You could visualize the axial skeleton as two containers on a stick; i.e., the skull and thoracic cage attached to the vertebral column. But the skeleton’s other division, the appendicular skeleton (ap-en-DIK-û-lar; append-, hang upon), could be thought of as a set of powerful tools. Your upper limbs (arms, forearms, and hands) and lower limbs (thighs, legs, and feet) help you to interact with your surroundings. The chief use of the upper limbs is to manipulate objects. But humans are bipeds (two-legged walkers) who chiefly use their lower limbs for locomotion and bodily support. As a set of tools, the limbs are so functionally sophisticated that at birth an infant doesn’t know how to work them. From the earliest twitches of a newborn’s fingers, to a toddler’s comical waddle, to a teenager’s nerve-wracking attempts learning to drive, people spend nearly the first two decades of life perfecting the use of their limbs.
The functional capabilities of the limbs are the result of the interaction of bones, muscles, and nervous tissue. In this chapter, we will discuss the bones of the limbs and the girdles that attach the limbs to the axial skeleton. The appendicular skeleton consists of 126 bones (about 60% of the body’s bones). Anatomists classify appendicular skeletal bones into four groups: the pectoral girdle, upper limbs, pelvic girdle, and lower limbs (Figure 11-1).
THE PECTORAL GIRDLE
The pectoral girdle (PEK-to-ral; pector-, breast) or shoulder girdle, connects the upper limbs to the thoracic cage of the axial skeleton. A “girdle” is a belt-like structure; indeed, the pectoral girdle forms a semi-circular ring around the superior part of the thoracic cage. The right and left halves of the pectoral girdle each consist of a clavicle (KLAV-i-kl), or collarbone, and a scapula (SKAP-ū-la), or shoulder blade. Each scapula articulates with the bone in the proximal end of the arm. In addition, the pectoral girdle serves as an attachment site for skeletal muscles that move the upper limbs and the head.
Think about the flexibility and wide range of motion that your shoulders display when you shrug, throw a ball, or lift something over your head. These actions would be impossible (or at least difficult) if the pectoral girdle’s bones were rigidly attached to the thoracic cage. (To appreciate this, lock one shoulder firmly in place, and using the arm on the same side try to touch the end of a pencil to your nose!) The pectoral girdle articulates with the thoracic cage only at the manubrium. Ligaments and skeletal muscles in the shoulders, back, and anterior part of the chest hold the rest of this girdle in place. In a sense, much of the pectoral girdle “floats” above the thoracic cage, and these relatively loose attachments are what give your shoulders their exceptional mobility.
Figure 11-1. Appendicular skeleton
CLAVICLES
The two clavicles are slender, curved bones that anchor the pectoral girdle to the sternum (Figure 11-2).
Each clavicle lies horizontally along the anterior, superior surface of the thoracic cage. A clavicle’s lateral, acromial end (a-KRŌ-mē-al; acro-, tip; –omos, shoulder) is flattened and articulates with the acromial process of the scapula. The acromial end of the clavicle looks like the flat handle of a key, which can help you remember the root meaning of clavicle (clavic-, key). Just as a key inserts into a lock, the medial, sternal end of the clavicle inserts into the clavicular notch of the manubrium. In a sense, the manubrium is the “lock” in the axial skeleton that holds the pectoral girdle in place.
Clavicles act as lateral supports. As the shoulder attempts to move anteriorly or medially, the clavicle presses against the sternum, restricting excessive movement. Pushing objects with the arms exerts compression stress on the clavicles, and it will cause the body of the clavicle to thicken over time. However, excessive compression force, as could occur when breaking a fall with an outstretched arm, can break the clavicle. In the same way that a broken rafter (brace) can cause a roof to collapse, a broken clavicle can cause the shoulder to collapse.
Note: The clavicles of a bird articulate with each other, but not the sternum, thus forming the bird’s “wishbone.” This arrangement allows a greater range of motion in the pectoral girdle that a bird needs to beat its wings.
SCAPULAE
The two scapulae are large, triangular-shaped, flat bones lying on the posterior surfaces of ribs 2-7. Scapulae anchor the upper limbs to the pectoral girdle and serve as attachment sites for skeletal muscles of the back and shoulder. A scapula looks like the blade of an axe or shovel, which is how it gets its common name, the shoulder blade (Figure 11-3).
The anterior surface of the scapula rests over the posterior ribs and forms a slightly concave depression, the subscapular fossa. A major skeletal muscle that helps rotate the arm attaches to this fossa. The sharp edges of the body are called borders. The lateral border is closest to the axillary (armpit) region, the medial border is closest to the vertebral column, and the superior border forms the more horizontal edge superiorly.
Figure 11-2. The pectoral girdle
Laterally, the superior border ends in an abrupt indentation, the suprascapular notch, through which a nerve passes. Lateral to the scapular notch, a coracoid process (KOR-a-koyd; corac-, beak) extends anteriorly and laterally; this process is an attachment site for several arm muscles. A circular depression, called the glenoid cavity (GLEN-oyd; glen-, socket), at the lateral angle receives the head of the humerus (arm bone).
