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Chapter 9: e Appendicular Skeleton
Chapter Introduction
e limbs of the body are the most complex regions of musculoskeletal anatomy; however, just as we discuss in eve chapter, they have evolved as a marriage of structure and function. e appendicular skeleton, or the skeleton of the limbs, has 126 adult bones (Figure 9.1). In Chapter 8, when we learned the bones of the axial skeleton, we discussed that the division between axial and appendicular is based in function rather than location. For example, the scapulae , the shoulder blades, are located on the trunk of the body, lying over the back of the ribcage. ey are not, however, pa of the axial skeleton; the scapulae are considered pa of the appendicular skeleton, and covered in this chapter, because they assist with the movement of the arm. e bones of the pelvis are considered appendicular for the same reason. Because humans stand upright and walk bipedally, the upper limbs and lower limbs have vastly di erent functional requirements. Early humans were walkers and runners; thus, the bones of the lower limbs are adapted for weight-bearing suppo , stability, and locomotion. In contrast, our upper limbs were not required for bearing the weight of the body. Instead, our arms evolved for throwing and car ing and are therefore highly mobile and can be utilized for a wide variety of activities. e large range of upper limb movements, coupled with the ability to easily manipulate objects with our hands and opposable thumbs, has allowed humans to construct the modern world in which we live.
Figure 9.1 e Appendicular Skeleton e appendicular skeleton consists of the shoulder, pelvis, limb, wrist, ankle, hand, and foot bones.
e bones of the appendicular skeleton develop via endochondral ossi cation (a process of developing bone from a ca ilage model discussed in more detail in Chapter 7 ) with one exception, the clavicle, which forms by intramembranous ossi cation. e development of the appendicular skeleton is one that begins ve early in emb onic development but isn’t completed until approximately age 25. erefore, it is likely that some of the readers of this book are still in the process of developing and growing their appendicular skeletons! e major events of skeletal development are outlined in Figure 9.2.
Figure 9.2Timeline of Skeletal Development
is chapter is divided into our exploration of the upper and then lower limbs. Each limb has a “girdle”—the suppo ive framework that ties it to the axial skeleton—and then the free limb. e limbs follow a pattern (illustrated in the “Anatomy of a Limb” feature); each sta s with a single bone in the proximal limb, then a hinge joint (the elbow and knee). e distal limb is composed of two bones, then a more complex joint that contains sho bones (wrist and ankle) before the hands and the feet, which share many structural prope ies. Learning the limbs is
9.1aClavicle
e clavicle has several impo ant functions. e clavicle is technically a long bone, but it shares function with long bones and at bones. Its function in movement is to suppo the scapula and to hold the shoulder joint superiorly and laterally away from the ribcage, allowing for maximal freedom of motion for the arm. e clavicle also se es an essential function to protect the underlying ne es and blood vessels as they pass between the trunk of the body and the upper limb.
e sternal end of the clavicle has a triangular shape and a iculates with the manubrium po ion of the sternum (Figure 9.4). is juncture between the medial end of the clavicle and the lateral edge of the sternum forms the sternoclavicular joint. is joint allows considerable mobility, enabling the clavicle to move in many directions during shoulder movements. e lateral or acromial end of the clavicle a iculates with the acromion of the scapula, the po ion of the scapula that forms the bony tip of the shoulder.
Figure 9.4 e Clavicle
e clavicle connects the acromion of the scapula to the sternum of the axial skeleton. erefore, it has a lateral acromial end and a medial sternal end.
e clavicle is one of the bones that can be used in forensics when t ing to decide if an adult skeleton is from a biologically male body (that is to say, a body that matured under the in uence of higher levels of testosterone) or a biologically female body (that is to say, a body that matured under the in uence of higher levels of estrogen). Whereas some clavicles (those from biological females) tend to be sho er, thinner, and less cu ed, other clavicles (those from the bodies of biological males) tend to be longer and have a larger diameter, greater cu atures, and rougher su aces where muscles attach.
