< 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).
Fig. 2-1. Deltoid muscle.
The lateral 1/2 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).
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 (Figs. 2-1,19). Touch yours.
The action of the deltoid muscle (insertion: deltoid tuberosity) is abduction of the humerus (Fig. 2-1). 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’s action.
The large origin of the deltoid muscle also induces flexion of the humerus (origin: lateral 1/2 of clavicle) and extension of the humerus (origin: spine of scapula) (Fig. 2-1). 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 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 such as hip 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 for joint movement pass the center of the head of humerus. According to the principle, 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, the teres major and latissimus dorsi generate three actions of the humerus because the two muscles neither pass nor are parallel to any of the three axes. The three actions are adduction (Z-axis), medial rotation (Y-axis), and extension (X-axis). Extension occurs 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 “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 (Figs. 2-2,15).
The teres MInor and DELToid muscle innervated by the 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, the number of structures that pass through the quadrangular space (axillary nerve (Fig. 2-15), posterior circumflex humeral artery) is greater by one than that of what passes through the triangular space (circumflex scapular artery) (Fig. 2-17).
Unlike the subscapular nerve (to subscapularis, teres major), the axillary nerve (Fig. 2-15) 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). The three muscles’ portions that are attached to the multiple ribs look like a saw (serrate in Latin).
Due to the broad origin (R1–R8) of the serratus anterior, its nerve has to travel a “long” way on the “thoracic” wall. Thus, it is called 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 secreted when a baby sucks the nipple. The areola, which surrounds the nipple (Fig. 2-7), secretes oil to help the baby suck.
The subcutaneous tissue that supports the mammary glands determines the size of a breast. A woman with small breasts usually has a normal amount of mammary glands, but small amount of 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 R2–R6 (Fig. 2-6), 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 (Fig. 2-6). Its origins are from R2–R6 (level of the breast), the sternum, and the medial 1/2 of clavicle.
The lateral 1/2 of 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 the humerus) are similar to that of the latissimus dorsi (adduction, medial rotation, extension of the 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 (Fig. 2-26) to firmly fixate the scapula to the ribs (Fig. 1-4). Subclavius with the origin of R1 and the insertion of clavicle holds the clavicle in place (Fig. 2-19). Therefore, it can be said that the two muscles, pectoralis minor and subclavius, play the role of ligament (holding the bones) (Fig. 2-58).
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 (Fig. 2-14). 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 neurons. (Note that neuron is synonymous to nerve cell.)
Fig. 2-10. Somatic nerves.
A somatic motor nerve controls the skeletal muscle, while a somatic sensory nerve receives impulse from the receptor near the skeletal muscle (to be specific, skin, subcutaneous tissue, or skeletal muscle itself).
A neuron 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.
The neuron with multiple dendrites is called a multipolar neuron. The neuron of somatic motor nerve is a multipolar neuron. It has numerous very short dendrites, which are omitted in this simple drawing (Fig. 2-10).
Fig. 2-11. Development of a sensory neuron.
At the early stage of sensory nerve development, a neuron has a single, long dendrite (bipolar neuron). As the development goes on, the dendrite and the axon are fused. It then becomes a pseudounipolar neuron.
A nerve cell body located in the peripheral nervous system is called a ganglion. The sensory nerve has the ganglion (Fig. 2-10); 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 motor nerve and somatic sensory nerve exist along the central and peripheral nervous systems (Fig. 2-10). Thus, now students can understand the above 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 simple sentence, “Action is Anterior. A is A.”
After the two roots meet, the somatic motor and sensory nerves coexist in the trunk of spinal nerve, anterior ramus, and posterior ramus (Fig. 1-2).
All role of the posterior ramus is to innervate the deep back muscles (Fig. 1-11) and the covering skin (Fig. 1-2). Therefore, the posterior ramus is apparently thinner than the anterior ramus.
Fig. 2-13. Trunks, divisions, cords of brachial plexus.
The anterior rami of CN5–TN1 form the brachial plexus (Fig. 1-17). The brachial plexus contains both the somatic motor and sensory nerves (Fig. 2-12). The plexus also contains visceral motor nerve (for innervating sweat gland of upper limb, etc.) (Fig. 3-17). But for now, only the somatic motor nerve of the brachial plexus 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).
R1 that is a boundary between the neck and the axilla (Fig. 2-19) is in contact with the inferior trunk. It signifies that the brachial plexus resides in both the neck and the axilla.
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. It is stated again that only the somatic motor nerve is dealt with (Fig. 2-10); therefore, 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 (Fig. 2-22). 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. That is why the two proximal nerves are drawn in dotted lines.
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 anterior, lateral to the scapula (Fig. 2-20). Lastly, the axillary and radial nerves run posterior to the humerus (Figs. 2-24,28).
