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< Scapular region >
The scapular region falls into the upper limb. However, the scapular region is usually dissected together with the back, so the region is often misunderstood to be included in the back.
Strictly speaking, the deltoid muscle belongs to the deltoid region. However, the authors assume that the deltoid muscle is a scapular region muscle for two reasons. The deltoid muscle is innervated by the scapular region nerve (axillary nerve) (Fig. 2-15). Also, the deltoid muscle must be dissected prior to the scapular region muscles, located underneath the deltoid muscle (Fig. 2-2).
In the comic strip above and in Fig. 1-3, the topography of the clavicle and the spine of scapula differs from reality. Readers should note that the two bone structures are placed nearly on a horizontal plane (Fig. 2-1). Touch yours.
The lateral half of the clavicle and the spine of scapula are the origins of the deltoid muscle as well as the insertions of the trapezius (Fig. 1-3).
Fig. 2-1. Deltoid muscle.
The action of the deltoid muscle (insertion: deltoid tuberosity) is abduction of the humerus. Alternative descriptions, such as abduction of the arm or abduction of the shoulder joint, are also correct. However, abduction of the humerus is recommended over other descriptions, because it is simple to use the muscle’s insertion (bone name) to describe the muscle action.
The large origin of the deltoid muscle also induces flexion of the humerus (origin: lateral half of clavicle) and extension of the humerus (origin: spine of scapula). By swinging movement of the humerus, a sprinter can run effectively. When the sprinter is in a crouching position, the deltoid muscle becomes prominent as it flexes the humerus.
Fig. 2-2. Scapular region muscles.
All of the scapular region muscles originate from the scapula. Among them, three are named after their detailed origins: the supraspinatus from the supraspinous fossa, the infraspinatus from the infraspinous fossa, and the subscapularis from the subscapular fossa. The remaining two are the teres minor and major.
The relationship between the axis of the synovial joint (Fig. 2-59) and the direction of the muscle is very important. A muscle that passes through a certain axis or lies parallel to the axis cannot induce any movement along the given axis.
The ball and socket joint, the most mobile type of the synovial joint (Fig. 8-45), can move along three axes. A muscle that passes through both the X and Y axes can induce movement only along the Z axis.
Fig. 2-3. Directions of scapular region muscles.
In case of the shoulder joint, three axes pass the center of the head of humerus. As explained above, the supraspinatus causes abduction (Z axis), the infraspinatus and teres minor give rise to the lateral rotation (Y axis), and the subscapularis induces medial rotation (Y axis).
In comparison, teres major and latissimus dorsi generate three actions of the humerus: adduction (Z axis), medial rotation (Y axis), and extension (X axis). The three actions are possible, because the two muscles don’t pass on or parallel to any of the three axes. Extension is induced since the origins of two muscles are posterior to the X axis (Figs. 1-3, 2-4,5).
The supraspinatus, infraspinatus, teres minor, subscapularis make up the “rotator” cuff that rotates the humerus. An exception is the supraspinatus which abducts the humerus (Figs. 2-2,3).
The rotator “cuff” embraces the shoulder joint. Cuff is a kind of band that hugs something. (In clinics, a balloon that embraces the arm to measure blood pressure is called a cuff.) The teres major and latissimus dorsi do not hold the shoulder joint (head of humerus) (Figs. 2-2,3); therefore these are not members of the rotator cuff.
Innervation in the scapular region has a pattern that two muscles are innervated by one nerve: the supraspinatus and infraspinatus are innervated by the suprascapular nerve, the teres minor and deltoid muscle by the axillary nerve, and the subscapularis and teres major by the subscapular nerve (Figs. 2-2,15).
The suprascapular nerve is named so because it passes the suprascapular notch to approach the two muscles (Fig. 2-2).
The teres MInor and DELToid muscle innervated by Axillary nerve can be remembered using the phrase “MIssissippi DELT-A.”
The supraspinatus is the muscle that initiates abduction of the humerus (Figs. 2-2,3) and the deltoid muscle is the muscle that completes the abduction (Fig. 2-1). The two muscles are controlled by different nerves.
Fig. 2-4. Triangular and quadrangular spaces.
The two teres muscles, the long head of triceps brachii (Fig. 2-28), and the humerus are boundaries of the triangular and quadrangular spaces. The Triangular space is bordered by three Ts (Teres minor, Teres major, Triceps brachii).
As the quadrangular space has one more angle, structures that pass through the quadrangular space (axillary nerve (Fig. 2-15), posterior circumflex humeral artery) are greater by one (circumflex scapular artery) (Fig. 2-17).
Unlike the subscapular nerve (to subscapularis, teres major), the axillary nerve has to pass through the quadrangular space in order to reach the teres minor, which is located posterior to the teres major.
Fig. 2-5. Muscles around scapula.
The figure above is not on a real horizontal plane; these muscles cannot be seen simultaneously on the same plane. However, this schematic illustration is helpful in summarizing the origins, insertions, and actions of the muscles, in relation to the scapula. An example is the rotator cuff to rotate the humerus (Fig. 2-3).
Three muscles (levator scapulae, rhomboid minor and major) hold the medial side of the scapula together with the serratus anterior. The serratus anterior protracts the scapula anteriorly (Fig. 1-4), which is required for a full punch. Hence, a nickname of the serratus anterior is the “boxer’s muscle.”
Serratus anterior makes a set together with the “serratus posterior” superior and inferior. Ribs are the origin of the serratus anterior, while ribs are the insertion of the two serratus posterior muscles (Fig. 1-10). These muscles’ portions that are attached to the ribs look like saw (serrate in Latin).
Due to the broad origin (R1–R8) of the serratus anterior, its nerve travels a “long” way on the “thoracic” wall. Thus, it is innervated by the “long thoracic” nerve (Fig. 2-14).