9.1bScapula
Along with the clavicle, the scapula forms the other half of the pectoral girdle and thus plays an impo ant role in anchoring the upper limb to the body. e scapula is located on the posterior side of the shoulder. It is surrounded by muscles on both its anterior (deep) and posterior (supe icial) sides, and thus does not a iculate directly with the ribs.
e scapula has several impo ant landmarks (Figure 9.5). e three borders of the scapula, named for their positions within the body, are the superior border of the scapula, the medial border of the scapula, and the lateral border of the scapula. e two corners of the triangular scapula are called angles , the third corner forms the joint su ace. e superior angle of the scapula is located between the medial and superior borders, and the inferior angle of the scapula is located between the medial and lateral borders. e inferior angle is a
pa icularly signi cant bone feature, as the most inferior po ion of the scapula it se es as the attachment point for several powe ul muscles involved in shoulder and upper limb movements. e remaining corner of the scapula, between the superior and lateral borders, is the location of the glenoid cavity. is shallow depression cups the head of the humerus, the proximal bone of the arm. Together the humerus and glenoid cavity to form the glenohumeral joint.
Figure 9.5 e Scapula
e scapula is shown here from its anterior side, which faces the ribcage, and its posterior side, which faces muscles and skin of the back.
e scapula also has two prominent projections. Toward the lateral end of the superior border, between the suprascapular notch and glenoid cavity, is the hooklike coracoid process (see Figure 9.5, anterior view). is process projects anteriorly and cu es laterally. In the superior view (see Figure 9.3C), the coracoid process cu es under the lateral end of the clavicle. e coracoid process se es as the attachment site for muscles of the anterior chest and arm. A strong ligament that ties the clavicle to the coracoid process adds stability to the shoulder. On the posterior aspect, the spine of the scapula is a long and prominent ridge that runs across its upper po ion. Laterally the spine ends in the broad, at acromion. e acromion forms the bony tip of the superior shoulder region and a iculates with the lateral end of the clavicle, forming the acromioclavicular joint (see Figure 9.3). Together, the clavicle, acromion, and spine of the scapula form a V-shaped bony line that provides for the attachment of neck and back muscles that act on the shoulder (see Figure 9.3C).
e scapula has three depressions, each of which is called a fossa (plural = fossae). Two of these are found on the posterior su ace, one above and one below the scapular spine. ey are named for their positions relative to the spine, with the supraspinous fossa, named for combining the words supra (superior) and spinous (spine). e much larger infraspinous fossa gets its name by combining infra (inferior) with spinous (spine). If we turn the scapula over and look at its anterior (deep) su ace, as if we were sitting between the scapula and the rib cage, we nd the subscapular fossa. All of these fossae provide space for the muscles that cross the shoulder joint to act on the humerus.
9.2Bones of the Arm
Universally across the animal kingdom, limbs are constructed using a ve common layout. Proximally there are fewer, larger bones, and as you move distally down the limb, each region is composed of greater numbers of smaller bones (see the “Anatomy of a Limb” feature). is format allows for stability closer to the trunk and dexterity fu her away. In humans, the upper limbs and lower limbs are each divided into three regions. For the upper limbs, these regions consist of the brachium, located between the shoulder and elbow joints; the antebrachium, which is
(A) e humerus is the single bone of the brachium. (B) Its distal end has a iculating su aces where it forms the elbow joint with the radius and ulna.
Proceeding down toward the diaphysis of the humerus, the wide proximal region of the humerus narrows again as the epiphysis becomes the diaphysis or shaft of the humerus. is narrowing is a common site of arm fractures and therefore is called the surgical neck since it sometimes is a site of fracture-repairing surgeries. e shaft is mostly smooth and unremarkable, except for a roughened protrusion on the lateral middle called the deltoid tuberosity. As its name indicates, it is the site of attachment for the deltoid muscle.
Distally, the humerus contributes to the elbow joint and therefore has a number of anatomically-relevant features. Let’s sta with the anterior view. e medial and lateral sides of the humerus each have a prominent bony projection called an epicondyle. Both of these can be palpated (felt) if you wrap your hand around your arm just above the elbow joint. e medial epicondyle of the humerus is quite a bit larger than the lateral epicondyle of the humerus (Figure 9.7B). e roughened ridge of bone above the lateral epicondyle is the lateral supracondylar ridge. All of these areas are attachment points for muscles that act on the forearm, wrist, and hand. e powe ul grasping muscles of the anterior forearm are attached at the medial epicondyle, which is therefore larger and more robust than the lateral epicondyle that provide attachment for the weaker posterior forearm muscles.