Here we offer 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, both of 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.
Regarding the thoracoacromial artery, the “acromial” branch goes to the “acromion” of the scapula in accordance with the name (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 clavicular and deltoid branches.
The lateral thoracic artery accompanies the long thoracic nerve to the serratus anterior (Fig. 2-5).
The subscapular artery 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 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” (Fig. 2-2) and “pectoralis” muscles (Figs. 2-8,9), 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). Consequently, the base of axilla is concave; that 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. 6-49).
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 are removed. However, the cutaneous veins entering the axilla, which are not correspondent to the arteries are 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 hand. The two veins anastomose via the median cubital vein in front of the cubital fossa (Fig. 2-29).
Commonly, the median cubital vein is used for obtaining blood samples, while the dorsal venous network is used for administering medications (Fig. 2-22).
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 cephalic and basilic 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 vein” is derived from the ancient anatomists’ misunderstanding.
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 >
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 ankle joints (Fig. 8-27).
Fig. 2-24. Humerus, arm muscles.
In the longitudinal plane of humerus, two epiphyses and one diaphysis, between which epiphyseal lines intervene, are identifiable.
The epiphysis consists of (outer) compact bone and (inner) spongy bone. The spongy bone is much more complicated with the trabeculae than in the figure above. The diaphysis is composed of (outer) compact bone and (inner) 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 (Fig. 2-14) innervates the anterior arm “muscles” then becomes the lateral “cutaneous” nerve of forearm. Radial nerve, responsible for the posterior arm muscle, is in contact with the humerus (groove for radial nerve) (Fig. 2-28).
In the arm, the median and ulnar nerves are on vacation, having no special role. The nerves descend in the medial intermuscular septum which is relatively safe from outside impact.
Fig. 2-25. Biceps brachii.
The short head starts from the coracoid process. 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 long head passes between the insertion tendons of the latissimus dorsi (Figs. 1-3, 2-5) and the pectoralis major (Fig. 2-8). (The pectoralis major covers the long head.) Due to the friction of the tendons, they easily get the inflammation when excessively used.
The tendon of biceps brachii passes the cubital fossa (Fig. 2-30), and 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 and to remove a cork.
Even though the biceps brachii passes the shoulder and elbow joints, the muscle hardly flexes the shoulder joint.
From the biceps brachii tendon, the bicipital aponeurosis arises to fuse with the antebrachial fascia of forearm (Figs. 2-25,32). The bicipital aponeurosis, which is a part of 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 easily palpable in one’s deltopectoral triangle (Fig. 2-22).
Fig. 2-27. Brachialis.
The brachialis from the humerus to the coronoid process of ulna and ulnar tuberosity 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 (Fig. 2-24) 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 of humerus (Fig. 2-33). The floor of the cubital fossa is the brachialis (Fig. 2-27) and 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 nothing 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. It is because the radial nerve innervates the supinator that presents in the cubital fossa (Fig. 2-29) even though it belongs to the posterior forearm muscles (Fig. 2-42).
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 state of flexion.
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 (Fig. 2-38), 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.
To begin with, 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 ones 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 is very mobile saddle joint (Fig. 2-56). Consequently, the muscles affecting the wrist joint arrive at the 2nd to 5th metacarpal bones (Figs. 2-36,45,47).
Fig. 2-35. Palmaris longus and brevis.
The third muscle with its origin at the medial epicondyle is the palmaris longus, accompanied by the median nerve. As the main motor nerve of the anterior forearm muscles (Fig. 2-32), the “median” nerve head for the middle finger that is ”median” plane in the hand (Fig. 2-60).
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 is to the scalp skin (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 ulnar 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 ulnar nerve (Fig. 2-52). That is why the ulnar nerve innervates the flexor carpi ulnaris. The ulnar nerve also controls the ulnar half of flexor digitorum profundus (Figs. 2-38,57). The flexor carpi ulnaris 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 (Fig. 2-36). If one extends one’s wrist vigorously, 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 happens frequently. So without the pisiform, the flexor carpi ulnaris tendon would be injured by the triquetrum.
The pisiform is strung to the tendon (Fig. 2-36). Attempt to move the pisiform right and left.
Fig. 2-38. Flexor digitorum superficialis (Left) and profundus (Right).
Deep anterior forearm muscles originate from the ulna, radius, and interosseous membrane. Exactly, the flexor digitorum superficialis originates from the medial epicondyle of humerus as well.
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 of the 2nd–5th fingers, 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 the anatomical position, the thumb is rotated at 90 degrees. So, the movement direction of the thumb is different from that of the other fingers.
Fig. 2-40. Flexor pollicis longus.