< Pectoral region >
We have come to the pectoral region. An adult female has two breasts in which the mammary glands and the subcutaneous tissue are contained.
Fig. 2-6. Breast.
In each breast, less than 20 mammary glands produce milk that flows through the corresponding lactiferous ducts. The milk is stored in lactiferous sinuses and then excreted when a baby sucks the nipple. The areola, which surrounds the nipple (Fig. 2-7), secretes oil to help a baby suck on the mother’s breast.
The subcutaneous tissue that supports the mammary glands determines the size of a breast. A woman with small breasts has a normal amount of mammary glands, but less than normal subcutaneous tissue. In the subcutaneous tissue, there are suspensory ligaments of breast to maintain the shape of the breast.
Fig. 2-7. Location of breast.
The level of the breast is from R2 to R6, and the level of the nipple, located at the center of the breast, is R4.
Fig. 2-8. Pectoralis major.
Pectoralis major underlies the breast. Its origins are from R2 to R6 (level of the breasts), the sternum, and the medial half of the clavicle.
The lateral half of the clavicle is occupied by the deltoid muscle (Fig. 2-1). In the surface anatomy, we can see a slight depression between the deltoid muscle and the pectoralis major, which is called the deltopectoral triangle. The deltopectoral triangle is also the entry site for the cephalic vein (Fig. 2-22).
Interestingly, the actions of the pectoralis major (adduction, medial rotation of humerus) are similar to that of the latissimus dorsi (adduction, medial rotation, extension of humerus) (Figs. 1-3, 2-3) due to their identical insertion (intertubercular groove) (Figs. 2-5,18).
Fig. 2-9. Pectoralis minor, subclavius.
Pectoralis minor is hidden beneath the pectoralis major because the origin of the pectoralis minor (R3–R5) is covered by that of the pectoralis major (R2–R6).
Pectoralis minor ends at the coracoid process to firmly fixate the scapula to the ribs (Fig. 1-4). Subclavius with the origin of R1 and insertion of the clavicle holds the clavicle in place. Therefore, it can be said that the two muscles, pectoralis minor and subclavius, play the role of ligament (holding the bones).
The brachial plexus passes between the clavicle and R1 (Fig. 3-11). Naturally, the nearby subclavius is innervated by a nameless short branch of the brachial plexus. The larger two pectoralis muscles are innervated by renowned branches of the brachial plexus (Fig. 2-20).
< Axilla >
The most distinguished component of the axilla is the brachial plexus. Prior to discussing the brachial plexus, let’s first learn about somatic nerves and nerve cells.
Fig. 2-10. Somatic nerves.
A somatic motor nerve controls the skeletal muscles, while a somatic sensory nerve receives signals from the receptors near the skeletal muscles (to be specific, skin, subcutaneous tissue, and the skeletal muscle itself).
A nerve cell has one nerve cell body, one or more dendrites, and one axon. The dendrite conveys impulse to the nerve cell body, while the axon conveys the impulse from the nerve cell body. One should be able to distinguish the two as shown in illustrations as above.
A motor nerve cell contains numerous dendrites. These dendrites are very short, and they are omitted in this simple drawing. The motor nerve cell with multiple dendrites and an axon is called a multipolar neuron. (Note that neuron is synonymous to nerve cell.)
Fig. 2-11. Development of a sensory nerve cell.
Before development, a sensory nerve cell has a single, long dendrite. Thus, one can state that the sensory nerve cell used to be a bipolar neuron which has a single dendrite and a single axon. After the development, the dendrite and the axon are fused. The sensory nerve cell then becomes a pseudounipolar neuron.
A nerve cell body located in the peripheral nervous system is called a ganglion. A sensory nerve has a ganglion; an example would be the spinal ganglion of spinal nerve (Figs. 1-2,19, 2-12).
The nervous system consists of the central nervous system (brain and spinal cord) and the peripheral nervous system (cranial nerve and spinal nerve).
Fig. 2-12. Somatic nerves in spinal nerve.
Somatic nerves exist along the central and peripheral nervous systems (Fig. 2-10). Likewise, students should be able to draw a realistic figure in which somatic nerves exist along the spinal cord (central nervous system) and the spinal nerve (peripheral nervous system).
The somatic motor nerve passes the anterior root, while the somatic sensory nerve passes the posterior root (Fig. 1-2). One can memorize it with a simple sentence, “Action is Anterior.” A is A.
After the two roots meet, the somatic motor and sensory nerves coexist in the trunk of the spinal nerve, anterior ramus, and posterior ramus (Fig. 1-2).
Fig. 2-13. Trunks, divisions, cords of brachial plexus.
The anterior rami of CN5–TN1 form the brachial plexus (Fig. 1-17). We can thus say that the brachial plexus contains both the somatic motor and sensory nerves (Fig. 2-12). The plexus also contains visceral motor nerves (for innervating sweat gland of upper limb, etc) (Fig. 3-18). But for now, only the somatic motor nerve will be dealt with.
The five anterior rami unite and split repeatedly to form three trunks, six divisions, and three cords. Do you know the old film actor, Robert Taylor? The sentence “Robert Taylor Drinks Coffee” represents the “Rami, Trunks, Divisions, Cords” of brachial plexus.
While the superior, middle, and inferior trunks are almost horizontal, the cords are nearly vertical. Therefore, the cords are named lateral, medial, and posterior based on their spatial relationship with the axillary artery (Fig. 2-16).
An important criterion in the brachial plexus is the divisions. Each trunk bifurcates into anterior and posterior divisions. Three anterior divisions form the lateral and medial cords for the anterior muscles such as pectoral region muscles (Figs. 2-8,9). In contrast, three posterior divisions form the posterior cord for the posterior muscles such as the scapular region muscles (Fig. 2-2).