e distal end of the humerus has two smooth areas for a iculation with the ulna and radius bones of the forearm to form the elbow joint. e more medial of these areas is the trochlea which a iculates with the ulna bone. Lateral to the trochlea is the capitulum, a knoblike structure located on the anterior su ace of the distal humerus. e capitulum a iculates with the radius bone of the forearm. ese two a iculation su aces can be compared in Figure 9.7B. e trochlea, which means “pulley” in Latin, is a gliding su ace that spans the anterior and posterior sides of the ulna which, as you will read about in the next section, pivots around this smooth su ace. e proximal end of the ulna cups the trochlea and so there are small depressions in the humerus to provide space to accommodate the ulna and radius when the elbow is fully bent ( exed). e processes of the ulna t into the coronoid fossa on the anterior side and the olecranon fossa on the posterior. Above the capitulum is the radial fossa, which provides space for the head of the radius when the elbow is exed.
Student Study Tip
e root “capit” means head, and the capitulum’s shape resembles a head.
9.2bRadius and Ulna
e forearm ts the limb pattern by having two bones (see the “Anatomy of a Limb” feature). e ulna is the medial bone of the forearm. It runs parallel to the radius, which is the lateral bone of the forearm (Figures 9.8A and 9.8B). e proximal end of the ulna resembles the letter C with its large trochlear notch (Figure 9.8C). is region ts around the trochlea of the humerus like a baseball glove around a baseball to form the hinge of the elbow joint. e anterior edge of the trochlear notch forms a prominent lip of bone called the coronoid process of the ulna. e posterior and superior po ions of the proximal ulna make up the olecranon process, which forms the bony tip of the elbow. Funnily enough, if you hit your “funny bone,” which isn’t funny at all but actually quite painful, you’re not hitting your humerus, but rather a vulnerable place where a major ne e, the ulnar ne e, passes along your olecranon process.
Student Study Tip
e coronoid process can be easily mixed up with the coracoid process, but you can use their spellings to help. Coro n oid goes with the ul n a; both contain the letter N. Cora c oid goes with the s c apula; both contain the letter C.
Figure 9.8 e Radius and Ulna
e radius and ulna make up the antebrachium. (A) e medial side of the radius and lateral side of the ulna are attached to each other by an interosseous membrane. (B) is view of the ulna shows the large olecranon process that ts into the olecranon fossa of the humerus when the arm is extended at the elbow. (C) A close-up view of the proximal ulna shows the trochlear notch, which cups the distal end of the humerus in the elbow joint.
e forearm is capable of supination and pronation, which allows us to ip the orientation of the hand. is movement is accomplished by the rotation of the radial head, which causes the distal radius to move over and across the distal ulna.
On the medial side of the distal radius is the ulnar notch of the radius. is shallow depression a iculates with the head of the ulna, which together form the distal radioulnar joint. At their distal ends, each bone has a delicate inferior projection that reaches toward the wrist. ese are the styloid process of the radius and the styloid process of the ulna (Figure 9.10A). e styloid process of the radius provides attachment for ligaments that suppo the lateral side of the wrist joint. e styloid process of the radius projects quite far distally, and limits lateral the range of motion of the hand. e styloid process of the ulna se es as an attachment point for a connective tissue structure that connects the distal ends of the ulna and radius.
Figure 9.10Bones of the Wrist and Hand
(A) e wrist is composed of eight carpal bones arranged in two rows. e metacarpal bones form the palm of the hand, and the phalanx bones (phalanges) compose the thumb and ngers. (B) A coronal section through the wrist illustrates the radiocarpal joint. e radius a iculates with the scaphoid and lunate. e ulna is shielded from the carpals by the a icular disc and does not directly a iculate with them.
e distal end of the radius has a smooth su ace for a iculation with two carpal bones to form the radiocarpal joint or wrist joint (see Figure 9.10A). e ulna does not technically contribute to the wrist joint; it is slightly sho er and separated from the carpal bones by a broca ilage disc (Figure 9.10B).