Flexor pollicis longus flexes mainly the 1st distal phalanx, and accessorily the 1st proximal phalanx and the 1st 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 as the insertion, while the ulna is fixed on the humerus with the olecranon (Fig. 2-28). In other words, the proximal 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 the fibrous joint). Syndesmosis is slightly mobile unlike suture (another kind of the fibrous joint) (Fig. 4-14).
Pronation makes the styloid process of ulna disappear; instead, the head of ulna gets prominent. Dry bones will help you understand.
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). 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 as 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 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’ flexion function is more important when the forearm is pronated at 90 degree angle. Beer Raising muscle is another nickname of the BrachioRadialis.
Usually, the insertion of a muscle is close to the joint. Exceptionally, the insertion is far from the joint (e.g., brachioradialis).
Fig. 2-44. Extensor digitorum, extensor digiti minimi.
Unlike the flexor digitorum superficialis and profundus, the extensor digitorum is single; thus 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–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. Imagine two egg shells split in half longitudinally, overlapped on each other.
The ellipsoid joint moves in two directions: abduction/adduction and extension/flexion. 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 moving wrist joint.
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 flexor carpi 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 the thumb is hyperextended, 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 with thumb rotated from anatomical position.
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 (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 (with 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 with thumb in anatomical position.
Abductor pollicis longus is located more anteriorly than the extensor pollicis brevis and longus (Fig. 2-48) and ends at the 1st metacarpal bone. If one puts a pencil on one’s palm transversely then elevates the pencil with one’s thumb, the thumb is abducted by the abductor pollicis longus and 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 the distal tendon pops out to be seen.
The extensor digitorum (Fig. 2-44) is analogous to the rein of all four horses. The extensor digiti minimi (Fig. 2-44) and extensor indicis (Fig. 2-51) are the reins of the right and left horses.
It is difficult to extend the middle or 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 >
Do not be confused by the names of carpal bones (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.
A tennis wristband, which enforces the flexor retinaculum, aids strong flexion of fingers even with the wrist flexed. Nevertheless, excessive flexion of 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 tendons of posterior forearm muscles that are surrounded by a synovial sheath too. However, there is no prominent tunnel like the carpal tunnel. Perhaps, this is because of the small size of extensor muscles of fingers (Fig. 2-52).
Exactly, 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 (Fig. 2-35).
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. The ulnar nerve is an outsider in the forearm.
< Hand >
As shown above, the hand is intentionally drawn upside down.
Fig. 2-54. Thenar eminence muscles, 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 which would be flexor digiti minimi longus. The function of absent 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 noteworthy saddle joint in our body is the 1st carpometacarpal joint, which assisted in flexion/extension and abduction/adduction of the thumb. It also enables opposition of the thumb (Fig. 2-54).
To hold a Computer Mouse with opposition, the 1st and 5th CarpoMetacarpal joints need to move. Therefore, the opponens pollicis reaches the 1st metacarpal bone (Fig. 2-54); the opponens digiti minimi reaches the 5th metacarpal bone (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 allow the hand to make an L-shape. For more memorization, the Lumbrical muscles insert at Lateral side of the extensor expansions.
There are three flexors of the 2nd to 5th fingers. The flexor digitorum profundus, flexor digitorum superficialis (Figs. 2-38,39), and lumbrical muscles are responsible for flexion of the distal, middle, and proximal phalanges, respectively. Three muscles sequentially contract to make a fist.
The 1st and 2nd lumbrical muscles and their origins (radial half of the flexor digitorum profundus) are innervated by the median nerve (Fig. 2-52).
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 which suddenly expands the articular cavity.
Fig. 2-60. Palmar and dorsal interossei.
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).
The term “interosseus” derives from the muscle origins between the metacarpal bones. There are three palmar interossei, rather than four (Fig. 2-60), because of the adductor pollicis. Since the adductor pollicis does not originate only from metacarpal bone, the adductor is not an interosseus (Fig. 2-54).
There are four instead of six dorsal interossei (Fig. 2-60) because of the abductor pollicis brevis and the abductor digiti minimi. Since these two abductors originate from the carpal bones, they are not named as interosseus (Figs. 2-54,55).
Among the anterior forearm muscles, the minority innervated by the ulnar nerve is the flexor carpi ulnaris (Fig. 2-36) and the ulnar half of flexor digitorum profundus (Figs. 2-38,57). Among the palm muscles, the minority innervated by the median nerve is introduced in the below comics.
In the palm muscles, two lateral lumbrical muscles (Fig. 2-57) and three 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 the 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. 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 nerve opens and closes 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 (Fig. 2-52) 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.
The Deep Palmar Arch (DPA) is Deep to the Adductor Pollicis (DAP) near the metacarpal bones (Fig. 2-54).
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.
Unlike the comics above, nowadays, the sheared radial artery is reconstructed by surgeon even if the ulnar artery is intact, in order to preserve the optimal circulation.