Fig. 2-14. Branches of the brachial plexus.
In this figure, cords are drawn horizontally to maintain the simplicity. For the same reason, pure sensory branches are omitted.
Let us discuss the anterior branches (represented by solid lines in the figure). The lateral and medial pectoral nerves, from the lateral and medial cords, anastomose like blood vessels. The lateral pectoral nerve supplies only the pectoralis major, while the medial pectoral nerve innervates both the pectoralis major and minor (Fig. 2-20). Lateral pectoral nerve to Less muscles; Medial pectoral nerve to More muscles.
The musculocutaneous nerve from the lateral cord travels to the anterior arm muscles (Fig. 2-24), while the median and ulnar nerves travel to anterior forearm muscles (Fig. 2-32) and palm muscles (Fig. 2-52).
Dorsal scapular and suprascapular nerves, along with nerves from the posterior cord, supply the posterior muscles.
Fig. 2-15. Posterior branches of brachial plexus.
The arising locations of the posterior branches (dotted lines in Fig. 2-14) topographically correspond to their target muscles. The dorsal scapular nerve runs medial (dorsal) to the scapula (Fig. 1-9), the suprascapular nerve (passing suprascapular notch (Fig. 2-2)) runs superior and posterior to scapula, and the subscapular and thoracodorsal nerves run lateral (ventral) to the scapula (Fig. 2-20). Lastly, the axillary and radial nerves run posterior to the humerus (Figs. 2-24,28).
Here we offer a mnemonics: From medial to lateral, the posterior branches (DOrsal scapular, SUPrascapular, Subscapular, Thoracodorsal, Axillary, and Radial nerves) are kept in mind by “DOminant SUPer STAR in the backstage.”
The five distal branches (Musculocutaneous, Axillary, Median, Radial, Ulnar nerves) (Fig. 2-14) can be recalled by “My Aunt Mary Requires Umbrella.” She might be Mary Poppins.
Fig. 2-16. Three parts of the axillary artery.
The axillary artery connects the subclavian artery to the brachial artery. The corresponding boundaries are the lateral border of R1 and the inferior border of teres major which are also the boundaries of the axilla (Fig. 2-19). The pectoralis minor divides the axillary artery into three parts.
Fig. 2-17. Branches of the axillary artery.
Coincidentally, the 1st, 2nd, and 3rd parts give rise to one, two, and three branches, in that order. Those branches have the following features with the companion nerves.
The superior thoracic artery does not accompany a nerve, but the lateral thoracic artery accompanies the long thoracic nerve to the serratus anterior (Fig. 2-5).
In accordance with the name, the acromial branch of thoracoacromial artery goes to the acromion of the scapula (Figs. 2-1,2). The pectoral branch follows the lateral pectoral nerve to the pectoralis major (Fig. 2-20). You can easily guess the destination of the deltoid branch, the deltoid muscle.
The subscapular artery, a branch of the 3rd part, bifurcates. The thoracodorsal artery accompanies the thoracodorsal nerve to the latissimus dorsi (Fig. 2-20), and the circumflex scapular artery passes the triangular space by itself (Fig. 2-4).
Fig. 2-18. Proximal part of the humerus.
The anterior and posterior circumflex humeral arteries surround the surgical neck of humerus to anastomose with each other (Fig. 2-17). (The surgical neck below the greater and lesser tubercles is prone to fracture.) The posterior circumflex humeral artery passes the quadrangular space (Fig. 2-4) with the axillary nerve (Fig. 2-15).
The six branches of the axillary artery (Superior thoracic, Thoracoacromial, Lateral thoracic, Subscapular, Anterior and Posterior circumflex humeral arteries) can be recalled with “SomeTimes Life Seems A Pain.” (Anytime studying anatomy seems a pain.)
Fig. 2-19. Boundary of the axilla.
The axilla is a pyramid with an apex, four walls, and a quadrangular base. The triangular apex is formed by the clavicle, R1, and the superior border of scapula. Through the apex, the brachial plexus, axillary artery, and axillary vein enter the axilla from the neck (Figs. 2-13,16, 3-11).
Fig. 2-20. Clavipectoral fascia, adjacent structures in axilla.
The posterior wall of the axilla is the scapular region and the anterior wall is the pectoral region. The sagittal plane of the axilla shows the scapular and pectoral regions containing the scapula and pectoralis muscles, respectively.
In the anterior wall, the two fasciae of subclavius and the pectoralis minor are connected by the “costocoracoid” membrane. Its name is derived from the fact that it runs from the “ribs” to the “coracoid” process like the adjacent pectoralis minor (Fig. 2-9). The extension of the fascia of pectoralis minor to the axillary fascia is called the suspensory ligament of axilla.
Clavipectoral fascia is the sum of the fascia of subclavius, the costocoracoid membrane, the fascia of pectoralis minor, and the suspensory ligament of axilla.
The clavipectoral fascia holds the base of axilla (axillary fascia, subcutaneous tissue, skin) between the latissimus dorsi (Fig. 1-3) and the pectoralis major (Fig. 2-8). This is why the axilla is called the armpit in everyday life.
In this sagittal plane, the lateral cord of brachial plexus is superior to the medial cord because the cords lay oblique. Compare this sagittal plane to the corresponding anterior view (Fig. 2-13).
The lateral pectoral nerve travels through the costocoracoid membrane to supply the pectoralis major. The medial pectoral nerve travels through the fascia of pectoralis minor to supply the pectoralis minor and major (Fig. 2-14).