9.2cBones of the Wrist and the Hand
e wrist and base of the hand are formed by a series of eight small carpal bones (see Figures 9.10A and 9.10B). e carpal bones are arranged in two rows, forming a proximal row of four carpal bones and a distal row of four carpal bones. e bones in the proximal row, running from the lateral (thumb) side to the medial side, are the scaphoid (“boat-shaped”), lunate (“moon-shaped”), triquetrum (“three-cornered”), and pisiform (“pea- shaped”) bones. e small, rounded pisiform bone a iculates with the anterior su ace of the triquetrum bone. e pisiform thus projects anteriorly, where it forms the bony bump that can be felt at the medial base of your hand. e distal bones (lateral to medial) are the trapezium (“table”), trapezoid (“resembles a table”), capitate (“head- shaped”), and hamate (“hooked bone”) bones. e hamate bone is characterized by a prominent bony extension on its anterior side called the hook of the hamate bone.
e palm of the hand contains ve elongated metacarpal bones. ese bones lie between the carpal bones of the wrist and the bones of the ngers and thumb (see Figure 9.10). e metacarpal bones are numbered 1 – 5, beginning at the thumb. e rst metacarpal bone, at the base of the thumb, is separated from the other metacarpal bones. is allows it a freedom of motion that is independent of the other metacarpal bones, which is ve impo ant for thumb mobility. e remaining metacarpal bones are united together to form the palm of the hand.
Student Study Tip
A helpful mnemonic for remembering the arrangement of the carpal bones is “So Long To Pinky, Here Comes e umb.” is mnemonic sta s on the lateral side and names the proximal bones from lateral to medial (scaphoid, lunate, triquetrum, pisiform), then makes a U-turn to name the distal bones from medial to lateral (hamate, capitate, trapezoid, trapezium). us, it sta s and nishes on the lateral side.
e ngers and thumb contain 14 bones, each of which is called a phalanx bone (plural = phalanges). Digit 1 (the thumb, pollex) has two phalanges which are called a proximal phalanx bone and a distal phalanx bone. Digits 2 (index nger) through 5 (little nger) have three phalanges each; between the proximal and distal phalanges is a third bone, the middle phalanx (see Figure 9.10).
Digging Deeper:
Fractures of Upper Limb Bones
When we fall, our rst instinct is often to t to “catch” ourselves by stretching out a hand in the direction of the fall. is may be a hard-wired instinct to protect our more vulnerable organs, but often this fall onto an outstretched hand results in fractures of the upper limb bones.
Falls onto the hand or elbow can result in fractures of the humerus (see Figure A) as the force of the impact is transmitted up the limb. e surgical neck of the humerus is a frequent location of fracture, as the distal po ion of the humerus is driven into the proximal po ion. Falls or blows to the arm can also produce transverse or spiral fractures of the humeral shaft.
In children, a fall onto the tip of the elbow frequently results in a distal humerus fracture. In these, the olecranon of the ulna is driven upward, resulting in a fracture across the distal humerus, a supracondylar fracture.
Another frequent inju following a fall onto an outstretched hand is a Colles fracture (“collees”) of the distal radius (see Figure B). is involves a complete transverse fracture across the distal radius that drives the separated distal fragment of the radius posteriorly and superiorly. is is the most frequent forearm fracture and is a common inju in persons over the age of 50, pa icularly in older individuals with osteoporosis. It also commonly occurs following a high-speed fall onto the hand during activities such as snowboarding or skating.
e most commonly fractured carpal bone is the scaphoid, often resulting from a fall onto the hand. Deep pain at the lateral wrist may yield an initial diagnosis of a wrist sprain, but a radiograph taken several weeks after the inju , after tissue swelling has subsided, will reveal the fracture. Due to the poor blood supply to the scaphoid bone, healing will be slow and there is the danger of bone necrosis and subsequent degenerative joint disease of the wrist.
for stability during standing, walking, and running.
9.3aOs Coxae
Each adult os coxae is formed by three separate bones that fuse together during the late teenage years (Figure 9.12). e three bones of the childhood pelvis are the ilium , the ischium , and the pubis (Figure 9.12A). ese names are retained and used to de ne the three regions of the adult hip bone (Figure 9.12C). e superior ilium forms the largest pa of the hip bone. It is rmly united to the sacrum at the largely immobile sacroiliac joint (see Figure 9.11). e ischium forms the posteroinferior region of each hip bone; it suppo s the body when sitting. e pubis forms the anterior po ion of the hip bone. e pubis cu es medially where it joins to the pubis of the opposite hip bone at a specialized joint called the pubic symphysis.