Posteriorly, the scapula has a prominent ridge, the spine that ends laterally as the acromion (a-KRŌ-mē-on). The acromion forms the bony tip of the shoulder and articulates with the acromial end of the clavicle. The ditch-like depression superior to the spine is the supraspinous fossa (soo-pra-SPĪ-nus; supra-, above); whereas, the surface inferior to the spine forms the infraspinous fossa (in-fra-SPĪ-nus; infra-, below). Each of these fossae is an attachment site for a major skeletal muscle that rotates the arm.
THE UPPER LIMB
In the same way that the branches of a tree become progressively smaller farther away from the trunk, the bones in the upper limb become progressively smaller farther away from the pectoral girdle. Each upper limb consists of 30 bones, including 1 large bone in the arm, 2 slightly smaller bones in the forearm and 26 small bones in the hand (including the wrist). All of the bones in the upper limb, except for the wrist bones, are long bones; whereas the wrist bones are short bones.
Figure 11-3. The scapulae
HUMERUS
The humerus (HŪ-mer-us; “shoulder”) is the longest and thickest bone in the upper limb, and it is the only bone in the arm, extending from the shoulder to the elbow (Figure 11-4).
The humerus articulates with the scapula proximally (superiorly), and with the forearm bones distally (inferiorly). The proximal epiphysis of the humerus has a smooth, rounded humeral head that articulates with the glenoid cavity of the scapula.
The part of the humerus where the head meets the rougher surface of the epiphysis is the anatomical neck. Lateral to the anatomical neck on the superior end of the humerus is the greater tubercle, and slightly inferior and medial to this tubercle is the smaller lesser tubercle; both tubercles are attachment sites for skeletal muscles that move the arm. An intertubercular groove separates both tubercles and supports a muscle tendon. Inferior to the lesser tubercle, the humerus begins to narrow in a region called the surgical neck. This name suggests the medical significance of this region of the humerus, for it is a common site of fractures. More importantly, the surgical neck marks the position of the epiphyseal plate of a growing humerus.
There are two notable landmarks on the shaft of the humerus. About halfway along the shaft’s lateral side is a roughened region called the deltoid tuberosity, so named because the large deltoid muscle attaches to it. A major nerve leading to the forearm passes along a shallow, longitudinal depression, the radial groove, on the posterior surface of the shaft.
The distal epiphysis of the humerus is the location of the condyle of the humerus, which consists of a set of articular surfaces and their adjacent fossae. The most prominent articular surface on the medial, anterior, distal epiphysis is the trochlea (TRŌK-lē-a). This condyle gets its name (trochle-, pulley) because part of the ulna (a forearm bone), glides over the trochlea like a rope moving over the grooved wheel of a pulley. Next to the trochlea on the lateral, anterior surface of the epiphysis is the capitulum (ka-PIT-ū-lum; capit-, head), named for its resemblance to a tiny, baldhead. The head of the radius (another forearm bone) articulates with the capitulum.
Figure 11-4. The humerus
The condyle of the humerus also contains fossae that make room for the proximal ends of the forearm bones when the elbow joint bends or straightens. When you bend your forearm, the coronoid fossa, immediately superior to the trochlea, receives the coronoid process of the ulna. At the same time, the radial fossa, immediately superior to the capitulum, receives the head of the radius. On the posterior side of the distal epiphysis is the large olecranon fossa (ō-LEK-ra-non), which receives the ulna’s helmet-shaped olecranon process (-cranon, helmet-like) when the elbow is straight.
To either side of the condyle of the humerus, two very prominent epicondyles (epi-, along side of) serve as attachment points for a variety of muscles that move the upper limb. The medial epicondyle is a rounded knob located medially and slightly superiorly to the trochlea. You can easily feel this epicondyle projecting from the medial surface of your elbow. Between the medial epicondyle and trochlea is a sulcus (groove) through which the ulnar nerve passes; you probably know this nerve as your “funnybone,” because hitting it causes a sharp tingling sensation. The lateral epicondyle is lateral and slightly superior to the capitulum.
ULNA
The forearm contains two long bones, the radius and the ulna, and both of these articulate with the humerus proximally, and with carpals (wrist bones) distally. The radius and ulna are parallel to one another when the forearm is in anatomical position (Figure 11-5).
Note: The letter “L” in “ulna” can help you recall this bone’s position. The uLna runs mediaLly along your forearm, between the trochLea of the humerus and the Little finger.
The ulna (UL-na; “elbow”) is the larger, medial bone in your forearm. It extends from the elbow to the medial, proximal part of the wrist (the little finger-side). You can feel your ulna along its entire length using your finger, beginning past the medial epicondyle and moving distally.
The ulna is widest at its proximal end, where it displays two processes. The larger olecranon process forms the point of your elbow posteriorly. The olecranon process fits into the olecranon fossa on the posterior, distal end of the humerus when the elbow is straight. Distal to the olecranon pro cess on the anterior side of the ulna is the pointed coronoid process. Like the coronoid process of the mandible, the coronoid process of the ulna gets its name because it looks like a bird’s beak. The coronoid process fits into the coronoid fossa on the anterior, distal end of the humerus when the elbow is bent.