Thoracoacromial artery passes the costocoracoid membrane and its pectoral branch enters the pectoralis major (Fig. 2-17). In the opposite direction, cephalic vein passes the costocoracoid membrane to join the axillary vein (Fig. 2-23).
Fig. 2-21. Axillary lymph nodes.
The axilla also includes the following axillary lymph nodes. The subscapular, pectoral, and humeral nodes collect lymph from the scapular region, pectoral region, and arm, respectively. All lymph passes through the central node, the apical node (at the apex of axilla) (Fig. 2-19), and finally to the subclavian lymphatic trunk (Fig. 6-47).
During the dissection, students are advised to guess to where the discovered lymph nodes belong. After that, remove the lymph nodes to make nerves and arteries clear.
For the same reason, tributaries of the axillary veins should be removed. However, the cutaneous veins entering the axilla, which are not correspondent to the arteries should be preserved (Fig. 2-23).
< Cutaneous veins >
Fig. 2-22. Cephalic and basilic veins.
The two main cutaneous veins of the upper limb are the cephalic and basilic veins that originate from the dorsal venous network of the hand. The two veins anastomose via the median cubital vein in front of the cubital fossa (Fig. 2-29).
The cutaneous veins of the upper and lower limbs have bicuspid venous valves to prevent the blood from flowing downward due to gravity. In limbs, the portions of cutaneous veins above the venous valves bulge out. Similarly, just above the aortic valve, there are the aortic sinuses (Fig. 5-24) that also bulge out.
Fig. 2-23. Emptying of the cephalic and basic veins.
The cephalic vein passes through the deltopectoral triangle (Figs. 2-8,22), the costocoracoid membrane (Fig. 2-9), then goes into the axillary vein (Fig. 2-20).
The name “cephalic (head) vein” is derived from the ancient anatomists’ misunderstanding that the destination of cephalic vein would be the head.
Unlike the single axillary vein, the double brachial veins surround the brachial artery (Fig. 2-23). The ulnar and radial veins show the same pattern. Veins like these are called the accompanying veins. The accompanying veins are commonly found in upper and lower limbs since the venous return of blood from bulky muscles is enormous.
The basilic vein pierces the brachial fascia (Figs. 2-22,24) to drain into the border between the brachial and axillary veins. Consequently, the axillary vein collects two Brachial and one Basilic (three Bs) veins (Fig. 2-23). Variation of the veins is frequently encountered, so students need not be meticulous about finding exact pattern of veins in cadavers.
The dark red color of blood in the cutaneous veins appears bluish in living persons.
< Arm >
Fig. 2-24. Arm muscles.
Unlike daily life understanding, the word “arm” in anatomy is restricted to the area between the shoulder and elbow joints. Between the elbow and wrist joints is called the forearm (Fig. 2-32). Likewise, “Leg” in anatomy means only the region between the knee and the ankle joints (Fig. 8-27).
Sectioning of bone shows (outer) compact bone and (inner) spongy bone. Spongy bone is much more complicated with the trabeculae than in the figure above. Space between the trabeculae is the medullary cavity, which is filled with the bone marrow.
Arm muscles, enclosed by brachial fascia, are divided into the anterior and posterior ones by two intermuscular septa.
The musculocutaneous nerve innervates the anterior arm “muscles” then becomes the lateral “cutaneous” nerve of forearm. This is why its name is the “musculocutaneous.” Radial nerve, responsible for the posterior arm muscles, is in contact with the humerus (groove for radial nerve) (Fig. 2-28).
The median and ulnar nerves are on vacation in the arm. The nerves descend in the medial intermuscular septum which is relatively safe from outside impact.
Fig. 2-25. Biceps brachii.
The long head of biceps brachii starts from the supraglenoid tubercle and descends through the articular cavity (Fig. 2-59) of the shoulder joint and between the greater and lesser tubercles (intertubercular groove) (Fig. 2-18). The short head starts from the coracoid process. The biceps brachii tendon is directed deep to end at the radial tuberosity, which allows for supination and flexion of the forearm.
In a restaurant, the two actions of the biceps brachii are needed to pick up a piece of food with a fork and to take out a cork from a wine bottle.
Even though the biceps brachii passes the shoulder and elbow joints, the muscle hardly flexes the shoulder joint. Generally, a muscle mostly moves the joint closest to its insertion.
From the biceps brachii tendon, the bicipital aponeurosis arises to fuse with the antebrachial fascia of forearm (Fig. 2-32). The bicipital aponeurosis, which is the roof of the cubital fossa (Fig. 2-29) is clearly palpable in one’s body.
Fig. 2-26. Coracobrachialis.
The coracobrachialis from the coracoid process flexes the humerus. The coracoid process is the insertion of the pectoralis minor (Fig. 2-9) and the origin of the short head of biceps brachii (Fig. 2-25) and the coracobrachialis. The coracoid process is palpable in one’s deltopectoral triangle (Fig. 2-22).
Fig. 2-27. Brachialis.
The brachialis from the humerus to the coronoid process and tuberosity of ulna flexes the ulna. The words “of ulna” are needed to distinguish the coronoid process “of ulna” from the coronoid process “of mandible” (Fig. 4-12). Two coronoid processes are the insertion sites for the brachialis and the temporal muscle. A great portion of the brachialis is covered by the superficial biceps brachii (Fig. 2-25).
Fig. 2-28. Triceps brachii.
In the humerus, the groove for radial nerve is nearly vertical. Therefore, two heads of triceps brachii that originate from each side of the groove are named the medial and lateral heads.
The long head originates from the higher infraglenoid tubercle. The long head in fact covers the medial head entirely.
Generally, a long muscle covers a short muscle.
Basketball players have outstanding triceps brachii that allow strong and elaborate extension of the forearm. When one does push-ups, the triceps brachii extends the forearm, and the pectoralis major adducts the arm (Fig. 2-8).