Figure 9.12 e Child Pelvis (A) In childhood, there are three individual bones on each side of the pelvis—the ilium, the ischium, and the pubis. (B) An X-ray of child’s pelvis. (C) e lateral view and (D) anterior view of the adult os coxae. Each adult os coxae does not form until adolescence. In the adult os coxae, we still use the names ilium , ischium , and pubis to refer to their adult regions after fusion.
Ilium
When you place your hands on your hip, you can feel the arching, superior ridge of the ilium, the iliac crest (Figure 9.13). e iliac crest ends anteriorly with a rounded rough area, the anterior superior iliac spine, which can be felt at your anterolateral hip. Inferior to the anterior superior iliac spine is a rounded protuberance called the anterior inferior iliac spine. Both of these iliac spines se e as attachment points for muscles of the thigh. Posteriorly, the iliac crest cu es downward to end as the posterior superior iliac spine. Muscles and ligaments surround but do not cover this bony landmark, thus sometimes producing a depression seen as a “dimple” located on the lower back. Continuing inferiorly, we nd the posterior inferior iliac spine. is is located at the inferior end of a large, roughened area called the auricular su ace of the ilium. e auricular su ace a iculates with the auricular su ace of the sacrum to form the sacroiliac joint. Both the posterior superior and posterior inferior iliac spines se e as attachment points for the muscles and ve strong ligaments that suppo the sacroiliac joint. e large, inve ed U- shaped indentation located on the posterior margin of the lower ilium is called the greater sciatic notch because it provides a passageway for the ve large sciatic ne e, which carries much of the inne ation for the posterior leg.
Figure 9.13 e Ilium
e ilium forms the large fan-shaped superior po ion of the hip bone. (A) Lateral and medial views. (B) Anterior view.
From the crests, the two ilia have gentle sloping depressions on their anterior and medial su aces. Each of these is an iliac fossa and se es as a bowl to hold the contents of the lower abdominal cavity.
Ischium
e ischium forms the posterior po ion of the hip bone (Figure 9.14). e large, roughened area of the inferior ischium is the ischial tuberosity. is se es as the attachment for the posterior thigh muscles and also carries the weight of the body when sitting. Fitness instructors will often refer to the ischial tuberosities as the sit bones. You can feel the ischial tuberosity if you wiggle your pelvis against the seat of a chair. Projecting superiorly and anteriorly from the ischial tuberosity is a narrow segment of bone called the ischial ramus. e slightly cu ed posterior margin of the ischium above the ischial tuberosity is the lesser sciatic notch. e bony projection separating the lesser sciatic notch and greater sciatic notch is the ischial spine.
Figure 9.14 e Ischium
e ischium forms the posteroinferior po ion of the hip bone. (A) Lateral and medial views. (B) Anterior view.
e pubic bodies of each os coxae are joined at the pubic symphysis. Extending downward and laterally from the body is the inferior pubic ramus. e inve ed V-shape formed at the pubic symphysis is called the subpubic angle.
9.3bFeatures of the Whole Pelvis
e three areas of each hip bone—the ilium, pubis, and ischium—converge centrally to form a deep, cup-shaped cavity called the acetabulum (Figure 9.16). e acetabulum hosts the rounded head of the femur to form the hip joint. Another notable feature of the pelvis is a large opening formed as the rami of the pubis and ischium meet. e obturator foramen is largely lled in by a layer of connective tissue, but, similar to the sciatic notch, a large ne e passes through this space to reach the anterior leg.
Figure 9.16 e Os Coxae
In adulthood, the three pelvic bones fuse to form each of the os coxae bones. e three bones all contribute to the acetabulum, the bowl-shaped region of a iculation of the hip and femur. e ischium and pubis also meet to form an arch around a large opening, the obturator foramen.