Figure 11-5. Radius and ulna
The ulna has two distinct notches at its proximal end. Between the olecranon and coronoid processes is the deep, U-shaped trochlear notch, which articulates with the trochlea of the humerus. A much smaller and shallow radial notch on the lateral side of the coronoid process receives the head of the radius.
The distal end of the ulna is much narrower than its proximal end. The ulnar head is the knob like projection that you can feel and sometimes see just superior to the wrist on the medial, posterior side of your forearm. A short, pointed styloid process projects distally from the ulnar head’s posterior side, but it is not an attachment point for muscles or ligaments. Instead, a disc of fibrocartilage covers the styloid process and prevents it from rubbing against the carpal bones of the wrist.
RADIUS
The radius (RĀ-dē-us; “ray”) is the smaller lateral bone in the forearm. In a way, the radius is sort of like an upside-down ulna: the ulna is widest at its proximal end, whereas the radius is widest at its distal end. The narrow proximal end of the radius has a wheel-like radial head that articulates with the capitulum of the humerus and the radial notch of the ulna. Like a wheel, the radial head can “spin” against the capitulum and radial notch, allowing the radius to rotate along its length. Inferior to the radial head, the radius narrows to form the radial neck. Immediately inferior and medial to this neck is the radial tuberosity, a rough prominence to which a large muscle from the arm attaches.
The diameter of the radial shaft (diaphysis) gradually increases toward its distal end. Like the ulna, the radius has a pointed styloid process at the distal end, but it is on the lateral side instead of the medial side. The ulnar notch on the lateral, distal end of the radius receives the head of the ulna.
CARPALS
Besides helping you to manipulate your environment, your hands are exquisitely expressive tools of communication. In most cultures, people wave, chop, flutter, curl, clench, and clap their hands to intensify the emotion behind their speech. And of course, the deaf unfold a beautiful and richly expressive language solely through hand motions.
Although only three bones (the humerus, radius, and ulna) comprise three-quarters of the upper limb’s total length, the majority of bones in the upper limb are found in the hands. Each hand contains 26 bones: 8 carpal bones (wrist bones), 5 metacarpal bones (palm bones), and 14 phalanges (finger bones) (Figure 11-6).
The carpal bones (KAR-pal; “wrist”), or simply carpals, are tiny, short bones that form the joint between the hand and the forearm, and that allow the hand to bend at the wrist. The carpals are arranged in two irregular rows, including four carpals in the proximal row and four carpals in the distal row. The proximal row of carpals, from lateral to medial, includes the scaphoid, lunate, triquetrum, and the pisiform. The distal row of carpals articulates with the metacarpal bones, and from lateral to medial includes the trapezium, trapezoid, capitate, and the hamate.
In the proximal row of carpals, the scaphoid (SKAF-oyd) is the largest carpal. It is boat-shaped (scaph-, boat) and articulates with the distal end (including the styloid process) of the radius. Medial to the scaphoid is the lunate (LOO-nāt; “moon”), so-named because it looks like a crescent moon. The lunate articulates with the radius. Medial to the lunate is the triquetrum (trī-KWĒ-trum; “three-cornered”), a three-sided bone that articulates with a pad of fibrocartilage located at the distal end of the ulna. Lateral to the triquetrum is the pisiform (PIS-i-form), the smallest carpal, so named because it looks like a small pea (pisi-, pea). You can feel your pisiform bone as a small knob on the medial, proximal corner of your hand’s anterior surface.
Figure 11-6. Bones of the hand
In the distal row of carpals, the trapezium (tra-PĒ-zē-um) is the most lateral in position. Its name suggests that this bone looks like a small, four-sided table (trapez-, table). You can remember the trapezium as the carpal closest to the thumb because trapezium rhymes with thumb. The trapezium’s proximal side articulates with the scaphoid.
Medial to the trapezium is the trapezoid (TRAP-a-zoyd), another carpal that looks like a four-sided table. Medial to the trapezoid is the capitate (KAP-i-tāt), the longest carpal bone. This carpal is so-named (capit, head) because its smooth, rounded proximal end looks like a tiny head. Medial to the capitate is the hamate (HAM āt; hook), named for a hook-like projection on its anterior surface.
Your hand is able to move at the wrist because carpal bones can slide over one another. A small degree of wrist movement is due to skeletal muscles pulling directly on some of the carpals. Most wrist movement, however, results when skeletal muscles in the forearm pull on metacarpals inferior to the carpals.
Note: The carpals are “low” (inferior) on the upper limb, so you can remember their names (starting with the scaphoid and ending with the hamate) using the first letters of the phrase: Swing Low To Place The Tiny Carpals Here.
METACARPALS
Five metacarpal bones (MET-a-kar-pal; meta-, beyond) extend distally from the carpals to form the palm of the hand. These tiny long bones are numbered 1 (the most lateral) through 5 (the most medial). The proximal end of a metacarpal is its base, which articulates with a distal carpal bone. The bases of adjacent metacarpals also articulate with each other. A metacarpal’s rounded, distal end, the head, is larger than its base and articulates with the proximal bone of a digit (finger). When you make a fist, the head of each metacarpal forms one of the knuckles (revealing how the word “knucklehead” came about). The region between the base and the head is the body of the metacarpal.