< Cubital fossa >
Fig. 2-29. Cubital fossa.
Cubital fossa is the triangular depression bordered by the pronator teres (Fig. 2-33), the brachioradialis (Fig. 2-43), and an imaginary line between the medial and lateral epicondyles (Fig. 2-33). The floor of the cubital fossa is the brachialis (Fig. 2-27) and the supinator (Fig. 2-42).
Fig. 2-30. Structures of cubital fossa.
In the cubital fossa, the brachial artery bifurcates into ulnar and radial arteries (Fig. 2-62). The brachial artery is accompanied by the median nerve; the median nerve serves barely any function in the arm (Fig. 2-24), but serves many purposes in the forearm and the hand (Figs. 2-32,52).
Oddly, the radial nerve, responsible for the posterior arm and the posterior forearm, passes anterior to the lateral epicondyle, so it is slightly visible in the cubital fossa.
In contrast, the ulnar nerve does not appear in the cubital fossa since it passes behind the medial epicondyle.
Fig. 2-31. Flexion of the upper limb.
Let us make clear what flexion and extension in the upper limb are. In the case of the elbow joint, flexion is to move the forearm forward, narrowing the angle at the cubital fossa. (The angle is 180 degrees in anatomical position.) Extension is returning to the anatomical position from the flexed state.
Flexion also refers to the movement of the arm forward from the anatomical position; the backward movement from the anatomical position is called hyperextension. Flexion has a wider range of movement than hyperextension.
The ranges of the wrist joint for moving forward and for moving backward are similar. However, wrist flexion is defined as the forward movement, because it matches the direction of the elbow joint, metacarpophalangeal joints, and interphalangeal joints (Fig. 2-57). Simply put, in all upper limb joints, flexion is the forward movement.
< Forearm >
Fig. 2-32. Forearm muscles.
Forearm muscles, encircled by the antebrachial fascia, are categorized into the anterior and posterior forearm muscles. Borders are the two bones, the interosseous membrane, and a narrow intermuscular septum. The anterior forearm muscles are innervated mainly by the median nerve and partly by the ulnar nerve, while the posterior forearm muscles are innervated solely by the radial nerve.
In the horizontal plane, anterior forearm muscles occupy not only the anterior part but also the medial part of the forearm. This is because the superficial anterior forearm muscles originate from the medial epicondyle of humerus (Figs. 2-33,34,35,36). When one’s wrist joint is flexed intensively, the wrist joint tends to be adducted for this same reason.
Fig. 2-33. Pronator teres.
The origin of the pronator teres is the medial epicondyle. An additional origin is the coronoid process of ulna (Fig. 2-27), which allows the pronation of the forearm.
Fig. 2-34. Flexor carpi radialis.
In this book, metacarpal bones are depicted as the five lines to distinguish them from the phalanges and extensor expansions (Fig. 2-44). Such depiction is also applied for the illustration of the metatarsal bones in the foot (Figs. 8-29,30).
Flexor carpi radialis also has its origin at the medial epicondyle. The muscle is accompanied by the radial artery (Fig. 2-63). One can easily feel pulse of one’s radial artery between the flexor carpi radialis and the styloid process of radius.
The carpometacarpal joints are ellipsoid but hardly mobile, so the muscles ending at the metacarpal bones actually move the wrist joint (Figs. 2-31,46). As an exception, the 1st carpometacarpal joint which is the very mobile saddle joint (Fig. 2-56). Consequently, the muscles affecting the wrist joint arrive at the 2nd to 5th metacarpal bones (Figs. 2-34,36,45,47).
Fig. 2-35. Palmaris longus.
The third muscle with its origin at the medial epicondyle is the palmaris longus, accompanied by the median nerve that occupies the center of the anterior forearm muscles as the main motor nerve (Fig. 2-32).
The tendon of the palmaris longus is expanded to become the palmar aponeurosis. The palmar aponeurosis is firmly attached to the palm skin as the epicranial aponeurosis to the skin of the scalp is (Fig. 4-5). It is due to the palmar aponeurosis that you cannot pinch the skin of your palm. The palmaris longus’ action is the flexion of the palm (flexion of the wrist).
Palmaris brevis starts from the palmar aponeurosis and ends at the skin, like the facial muscles do (Fig. 3-1). This small muscle protects the underlying ulnar artery and nerve in the hand (Fig. 2-52).
Fig. 2-36. Flexor carpi ulnaris.
Lastly, the flexor carpi ulnaris also has its origin at the medial epicondyle. The muscle is accompanied by the ulnar artery and the ulnar nerve (Fig. 2-52), and approaches the pisiform then the 5th metacarpal bone.
Pisiform is a sesamoid bone that slides on the triquetrum to protect the flexor carpi ulnaris tendon from wearing out. If one extends one’s wrist vigorously, and the wrist will be abducted because of the muscle origin (lateral epicondyle, lateral supracondylar ridge) (Fig. 2-45). In our daily lives, this extension of the wrist joint happens frequently. So without the pisiform, the flexor carpi ulnaris tendon would be injured by the triquetrum.
Fig. 2-38. Flexor digitorum superficialis (left) and profundus (right).
Deep anterior forearm muscles originate from the ulna, the radius, and the interosseous membrane.
When compared to the origin of the flexor digitorum superficialis, the origin of the flexor digitorum profundus lies more distal. Therefore, the former wraps the latter in the origin side. However, their insertions are an exception to the wrapping rule. The flexor digitorum superficialis and profundus end at the middle and distal phalanges, respectively.
Fig. 2-39. Tendons of flexor digitorum superficialis and profundus.