LO 9.3.
e space internal to the bony pelvis houses the organs of the lower abdomen and pelvic cavities. Examine the inside of the pelvis using either the bones in a classroom or Figure 9.16. e internal space can be divided in two by the narrowest internal pa of the pelvis, the pelvic brim, which is formed by the pubic symphysis anteriorly, and the pectineal line of the pubis, the arcuate line of the ilium, and the sacral promonto (the anterior margin of the superior sacrum) posteriorly. e space superior to the pelvic brim is the greater pelvis. is broad area is occupied by po ions of the small and large intestines and, because it is more closely associated with the abdominal cavity, it is sometimes referred to as the false pelvis. More inferiorly, the narrow, rounded space of the lesser pelvis contains the bladder, the rectum, and the reproductive organs and thus is also known as the true pelvis. e roof of the lesser pelvis is called the pelvic inlet (Figure 9.17) because it is the entrance to the pelvis. Similarly, the inferior oor of the lesser pelvis is called the pelvic outlet. is opening is de ned by the pubic symphysis anteriorly, and the ischiopubic ramus, the ischial tuberosity, and the inferior tip of the coccyx posteriorly. e pelvic inlet and outlet become impo ant landmarks when considering bi h as the fetus moves down from the greater pelvis into the lesser pelvis and then passes out of the body through the pelvic outlet. e pelvic inlet and outlet are the narrowest pa s of the passage and therefore present anatomical constraints to human bi h. is process is considered in more detail in Chapter 27.
Figure 9.17 e Pelvic Inlet and Outlet
e passageway through the pelvis narrows in two places—the pelvic inlet and the pelvic outlet. ese landmarks form the borders of the greater and lesser pelvis. (A) Lateral view. (B) Anterior oblique view. (C) Coronal section.
Biological Sex and the Pelvis
Bones are highly dynamic structures and change over the course of the lifespan in relationship to their use, as well as to other physiological factors such as hormones and diet. Bigger bodies have bigger, wider pelvises and more muscular bodies exe more tension and force on the pelvis, and so these bones—like all bones—will become bigger and rougher in response. A key physiological factor in determining the shape of the pelvis is the hormone
Some key di erences between the formats of the arm and leg are found at the knee and the ankle. e leg has a sesamoid bone at the knee; the patella, or kneecap, a iculates with the distal femur. e ankle is formed by just two bones, rather than the eight of the more complex wrist.
9.4aFemur
e femur is the single bone of the thigh region (Figure 9.19). It is the longest and strongest bone of the body, and accounts for approximately one-qua er of a person’s total height. e rounded, proximal end is the head of the femur, which a iculates with the acetabulum of the hip bone to form the hip joint. If you can look at a femur from its medial side, at the center of the head, normally tucked deep into the acetabulum, is a small indentation called the fovea capitis. A ligament attaches here to tie the head of the femur to the acetabulum. An a e also uses the fovea capitis as an ent point into the femur.
Figure 9.19 e Femur and the Knee Joint
e femur is the single bone of the thigh region. Its head ts into the acetabulum of the os coxae to form the hip joint. Inferiorly, the femur a iculates with the tibia at the knee joint. e patella a iculates with the distal end of the femur.
Fitting our standard pattern, the head of the femur gives way to a narrower region, the neck of the femur. Just like the neck region of other bones, the femoral neck is a common site of fracture. e neck is sandwiched between two much larger regions of bone. To its medial side, of course, is the femoral head. To its lateral side is the greater trochanter—a large, upward-pointing, bony projection located above the base of the neck. Multiple muscles that act across the hip joint attach to the greater trochanter, which, because of its projection from the femur, gives additional leverage to these muscles. e greater trochanter can be felt just under the skin on the lateral side of your upper thigh. e bony out pocket continues around the posterior side of the bone (the inte rochanteric crest) and gives way to the lesser trochanter, a small, bony prominence that lies on the medial aspect of the femur, just below the neck. A single powe ul muscle attaches to the lesser trochanter. Running between the greater and lesser trochanters on the anterior side of the femur is the roughened inte rochanteric line (Figure 9.20, anterior view).
Running along the posterior shaft of the femur, the gluteal tuberosity is a roughened area extending inferiorly from the greater trochanter. More inferiorly, the gluteal tuberosity becomes continuous with the linea aspera (the name means “rough line”). is is the roughened ridge that passes distally along the posterior side of the mid- femur. Multiple muscles of the hip and thigh regions make long, thin attachments to the femur along the linea aspera (Figure 9.20, posterior view).
Student Study Tip
e gluteal tuberosity is in the gluteal region and is an attachment point for the gluteal muscles. Understanding where a name came from can help you remember it more easily.
Figure 9.20 e Distal Femur
Looking at the femur from an inferior view, we can see the two condyles and the ca ed-out space—the intercondylar fossa—between them. is space accommodates two major ligaments of the knee.