Tendons of skeletal muscles in the forearm attach to the heads of metacarpals, allowing the hand to bend at the wrist when the muscles contract. Other skeletal muscles connect the metacarpals to bones in the digits. Contracting different groups of these muscles enable you to spread your fingers apart or pull them together.
PHALANGES
The five slender, distal extensions of your upper limbs are called digits (DIJ-its), or fingers. The hand’s digits are numbered in the same manner as the metacarpals. Digit 1, the pollex (POL-eks), or the thumb, is the most lateral, and digit 5, the little finger, is the most medial. The bones that support each finger are called phalanges (fa-LAN-jēz; “lines of soldiers”); each bone in a finger is a phalanx (FĀ-lānks). The pollex has two bones, a proximal phalanx and a distal phalanx. All other digits have three phalanges, including a middle phalanx between the proximal and distal phalanx.
Note: Recall that the palm faces forward and the hand’s dorsal surface faces backward in anatomical position. Accordingly, anatomists describe the anterior view of the hand as the palmar view and the posterior view of the hand as the dorsal view.
THE PELVIS AND PELVIC GIRDLE
Whereas the pectoral girdle joins the upper limbs to the axial skeleton, the pelvic girdle (PEL-vik) joins the lower limbs to the axial skeleton. The pelvic girdle lies between the sacrum and the femurs (thighbones), and consists of two hipbones, or coxal bones (KOK-sal; coxa-, hip).
Each hipbone lies on opposite sides of the body’s midline and articulates with the opposite hipbone, anteriorly, at a joint called the pubic symphysis. Each hipbone articulates with the sacrum, posteriorly. The pelvic girdle is not synonymous with the bony pelvis, but is a significant part of it. The bony pelvis is the massive bowl-shaped ring of bone that includes the pelvic girdle, sacrum and coccyx.
By this point, you may have noticed that the pelvic girdle looks massive and rigid compared to the shoulder girdle. Why is this so? Consider that when you lift one leg off the ground as you walk, the pelvic bone on the opposite side temporarily bears all the body’s weight above the sacrum plus the weight of the raised leg. Hence, a pelvic bone needs to be strong, and it needs a rigid articulation with the sacrum to transmit weight to the lower limb. The bones of the shoulder girdle, on the other hand, bear relatively little weight. They are loosely attached to the thoracic cage, permitting the wide range of shoulder motion that’s needed for you to make full use of your upper limbs. In short, the pelvic girdle emphasizes strength over flexibility, while the shoulder girdle emphasizes flexibility over strength.
Note: The bones of the pelvic girdle are sometimes called innominate bones (i-NOM i-nāt; in-, without; nomin, name); or literally, bones with no name!
Bones of the Pelvic Girdle
Each hipbone of the pelvic girdle forms from the fusion of three bones: the large ilium, the medium size ischium, and the slightly smaller pubis. The united bones form two of the pelvic girdle’s most conspicuous features. A portion of the all three bones forms a large, cup-shaped socket, the acetabulum (as-i-TAB-ū-lum), on the lateral side of a hipbone. The acetabulum articulates with the ball-shaped proximal end of the femur (thighbone).
Immediately inferior to the acetabulum, curved extensions of the ischium and pubis unite to form a large opening, the obturator foramen (OB-too-rā-tor). This foramen is so named (obtur-, stop up) because it is covered over by a thick web of ligaments. The pectoral girdle is shown in Figure 11-7.
Note: “Acetabulum” (aceta-, vinegar) literally means “vinegar cup,” because it looks like the small bowl that was set on dinner tables during Roman and medieval times. The cup was filled with vinegar to clean grease from the fingers following a meal.
Figure 11-7. The pelvic girdle
ILIUM
The ilium (IL-ē-um; “the flank”) is the largest part of a hipbone, forming its entire superior half. The ilium has two main regions: the wing and the body. The first feature of the ilium that you probably notice is its large wing, or ala (Ā-la, “wing”). When you look at the pelvis, the paired alae (Ā-lē) looks like the outstretched wings of a butterfly. The wing of the ilium has several prominent features marking sites where skeletal muscles attach. The iliac crest is a thick ridge running along the superior and lateral edge of the wing. You can feel your iliac crests by pressing on the sides of your abdomen slightly inferior to the level of your navel. The anterior, medial surface of the ilium forms a large depression, the iliac fossa, which serves as a point of attachment for a skeletal muscle that moves the thigh.
The ilium has four spines that serve as attachment sites for skeletal muscles, and they serve as important landmarks during physical exams and pelvic surgeries. The names of the spines describe their location on the ilium. The anterior superior iliac spine is the most anterior projection on the hipbone, and marks the anterior end of the iliac crest. Inferior and slightly medial to this spine is the anterior inferior iliac spine, which is slightly superior and anterior to the acetabulum. The posterior superior iliac spine is the most posterior projection on the hipbone, and marks the posterior end of the iliac crest. Slightly inferior and anterior to this spine is the posterior inferior iliac spine. Immediately inferior to the posterior inferior iliac spine is the deep greater sciatic notch (sī-A-tik; “hip”). The body’s largest nerve (the sciatic nerve) passes through this notch.