This is because the flexor digitorum superficialis splits to terminate at the middle phalanx, while the flexor digitorum profundus passes the split to terminate at the distal phalanx. This can be memorized easily because the Superficialis Splits, while the Profundus Passes.
If one hyperextends the fingers, then hyperextends the wrist fully, the fingers will be flexed. This phenomenon occurs because the degree to which the flexor digitorum muscles can be lengthened is limited.
In anatomical position, the thumb is rotated at 90 degrees. So, the movement direction of the thumb is different from that of the other fingers. Nonetheless, there is a universal rule for their movements. Extension is to move fingers toward the corresponding nails, while flexion is to move fingers toward their fingerprints. Abduction is to move fingers away from the middle finger.
Fig. 2-40. Flexor pollicis longus.
Flexor pollicis longus flexes mainly the 1st distal phalanx, and accessorily the 1st proximal phalanx and the metacarpal bone.
Flexor pollicis longus and brevis (Fig. 2-54) of a couch potato are highly developed.
Fig. 2-41. Pronator quadratus.
The last anterior forearm muscle to discuss is the pronator quadratus which literally performs pronation together with the pronator teres (Fig. 2-33). When the two pronators contract, the radius rotates, while the ulna is fixed on the humerus with the olecranon as an axis (Fig. 2-28). Otherwise put, the ulna and radius form a pivot joint which is rotatable, while the humerus and ulna form a hinge joint which is not rotatable.
The interosseous membrane is a good example of syndesmosis (a kind of fibrous joint). Syndesmosis is slightly mobile unlike suture (another kind of the fibrous joints) (Fig. 4-14).
In anatomical position, one can touch his/her own styloid process of ulna. Pronation makes the styloid process disappear; instead, the head of ulna can be felt.
The posterior forearm muscles originating from the lateral epicondyle are the extensors. But there are two muscles (supinator, brachioradialis) that do not yield extension.
Fig. 2-42. Supinator.
The supinator has two origins for its function (supination) just as the pronator teres has two origins (Fig. 2-33). The supinator occupies a small portion of the floor of the cubital fossa (Fig. 2-29).
Although the forearm has the two pronators (teres and quadratus) and one supinator, supination is a stronger action than pronation, because of the big biceps brachii (Fig. 2-25). Using a right-handed person as an example, tightening the right-handed screw is more powerful than slackening it. The robust supination of the right forearm is also carried out to start up the engine of the car.
When the elbow is flexed, lateral rotation of the arm and supination of the forearm can easily be distinguished by doing them alternatively.
We refer to the position of lying down, supine position. The reverse is called prone position.
Fig. 2-43. Brachioradialis.
The Biceps brachii (Fig. 2-25), Brachialis (Fig. 2-27), and Brachioradialis (BBB) cause Bending of elBow (BB). What a plenty of Bs!
The BrachioRadialis is known as the Breaking Rule (or BetRayer) muscle. The rule is that elbow flexors are innervated by the musculocutaneous nerve (Fig. 2-24), However, the brachioradialis, as a posterior forearm muscle, is innervated by the radial nerve.
The brachioradialis’ function is more important when the forearm is pronated at a 90 degree angle. Beer Raising muscle is another nickname of the BrachioRadialis.
In most cases, the insertion of a muscle is close to the joint. There are rare cases in which the insertion is far from the joint.
Fig. 2-44. Extensor digitorum, extensor digiti minimi.
Unlike the flexor digitorum superficialis and profundus, the extensor digitorum is single; so the extension of fingers is not as powerful as their flexion. Consider the frequent activity of grasping objects with a hand.
The tendon of the extensor digitorum is expanded to become the extensor expansion of the 2nd to 5th fingers. Here, the word “extension” means the longitudinal enlargement, while the word “expansion” means the transverse enlargement. Two words have combined to become “extensor expansion.”
The extensor expansion is attached to the proximal, middle, and distal phalanges; therefore, the extensor digitorum extends the three phalanges at the same time (Fig. 2-57).
An additional muscle approaching the extensor expansion is the extensor digiti minimi.
Fig. 2-45. Extensor carpi muscles.
While there is only one extensor carpi ulnaris, there are a double extensor carpi radialis (brevis and longus).
The uniqueness of twin extensor carpi radialis can be explained with a theory.
Fig. 2-46. Ellipsoid joint (wrist joint).
The ellipsoid joint (e.g., wrist joint) has a concave ellipsoid articular surface on one side and a convex ellipsoid articular surface on the other side. It is like longitudinally half-split egg shells that are overlapped together.
The ellipsoid joint moves in two directions: flexion/extension and abduction/adduction. If one tries to circumduct the wrist joint excessively, it will not move as smoothly as the shoulder joint (ball and socket joint).
Fig. 2-47. Five muscles ending at metacarpal bones.
This figure shows the courses of the five muscles that end at the metacarpal bones (Figs. 2-34,36,45). The two axes of the wrist joint pass the carpal bones, which are proximal to the metacarpal bone (Fig. 2-46). Two flexors are anterior to the axis of flexion, extension, while three extensors are posterior to the axis. Three abductors are lateral to the axis of abduction, adduction, while two adductors are medial to the axis.
These muscles all work cooperatively. For instance, simultaneous contraction of the flexor carpi ulnaris and radialis results in the flexion of the wrist joint, and simultaneous contraction of flexor carpi ulnaris and extensor carpi ulnaris results in the adduction of the wrist joint.
Fig. 2-48. Anatomical snuffbox (lateral view).
When one hyperextend one’s thumb, and the tendons of the extensor pollicis brevis and longus become prominent. The depression between two tendons is called the anatomical snuffbox because the depression used to be the place to snuff the finely powdered tobacco.
Fig. 2-49. Extensor pollicis longus and brevis (rotated thumb).
