

As it enters the forearm, the radial n. splits into a superficial and a deep branch.  The superficial branch runs deep to the brachioradialis m., and crosses the wrist to innervate the lateral (radial) aspect of the hand.  The deep branch of the radial n. runs through the substance of the supinator m. and then emerges to supply motor innervation to the muscles of the posterior compartment of the forearm and sensory innervation to the overlying skin.

You may be able to separate the deep muscles of the posterior forearm to see the posterior interosseous a. running along the posterior surface of the interosseous membrane.

Many of the forearm muscles, both the radial and ulnar aa., and the ulnar and medial nn. pass into the hand to move the digits, and supply blood and motor innervation to the intrinsic mm. of the hand.  The forearm muscles that have tendons going into the hand (those that pass through the carpal tunnel) to move the digits are extrinsic mm. of the hand.  There are several small muscles within the hand that produce fine movements of the fingers, these are the intrinsic mm. of the hand.  

The radial and ulnar aa. cross the wrist and form two main anastomotic networks in the palm of the hand, the superficial and deep palmar arches.  Palmar and digital arteries branch from those arches to supply the palm of the hand and the digits.  

The intrinsic mm. of the hand are innervated by terminal branches of the median and ulnar nn. The radial nerve has no motor innervation in the hand. 

The palm of the hand is covered by a thick, fibrous palmar aponeurosis, which is difficult to dissect through.  Take care as you remove the palmar aponeurosis, because just deep to it are the palmar nn. and aa.  The palmar aponeurosis is he insertion site of the palmaris longus m.

Start skinning the hand from the transverse cut labeled (B) down to the wrist.  

You do not want to make the transverse cut around the wrist because you may cut nerves and vessels. 

Once you have removed the skin down to the wrist, go ahead and remove the flap.

Use forceps to hold up a skin flap and use your scalpel to cut at most 3-5mm below the skin surface.  

Once all the skin is off, you should be able to gauge how thick the palmar aponeurosis is and how much you can cut away.  The palmar aponeurosis is tough connective tissue, you’ll need to carefully remove it piecemeal to avoid damaging deeper structures.

Dissect three of the five digits, including the thumb.

Palmar and digital arteries:

As you dissect through the palmar aponeurosis, you’re looking for the superficial palmar arch, an arterial arch that forms an anastomosis between the radial and ulnar aa.  Later in your dissection, deep in the palm of the hand, you may also find the deep palmar arch, which is also an arterial anastomosis between the radial and ulnar aa.  Branches from the superficial palmar arch that radiate into the palm of the hand are the common palmar digital aa.  Branches from the deep palmar arch that radiate into the palm of the hand are the palmar metacarpal aa.  The common palmar and palmar metacarpal aa. join in the distal palm, forming the common palmar digital aa.  Those arteries bifurcate and send arteries up either side of the digits, the proper palmar digital aa. 

Palmar and digital nerves:

Accompanying the palmar and digital aa. are palmar and digital nn. that supply motor and cutaneous innervation to the hand.  The ulnar n. is the main motor nerve in the hand, but the median nerve also has important motor function in the hand.  For now just be aware that you will find nerves running along with the arteries.  Do your best to expose and safeguard them, and we’ll consider which nerves innervate which muscles as we work through the muscles.

The thenar eminence: 

The thenar eminence is the mound of muscle at the base of the thumb.  It is made of three muscles, and all three of those muscles are innervated by the recurrent median n., which branches from the median n. as it exits the carpal tunnel.  

The most superficial muscle of the thenar eminence is the abductor pollicis brevis m., and the next muscle as you move towards the middle of the palm is the flexor pollicis brevis m.  Deep to these two muscles is the opponens policis m.  As you consider the names and actions of these 3 muscles, recall that the thumb is rotated 90 degrees relative to digits 2-5.  So while abducting digits 2-5 spreads them apart from one another, abducting the thumb actually swings it anteriorly, perpendicularly away from the palm.  Opposition is the movement in which the thumb and pinky swing toward one another such that the the tips of digits 1 and 5 meet over the surface of the palm.