ISCHIUM
The ischium (ISH-ē-um or IS-kē-um; “hip”) is the second largest bone to become part of a hipbone, forming its posterior, inferior half. The ischium has three main regions: the body, the ischial tuberosity, and the ramus. If we compare the lateral view of the ischium to an elephant head and trunk, the ischium looks like the elephant’s head. The superior end of the ischial body joins with the ilium, while the anterior end joins with the pubis. Laterally, a portion of the ischial body forms another third of the acetabulum. A sharp ischial spine projects from the posterior rim of the ischial body. A thick ligament connects the ischial spine to the sacrum, helping to support the internal organs within the pelvic cavity.
The ischial tuberosity is the ischium’s most noticeable feature, and can be likened to the thickest part of an elephant’s trunk. The ischial tuberosity bends at the inferior end of the ischium, and this is where hamstring muscles from the thigh attach to the hipbone. A small indentation called the lesser sciatic notch is located between the ischial tuberosity and the ischial spine. This notch serves as a passageway for nerves and blood vessels. By the way, if you are sitting while reading this sentence, you are sitting on your ischial tuberosities.
The ramus of the ischium (ischial ramus) is like the tapering, distal end of the elephant’s trunk. It extends anteriorly from the ischial tuberosity to connect to the pubis. The ischial ramus and the body form the border for the posterior half of the obturator foramen.
PUBIS
The pubis (PŪ-bis), or pubic bone, is the smallest part of a hipbone, forming its anterior, inferior half.
Note: The pubis derives its name from the pubes (Pū-bēz), the hairs that grow around the region of the external genitalia following puberty, the onset of sexual maturity.
From a lateral view, the pubis looks like a smaller, mirror image of the ischium. The right and left pubic bones articulate at the pubic symphysis. The angle formed by the right and left pubic bones at the pubic symphysis is called the pubic arch.
TRUE PELVIS AND FALSE PELVIS
The bony pelvis has a complex three-dimensional structure, so it may seem hard to visualize the spaces in the body that the pelvis surrounds. To simplify matters, anatomists make a distinction between the false pelvis and the true pelvis. The wide-but-shallow false pelvis is the abdominal cavity space superior to the pelvic brim and inferior to the wings of the alae. You can follow the pelvic brim by tracing a line from the sacral promontory, along the inner margin of either ilium, and continuing along the superior edge of either pubis to the superior margin of the pubic symphysis. The narrower-but-deeper true pelvis is the pelvic cavity space inferior to the pelvic brim.
To be sure that you understand that the true and false pelves (PEL-vēz) represent spaces, it may help to visualize the bony pelvis as two stacked and tilted bowls. The space inside the rim of the upper bowl is analogous to the false pelvis; in the body, parts of the small and large intestines occupy this space. On the other hand, the space inside the rim of the lower bowl is analogous to the true pelvis; the urinary bladder and parts of the reproductive system occupy this space.
The true pelvis has an inlet and an outlet, which suggests the route in the female pelvis through which the fetus passes during birth. The larger, superior opening of the true pelvis is the pelvic inlet, which is formed by the pelvic brim. The pelvic outlet is the smaller, inferior opening of the true pelvis. You can outline the border of the pelvic outlet by tracing anteriorly from the tip of the coccyx, to either of the ischial spines and ischial tuberosities; then continue anteriorly along either of the inferior pubic margins to the inferior margin of the pubic symphysis.
There are notable differences between the pelvis of a male and the pelvis of a female. Some of these differences relate to differences in body size; however, a number of differences relate to the female’s childbearing capability.
Table 11-1 lists the characteristics of the male and female pelves and Figure 11-8 illustrates them.
Table 11-1. Comparison of Male and Female Pelvis
Figure 11-8. Differences in the male and female pelvis
THE LOWER LIMB
Genetic traits influence the size and shape of the lower limb bones, but the mechanical stress these bones receive every day ultimately determines their strength. Recall that mechanical stress triggers bone remodeling, a process that can make bones stronger. The body’s weight compresses the bones of the lower limbs whenever a person stands, a situation that is analogous to resting the head of a hammer on top of a nail head. But striking forces, such as those that occur when a person jogs, multiply many times over the weightbearing force on the lower limb bones, a situation that is analogous to striking the same nail head with a hammer. It should come as no surprise, therefore, that the lower limb bones grow many times stronger than the upper limb bones as a person matures.
Apart from the differences in thickness and strength, the arrangement of different-size bones in the lower limb is similar to that in the upper limb. The largest bones are proximal to the axial skeleton, while the smallest bones are distal to the axial skeleton. In addition, each lower limb consists of 30 bones, the same quantity as in each upper limb. Each lower limb consists of 1 bone in the thigh, 2 bones in the leg, 1 large sesamoid bone in the knee, and 25 bones in the foot (which includes the ankle). Notice that the foot contains one less bone than the hand; however, the sesamoid bone in the knee makes up the difference in number.
FEMUR
The femur (FĒ-mur; “thigh”) is the longest and thickest bone in the body, and it is the only bone in the thigh (Figure 11-9).