In the figure above, the thumb is laterally rotated at 90 degrees to demonstrate the anatomical snuffbox. While the extensor pollicis brevis goes to the thumb directly (taking the shortcut), the extensor pollicis longus goes to the index finger then bends to the thumb (long course). This is how the anatomical snuffbox is formed.
When one’s thumb is hyperextended, it can be seen that the brevis reaches the proximal phalanx, and the longus approaches the distal phalanx (Fig. 2-48). The long course (proximal origin and distal insertion) of the Extensor Pollicis Longus (EPL) reminds us of the long history of the English Premier League (EPL).
Fig. 2-50. Abductor pollicis longus (not rotated thumb).
Abductor pollicis longus is located more anteriorly (Fig. 2-48). If one puts a pencil on one’s palm transversely then elevate the pencil with one’s thumb, the thumb is abducted by this muscle and the abductor pollicis brevis (Fig. 2-54).
Fig. 2-51. Extensor indicis.
The deepest extensor is the extensor indicis. Most of its muscle belly is covered by other muscles; only insertion tendon pops out.
The extensor digitorum (Fig. 2-44) is analogous to the rein on all four horses. The extensor digiti minimi (Fig. 2-44) and extensor indicis are the reins to the left and right horses.
It is difficult to extend the middle or the ring finger alone since there are no individual muscles for them. Another factor for this phenomenon is the intertendinous connections of the extensor digitorum (Fig. 2-44).
< Wrist >
Are you still confused by the names of carpal bones? The best sentences to commit to memory are “She Looks Too Pretty (Scaphoid, Lunate, Triquetrum, Pisiform). TRAP TO CAtch Her (TRAPezium, TrapezOid, CApitate, Hamate).” For the orientation, the trapeziUM is the closest to the thUMb (Figs. 2-52,56).
Fig. 2-52. Two retinacula of wrist, carpal tunnel.
The carpal bones, which are concave anteriorly, form a carpal tunnel with the flexor retinaculum.
Pulley-like structures can be found in the eye (Fig. 4-47), knee (Fig. 8-5), and neck (Fig. 3-6). In the wrist, the flexor retinaculum plays the role of a pulley. Without the retinaculum, fingers cannot be flexed with the wrist flexed.
A tennis wristband, which enforces the flexor retinaculum, aids strong flexion of fingers even in the flexed state of wrist. Excessive flexions of the both fingers and wrist do not occur simultaneously due to the limited elongation of the extensor digitorum (Fig. 2-44).
Lots of tendons of anterior forearm muscles (e.g., flexor digitorum superficialis and profundus (Fig. 2-38)) pass the narrow carpal tunnel. Therefore, the tendons are surrounded by a synovial sheath, which is a balloon including lubricant, synovial fluid (Fig. 2-52). This is somewhat similar to the synovial membrane containing synovial fluid (Fig. 4-17).
Similarly on the dorsum of the wrist, extensor retinaculum holds the extensor tendons that are also surrounded by a synovial sheath. However, there is no prominent tunnel like the carpal tunnel. Perhaps, this is because of the fewer number of extensor muscles (Fig. 2-52).
In contrast to what is shown in Fig. 2-52, the extensor retinaculum between the ulna and radius is proximal compared to the flexor retinaculum between the carpal bones.
Fig. 2-53. Structures external to flexor retinaculum.
Since the palmaris longus is the only muscle superficial to the flexor retinaculum, it becomes clearly visible after wrist flexion. The opposition of the thumb and little finger makes the palmar aponeurosis and the attached palmaris longus bulge out more.
The carpal tunnel includes the radial artery and median nerve, but not the ulnar artery and ulnar nerve (Fig. 2-52). Small tips to memorize them: The radial artery mainly forms the deep palmar arch, while the ulnar artery mainly forms the superficial palmar arch (Fig. 2-63). While the median nerve mostly innervates muscles passing the carpal tunnel, the ulnar nerve innervates only few. Both the ulnar artery and ulnar nerve are outsiders.
< Hand >
As shown above, the hand is intentionally drawn upside down, so that a reader may easily compare our illustrations with one’s own hands.
Fig. 2-54. Thenar eminence muscles and adductor pollicis.
Fig. 2-55. Hypothenar eminence muscles.
Just as thenar eminence has three muscles (abductor, flexor, opponens), the hypothenar eminence has three muscles (abductor, flexor, opponens). The adductor pollicis is found deep to thenar eminence muscles. However, there is no such thing as the adductor digiti minimi. Instead, the palmar interosseus is in charge of adducting the little finger (Fig. 2-60).
Flexor digiti minimi brevis has no counterpart; there is no such muscle as the flexor digiti minimi longus. The function of flexor digiti minimi longus is performed by the little finger parts of the flexor digitorum superficialis and profundus (Fig. 2-38).
As the articular surfaces of a saddle joint, two saddles are in contact at the right angle.
Fig. 2-56. Saddle joint (1st carpometacarpal joint).
The only remarkable saddle joint in our body is the 1st carpometacarpal joint, which enables flexion/extension and abduction/adduction of the thumb. It also assists in opposition of the thumb (Fig. 2-54).
For holding a Computer Mouse with opposition, the 1st and 5th CarpoMetacarpal joints need to move. Therefore, the opponens pollicis reaches the 1st metacarpal bone and is located deeper than flexor pollicis brevis (Fig. 2-54). In this manner, the opponens digiti minimi reaches the 5th metacarpal bone and is hidden by the flexor digiti minimi brevis (Fig. 2-55). The 5th carpometacarpal joint is less mobile than the 1st carpometacarpal joint (Fig. 2-56) but more so than the rest (Fig. 2-34).