The hypothenear eminence:

The hypothenar eminence is the mound of muscle at the base of the pinky.  The names and positions of the three muscles of the hypothenar eminence are similar to those of the thenar eminence, except where the thenar muscles are “pollicis” (thumb), the hypothenar are “digiti minimi” (pinky), so that the three muscles of the hypothenar eminence are the abductor digiti minimi m. (sometimes with “brevis” added), flexor digiti minimi m., and opponens digiti minimi m..  the hypothenar muscles are all innervated by the ulnar n.  Again, recall that the movements of digits 2-5 are oriented 90 degrees different than the movements of digit 1.  

The middle compartment of the hand:

We’ll work from superficial to deep as we consider the muscles of the middle compartment of the hand.  

The lumbrical mm. are the first muscles you’ll come to in the center of the palm of the hand.  They are small and cylindrical (lumbricalis is the scientific name for the earthworm).  The lumbrical mm. do not attach to bone, but originate from the tendons of the flexor digitorum profundus m., and insert into the tendons of the extensor digitorum m. at a site called the extensor expansion.  Because they run from the flexor side of the palm to the extensor region of the digits the action of these muscles is to both flex the metacarpophalangeal joints, and extend the interphalangeal joints.  The first and second lumbricals are innervated by the median n., and the third and fourth lumbricals are innervated by the ulnar n.  

To dissect deeper into the palm of the and it will be helpful at this time to cut through the flexor retinaculum (transverse carpal ligament) and pull the tendons of the flexor digitorum superficialis and profundus out of the carpal tunnel.  This will give you more room to work into the deeper part of the palm.  Work a blunt probe or a scalpel handle (no blade) through the carpal tunnel, just deep to the transverse carpal ligament, and then cut the ligament by cutting down onto the probe or handle.  This will help ensure you don’t cut into the median n. or any of the muscle tendons.  Once the ligament is cut you should be able to pull the muscle tendons out of the carpal tunnel and “bowstring” them to give yourself more room to work in the palm.  You can also cut through the muscle bellies of the digital flexors, and reflect the free end distally so that you completely expose the deep palm.

Interosseus muscles:

There are two sets of muscles left to identify in the palm, the palmar interosseus mm., and the dorsal interosseus mm.  All interossei are innervated by the ulnar n.

You will first come to the palmar interosseus mm., of which there are three.  They attach to digits 2, 4, and 5.  The palmar interosseus attaching to digit 2 may be mostly covered by the adductor pollicis m.  Abduction and adduction of digits 2-5 are movements relative to digit 3 (known locally as the welcome to Rhode Island finger).  So adduction of digits 2-5 involves digits 1, 4, and 5 moving toward digit 3.

The dorsal interossei are most easily seen on the posterior (dorsal) surface of the hand.  Where the palmar interossei are adductors, the dorsal interossei are abductors (they spread the fingers).  Recall that the thumb (digit 1) and the pinky (digit 5) have their own abductors as part of the thenar and hypothenar muscle groups.  That leaves only digits 2, 3 and 4, but still there are 4 dorsal interossei.  Digit 3, which again is designated as the midline of the hand as concerns abduction and adduction of the fingers, has a dorsal interosseus on either side, so that it can move toward the radial (thumb) or ulnar (pinky) side of the hand.  This movement of the 3rd digit is termed radial or ulnar (lateral or medial) deviation of the finger.

So:

Palmar interossei ADduct - PAD

Dorsal interossei ABduct - DAB

Sensory innervation of the hand:

Sensory innervation of the hand fairly closely follows the pattern of motor innervation, except that while the radial n. has no motor input in the hand it does carry sensory output from the hand.  As with sensory innervation anywhere in the body you will find maps that show slight differences, which are no doubt reflective of the reality of variation in sensory innervation between individuals.