The femur articulates with the hipbone proximally and with the tibia (shinbone) distally. The head is ball-shaped and projects medially and superiorly from the proximal epiphysis. The femoral head fits snuggly into the acetabulum of the pelvic girdle. The fovea capitis (FŌ-vē-a KA-pi-tis; fov-, pit) is a small pit in the medial part of the femoral head. A short ligament extends from the fovea capitis that anchors the femoral head to the acetabulum.
Continuing distally along the femur, the neck is a constricted region connecting the femoral head to the remainder of the proximal epiphysis. Lateral and superior to the neck is a large process called the greater trochanter (trō-KAN-ter). You can feel the greater trochanter on the lateral side of your thigh, about a hand’s length inferior to the iliac crest. The lesser trochanter is a smaller process on the medial side of the epiphysis, inferior to the femoral neck. Both trochanters serve as attachment points for various hip and thigh muscles.
Figure 11-9. The femur
The shaft (diaphysis) extends medially at an oblique angle from the proximal epiphysis to end distally at the knee joint. As a result, both femurs converge toward the knees so that most of the body’s weight balances on a relatively narrow base directly over the feet. This arrangement facilitates walking and running. For example, stand in anatomical position and then start jogging in place naturally and you’ll notice your head and torso bob up and down along your body’s midline. Then try jogging with your knees spread one foot apart, and you’ll notice your body shifting in an awkward side-to-side motion. This would not be an efficient way to run.
The anterior surface of the femoral shaft is relatively smooth compared to its posterior surface. A roughened line, the gluteal tuberosity, on the superior, posterior surface of the shaft marks the attachment site for a large hip muscle. A long ridge, the linea aspera (LIN-ē-a, “line”; AS per-a, “rough”), extends distally from the gluteal tuberosity and marks the attachment site for large hip muscles.
The most prominent features at the distal end of the femur are the two large condyles that articulate with a large bone in the leg (inferior to the knee). The lateral condyle of the femur aligns with the greater trochanter superiorly, while the medial condyle aligns with the femoral head. The outer edges of the lateral condyle and the medial condyle form rounded processes, the lateral epicondyle and the medial epicondyle, respectively.
A deep intercondylar fossa exists between the condyles on the posterior side and holds ligaments that stabilize the knee joint. A smooth, shallow patellar surface unites between the condyles on the anterior side, and marks the site where the patella (kneecap) articulates with the femur. The patellar surface can help you decide from which side of the body an individual femur originated. The femoral head always faces medially, so if you hold the patellar surface toward you and the femoral head faces to your right, you are looking at a right femur.
PATELLA
The patella (pa-TEL-a; “little dish”), or knee cap, is a large, triangular sesamoid bone on the anterior side of each knee joint. In fact, the two patellae (pa-TEL-ē) are the body’s largest sesamoid bones. As with most sesamoid bones, the patella develops within a muscle’s tendon; in this case, the tendon extends from a large thigh muscle that straightens the knee. In addition to protecting the knee joint, the patella keeps the muscle tendon centered over the knee joint whenever the knee joint bends.
In anatomical position, the patella looks like an upside-down triangle. The anterior surface is convex (protrudes outward or anteriorly) and has a rough texture, serving as the attachment site for the thigh muscle’s tendon. A thick ligament extends from the patella’s apex and attaches to the proximal end of the tibia. When the knee bends during walking, this ligament pulls the patella inferiorly. When the knee straightens, the thigh muscle tendon pulls the patella superiorly over the knee joint. The patella’s posterior surface has a smooth texture that reduces friction between the patella and the femur.
TIBIA
The leg (the region between the knee and the foot) contains two long bones, the tibia and the fibula. The tibia and fibula are parallel and articulate with one another. Only the tibia, however, articulates with other bones (the femur and a large bone in the foot). In anatomical position, the tibia is located medial to the fibula.
The tibia (TIB-ē-a) is so named (tibi-, flute) because its pipe-like shape was thought to resemble a flute. The tibia is the largest bone in the leg and is the only leg bone that articulates with the femur. Consequently, the tibia is the weight bearing bone of the leg. The proximal epiphysis of the tibia exhibits two large condyles: the lateral condyle and the medial condyle. Each condyle has a smooth, concave articular surface on the posterior-superior side of the proximal epiphysis that articulates with a corresponding condyle on the distal end of the femur. Medial and lateral intercondylar tubercles form a double ridge called the intercondylar eminence that separates the tibial condyles from one another.
Slightly inferior and anterior to the tibial condyles, the rough tibial tuberosity marks the attachment site for the ligament leading from the patella. You can feel your tibial tuberosity as a small knob beneath the skin immediately inferior to the patella. Lateral to the tibial tuberosity and inferior to the lateral condyle, a small facet marks the site where the tibia articulates with the proximal end (head) of the fibula.
The distal epiphysis of the tibia is slightly narrower than the proximal epiphysis, and articulates with the talus bone of the foot. A large process, the medial malleolus (ma-LĒ-ō lus; “hammer”) projects medially and inferiorly from the distal epiphysis. The medial malleolus is the large knob that most people mistake for the “inside part of the ankle,” even though it is really part of the leg. The lateral side of the tibia’s distal epiphysis is a shallow depression, the fibular notch, which articulates with the distal end (lateral malleolus) of the fibula. The inferior surface on the distal epiphysis articulates with the talus bone of the foot.