Monkeys have tiny opponens, so its thenar and hypothenar eminences are not bulged out. This is a significant difference between humans and other primates.
Fig. 2-57. Lumbrical muscles.
Lumbrical muscles are special as they originate from the tendons of the flexor digitorum profundus (Fig. 2-38). They approach the extensor expansion of the 2nd to 5th fingers to allow flexion of the proximal phalanges and extension of the middle and distal phalanges. Note the three axes for three movements in the figure. The Lumbrical muscles starting with “L” allows the hand to make an L-shape, by flexing only the proximal phalanges.
There are three flexors of the 2nd to 5th fingers. The flexor digitorum profundus, flexor digitorum superficialis (Fig. 2-38), and lumbrical muscles are responsible for flexion of the distal, middle, and proximal phalanges, respectively. Three muscles sequentially contract to make a fist.
Fig. 2-58. Collateral ligaments.
Interphalangeal joints are hinge joints, not due to the shape of articular surfaces, but due to the presence of collateral ligaments. The ligaments prevent the adduction and abduction of the joint, while allowing the flexion and extension.
Fig. 2-59. Structures of synovial joint.
In general, a ligament connecting two bones is a thickened portion of the fibrous membrane of the articular capsule. When a joint accidentally exceeds its movement range, the ligament gets stretched out, which is called a sprain.
A cracking sound of the knuckles is caused by the movement of a finger joint to suddenly expand the articular cavity.
Unlike the interphalangeal joints which are hinge joints (Fig. 2-58), the metacarpophalangeal joints allow adduction and abduction. The palmar interossei adduct and flex the proximal phalanges. Conversely, the dorsal interossei abduct and extend them (Fig. 2-60).
Fig. 2-60. Palmar and dorsal interossei.
The term “interosseus” derives from the muscle origins between the metacarpal bones. There are three palmar interossei, rather than four, because of the adductor pollicis (Fig. 2-54). Since the adductor pollicis does not originate from metacarpal bones, the adductor is not an interosseus.
There are four instead of six dorsal interossei because of the abductor pollicis brevis (Fig. 2-54) and the abductor digiti minimi (Fig. 2-55). Since these two abductors originate from the carpal bones, they are not named as interosseus.
The palm muscles innervated by median nerve are memorized by the MEat LOAF. In summary, two lateral lumbrical muscles (Fig. 2-57) and thenar eminence muscles (Fig. 2-54) are contracted by the median nerve.
The ulnar nerve is responsible for the rest of the palm muscles. It controls the medial and deep palm muscles, which is rather unexpected considering that the course of the ulnar nerve is superficial to the flexor retinaculum (Fig. 2-52).
Fig. 2-61. Hand skin innervated by three nerves.
The radial nerve has no motor function in the palm. The only role it serves in the hand is the sensory innervation in the dorsum of the hand (Fig. 2-61).
In the hand, the median, ulnar, and radial nerves equitably occupy the skin as the cutaneous nerves. The keenest median nerve is used for touching important things such as money. For that, the sensitive 1st, 2nd, 3rd nails are also innervated by the median nerve.
A dermatome is an area of skin that is innervated by a spinal nerve. The dermatomes of the upper limb are as significant as the skin areas innervated by brachial plexus branches (Fig. 2-61). It is because a spinal nerve proximal to the brachial plexus can be damaged. An example is a herniated nucleus pulposus (Fig. 1-7) in the cervical vertebrae (Fig. 1-5).
In the fetal position, the dermatomes of upper and lower limbs can be easily drawn: CN5–TN1 in the upper limb (Fig. 2-13), and LN2–SN3 in the lower limb (Fig. 7-15).
< Arteries >
Fig. 2-62. Branches of brachial, radial, and ulnar arteries.
The brachial artery bifurcates into ulnar and radial arteries in the cubital fossa (Fig. 2-30).
Pulsations of the brachial artery, radial artery (Fig. 2-34), and ulnar artery (Fig. 2-36) are palpable because some parts of these arteries are not covered by the muscles and are close to the skin surface. On the other hand, the deep brachial artery is deep enough to make its way along the groove for radial nerve (Figs. 2-28,62).
There are anastomoses between the collateral arteries and recurrent arteries around the elbow joint. These small branches are hard to detect in routine dissection (Fig. 2-62).
Anastomoses between arteries (Figs. 2-62,63, 8-44), and anastomoses between veins (Fig. 2-22) exist throughout the whole body. Bloodstream through the anastomosis is called the collateral circulation. Anastomosis between arteries is especially crucial for protection of our body, that needs a constant supply of oxygen.
Anastomosis also exists between an artery and a vein, which is microscopic. Autonomic nerves open and close the arteriovenous anastomosis to control the amount of blood supply in individual organs. For instance, after dining, the arteriovenous anastomosis of the gastrointestinal tract is closed to increase blood flow in numerous capillaries (promoting digestion). Instead, the arteriovenous anastomosis in the brain is opened to decrease the blood flow, making you drowsy.
Fig. 2-63. Arteries of hand.
The main stream of radial artery is curved dorsally, so one can palpate the artery in the anatomical snuffbox (Fig. 2-48). The ulnar artery encounters the superficial palmar branch of radial artery to form the superficial palmar arch. The arch gives rise to the common palmar digital arteries. On the other hand, the radial artery encounters the deep palmar branch of ulnar artery to make the deep palmar arch that gives off the palmar metacarpal arteries.
All the arches and branches require the term “palmar” in their names, because there are “dorsal” carpal arch, “dorsal” metacarpal arteries, and “dorsal” digital arteries. These arteries are not illustrated in this book because they are too thin to be found during rapid cadaver dissection.
The two palmar arches are double anastomoses between the radial artery and the ulnar artery as a protection mechanism of the precious hand.