FIBULA
The fibula (FIB-ū-la) is the other bone in the leg, and unlike the tibia, it does not articulate with the femur or any foot bones. Hence, the fibula does not support any of the body’s weight that is transmitted through the femur. This is why a person can still walk—albeit painfully—with a broken fibula. As its Latin root suggests (fibul-, fastener or clasp), the fibula resembles the clasp of a safety pin. The fibula serves as an attachment site for many of the skeletal muscles in the leg.
Figure 11-10. Tibia and fibula
The head is the proximal epiphysis, and it articulates with a facet on the tibia’s lateral condyle. Continuing distally, the fibular shaft completely ends at the distal epiphysis, which is about the same size as the fibular head. The medial surface of the fibula’s distal epiphysis articulates with the fibular notch on the tibia’s distal epiphysis. The lateral malleolus projects inferiorly from the distal epiphysis. Most people may mistake this large knob for the “outside of the ankle,” but like the medial malleolus of the tibia, it is indeed part of the leg. See the leg bones in Figure 11-10.
The femur, tibia, and fibula make up four fifths of the lower limb’s total length. The patella doesn’t contribute to the lower limb’s length. Like the hand of the upper limb, however, most bones of the lower limb are found in the foot. Each foot contains 25 bones, including 7 tarsal bones, 5 metatarsal bones, and 14 phalanges.
TARSALS
The tarsal bones (TAR-sal; “flat surface”) make up the tarsus (ankle). They are short bones that form the joints between the foot and the leg that allow the foot to move at the ankle. The tarsals can be grouped according to their relative sizes. The largest and most proximal tarsal bones include the calcaneus and talus. The smaller and more distal tarsals include the navicular and four bones that articulate with the metatarsals: the cuboid and the lateral, intermediate, and medial cuneiforms.
Note: The following phrase may help you to remember the tarsals’ names and positions (starting with the calcaneus and ending with the medial cuneiform): Crazy Tarsals Never Can Live In Me.
The calcaneus (kal-KĀ-nē-us; “heel”) is the largest and the most posterior tarsal bone. The rounded posterior and inferior projection of the calcaneus is the calcaneal tuberosity, the part of the foot that strikes the ground first when a person walks. The calcaneus articulates with two other tarsal bones: the talus superiorly and the cuboid bone anteriorly. A large tendon extends from the calf muscle to attach on the posterior surface of the calcaneus.
Note: In anatomical position, the top of the foot (its dorsal surface) faces up while the bottom of the foot (its plantar surface) faces down. Thus, anatomists call a view looking at the top of the foot a dorsal (not superior) view and a view looking from below the foot a plantar (not inferior) view.
The talus (TĀ-lus; “ankle”) is the second largest tarsal bone and lies on the superior sur- face of the calcaneus. The anterior surface of the talus articulates with the navicular bone, and the superior surface articulates with the tibia; in fact, the talus is the only tarsal bone that articulates with the tibia. For this reason, the talus bears the brunt of the body’s weight and disperses it to the calcaneus and the navicular.
The navicular (na-VIK-ū-lar; “boat”) is a curved tarsal bone that, from a plantar view, looks a bit like a canoe. The navicular’s posterior surface articulates with the talus, and its anterior surface articulates with the three cuneiform bones.
Moving to the most distal tarsals, the cuboid (KŪ-boyd) is so-named because of its cube-like shape. The cuboid is the most lateral of the tarsal bones, and it articulates with the calcaneus posteriorly, with the lateral cuneiform and the navicular medially, and with the fourth and fifth metatarsals anteriorly.
The three cuneiform bones (kū-NĒ-i-form; cunei-, wedge; –form, resembles) are so named because they look like little wedges driven between the other bones of the foot. The lateral cuneiform articulates with the cuboid bone laterally, with the navicular bone posteriorly, with the intermediate cuneiform medially, and with the third metatarsal anteriorly. The intermediate cuneiform articulates with the navicular posteriorly, with the lateral cuneiform laterally, with the medial cuneiform medially, and with the second metatarsal anteriorly. The medial cuneiform is the most medial of the distal tarsals. It articulates with the navicular posteriorly, with the intermediate cuneiform laterally, and with the first metatarsal anteriorly. Collectively, the navicular and the cuneiform bones form the arch shaped instep of the foot (Figure 11-11).
Figure 11-11. Bones of the foot
The Metatarsals
Five metatarsal bones (MET-a-tar-sal; meta-, beyond) extend distally from the tarsals to form the middle portion of the foot. Like the metacarpals of the hand, the metatarsals are numbered 1 (most medial) through 5 (most lateral). The proximal end of a metatarsal is its base, which articulates with an anterior tarsal bone. The bases of adjacent metatarsals also articulate with each other. A metatarsal’s rounded, distal end, the head, is larger than the base and articulates with a bone of a digit (toe). The region between the base and the head is called the body of the metatarsal.
Some metatarsals, especially the first metatarsal, often have small sesamoid bones on the plantar surface of their heads. The function of the sesamoid bones is unclear, but they may help reduce friction or stabilize the position of the tendons leading to the phalanges. Tendons of skeletal muscles in the leg attach to the heads of metatarsals, allowing the foot to bend at the ankle. Other skeletal muscles connect the metatarsals to bones in the digits. Contracting different groups of these muscles enable you to spread your toes apart or pull them together.
PHALANGES
The five most distal extensions of the lower limbs are called digits or toes. The foot’s digits are numbered in the same order as the metatarsals. Digit 1, the hallux (HAL-uks) or great toe, is the most medial, and digit 5, the little toe, is the most lateral. As with the fingers of the hand, the bones that support each toe are called phalanges. The phalanges of the foot help a person maintain balance while standing. The hallux consists of two bones, a proximal phalanx and a distal phalanx. All the other digits of the foot consist of three phalanges, including a middle phalanx between the proximal and distal phalanx. Like the metatarsals, the proximal and middle phalanges have a base at the proximal end and a head at the distal end.
ARCHES OF THE FOOT
Have you ever left footprints along a sandy lake shore, or walked along a sidewalk with bare, wet feet? If so, you probably saw that your footprints had a crescent-shaped slice missing from the medial side of each sole. You were right if you concluded that parts of your soles aren’t touching the ground. Ligaments and muscle tendons in the foot pull the tarsals and metatarsals dorsally so that these bones do not align parallel to the ground. The upwardly bowed foot bones form arches, which are beneficial because they spread the body’s weight evenly across the foot, help absorb physical impacts, and provide leverage that makes walking easier.
As a physical structure, any arch has the ability to distribute weight evenly from its center to its ends. For example, the arch of a highway bridge distributes the weight of a passing car between the bridge’s ends. In the same way, because the foot has arches, the body weight bearing down on the talus spreads evenly between the calcaneus and the heads of the metatarsals. Each foot has three arches that redistribute the body’s weight: two longitudinal arches, which run lengthwise (posterior-to-anterior), are joined together by a transverse arch, which runs crosswise (lateral to-medial).
In addition to redistributing bodyweight, the arches of the foot help to put a “spring in your step” and protect soft tissues in the foot. The ligaments and various muscle tendons allow the foot’s arches to remain elastic. The arches flatten slightly when a person is standing or planting the foot while walking. However, the arches spring back to their original curvature when the person sits or lifts the foot off the ground. Since the arches keep the middle portion of the foot off the ground, the anterior tarsals and the metatarsals do not compress the nerves or blood vessels in the foot.
TOPICS TO KNOW FOR CHAPTER 11
(The Appendicular Skeketon)
acetabulum
acromial end of clavicle
acromion
ala of ilium
anatomical neck of humerus
anterior inferior iliac spine
anterior superior iliac spine
appendicular skeleton
base of metacarpal
base of metatarsal
body of metacarpal.
body of metatarsal
bony pelvis
calcaneal tuberosity
calcaneus
capitate
capitulum
carpals
clavicles
condyle of the humerus
coracoid process
coronoid fossa
coronoid process of ulna
coxal bones
cuboid
deltoid tuberosity
digits
distal phalanx
false pelvis
femoral medial condyle
femoral medial epicondyle
femur
fibula
fibular head
fibular notch
fibular shaft
foot arches
fovea capitis
glenoid cavity
gluteal tuberosity
greater sciatic notch
greater trochanter
greater tubercle
hallux
hamate
head of femur
head of metacarpal
head of metatarsal
hipbones
humeral head
humeral lateral epicondyle
humeral medial epicondyle
humerus
iliac crest
iliac fossa
ilium
infraspinous fossa
innominate bones
intercondylar eminence
intercondylar fossa
intermediate cuneiform
intertubercular groove
ischial spine
ischial tuberosity
ischium
lateral border of scapula
lateral condyle of femur
lateral cuneiform
lateral epicondyle of femur
lateral malleolus
lesser sciatic notch
lesser trochanter
lesser tubercle
linea aspera
longitudinal arches
lower limb
lunate
medial border of scapula
medial cuneiform
medial malleolus
metacarpals
metatarsals
middle phalanx
navicular
neck of femur
obturator foramen
olecranon fossa
olecranon process
patella
patellar surface of femur
pectoral girdle
pelvic brim
pelvic girdle
pelvic inlet
pelvic outlet
phalanges
phalanx
pisiform
posterior inferior iliac spine
posterior superior iliac spine
proximal phalanx
pubic arch
pubic bone
pubis
radial fossa
radial groove
radial head
radial neck
radial notch
radial tuberosity
radius
ramus
scaphoid
scapulae
shaft of femur
shoulder girdle
spine of scapula
sternal end of clavicle
styloid process of radius
styloid process of ulna
subscapular fossa
superior border of scapula
suprascapular notch
supraspinous fossa
surgical neck of humerus
talus
tarsals
tarsus
tibia
tibial lateral condyle
tibial medial condyle
tibial tuberosity
toes
transverse arch
trapezium
trapezoid
triquetrum
trochlea
trochlear notch
true pelvis
ulna
ulnar head
ulnar notch
EOC Questions
upper limb
wing of ilium
EOC Questions
