← Human Anatomy and Physiology I outline

Week 9 · Chapter

Chapter 9 — The Skeleton & Joints

Human Anatomy and Physiology I Free sample edition

The module's primary reading · ~45–55 minutes · Objective 4 (describe bone as a living tissue — its functions, the gross and microscopic anatomy of a long bone, the four bone cells, ossification, growth, remodeling, and the calcium-homeostasis loop — and sort the skeleton into its axial and appendicular divisions by region while classifying joints by structure, function, and movement), Objective 1 (use the language of the body correctly — the anatomical position, the directional-term pairs, the three body planes, and the body cavities and abdominal regions — distinguish anatomy from physiology, climb the levels of structural organization, list the functions and requirements of human life, and trace a homeostatic feedback loop through its receptor, control center, and effector, telling negative feedback from positive feedback) · Read it first this week; the lecture, the slides, the tutorial, the quiz, and the Build the Skeleton, Name the Joints lab all build on it.

In plain language

Your skeleton is 206 bones, and every one of them belongs to one of two groups. The axial skeleton (the bones down the middle of your body) holds up your head and trunk and guards your brain, spinal cord, heart, and lungs. The appendicular skeleton (your arms, your legs, and the bony rings that hook them on) does most of the moving. Where two bones meet you have a joint (a meeting place of bones), and each joint makes a trade between how far it can move and how firmly it holds. This week you sort the bones, name the joints, and learn the words for every move you make.

Why you already care about this

Stand up and try 3 moves. Shake your head no about 3 times. Bend one elbow until your hand touches your shoulder. Lift your other arm out to the side until it points at the wall. Three moves used 3 different joints, and every one of them was built for its job. Your head turned on a pivot at the top of your neck, your elbow folded on a hinge, and your shoulder swung its arm through a wide arc from a shallow socket.

Now think of someone carrying moving boxes up a flight of stairs. The thigh bone and the bone of the lower leg carry the body's weight on every step, while the arms hold a box out in front. A skeleton is not one frame; it is a central column plus four limbs, and the limbs move because the joints let them. This chapter gives you the map: where each bone sits, how many there are, and what each joint trades away to move the way it does.

By the end of this chapter you will be able to:

sort any bone into the axial or the appendicular skeleton, including the girdle bones;
add the regional counts up to 80, 126, and 206 with every step shown;
order the vertebrae as 7 cervical, 12 thoracic, and 5 lumbar, and name the parts of the thoracic cage;
place the radius and the tibia correctly from the anatomical position;
classify a joint by its structure and by its function, and name the parts of a synovial joint;
match each of the six synovial types to its example joint, and name a movement by its pair;
explain why the shoulder moves freely and dislocates easily while the hip holds firm.

Carry this sentence with you: girdles ride with their limbs — not with the spine.

Where we are

Already covered: Week 1, Body Organization, Homeostasis & Anatomical Terminology, gave you the anatomical position and the directional pairs, including medial and lateral. Week 5, Tissues, put cartilage and bone in the supportive connective tissue family and named the synovial membrane that lines a freely movable joint. Week 7, The Skeletal System: Bone Tissue & Structure, showed you a long bone, the five shape classes, and articular cartilage. Week 8, the midterm review, added the three-step read: name the structure, state its function, name the clinical connection and stop.
This week: the 206 bones in two divisions and by region, with the limbs and their girdles. Then the joints: their two sorts, the parts of a synovial joint, the six synovial types, the movement pairs, and the trade between moving freely and holding firm.
How they connect: Week 7 told you bones are levers and joints are the pivots; this week names the levers and the pivots. Week 1's medial and lateral now place real bones.
Where we're going next: Week 10, Muscle Tissue & the Physiology of Contraction, opens the unit on muscle, the tissue that pulls on these levers.

Here is what you lean on this week. In Week 1 you learned the anatomical position: standing upright, feet parallel, arms at the sides, palms facing forward. From that pose the thumb is lateral, away from the midline. In Week 5 you learned that cartilage has no blood vessels of its own and that a synovial membrane lines the cavity of a freely movable joint. In Week 7 you learned that articular cartilage covers the joint surfaces of a bone, cutting friction and absorbing shock. All three return this week.

1. Sort the skeleton: two divisions and the girdle rule

Picture a museum skeleton hanging on its stand. Take away the arms and the legs, and a column is left: the skull on top, the backbone down the middle, and the rib cage around the chest. Put the limbs back, and you see the bony rings at the shoulders and the hips that hook each limb onto that column.

Anatomists split the 206 bones of an adult skeleton along that line.

The axial skeleton is the division of the skeleton along the body's central axis: the skull, the hyoid and the ear ossicles, the vertebral column, and the thoracic cage, 80 bones in an adult.
The appendicular skeleton is the division of the skeleton made of the upper and lower limbs and the girdles that attach them to the central axis, 126 bones in an adult.

A girdle is the ring of bone that hooks a limb onto the axis. The girdles are where students lose the most points in this unit, so learn the rule now: if a bone attaches a limb, it belongs to the appendicular skeleton.

Worked example — Dataset F, the whole skeleton. You add the two divisions, then sort six bones from the museum skeleton.

Whole skeleton: 80 axial + 126 appendicular = 206 bones.
The skull sits on the central axis: axial.
A vertebra sits in the backbone, on the axis: axial.
A rib wraps the chest, part of the thoracic cage: axial.
The femur is in the leg: appendicular.
The scapula attaches the arm to the trunk, so it is a girdle bone: appendicular.
A hip bone attaches the leg to the trunk, so it is a girdle bone: appendicular.

In words: 80 bones run down the middle and 126 hang off it, and the girdle bones count with the limbs they attach.

Misconception check. Students often call the scapula or the hip bones axial, because those bones sit on the trunk and look like part of the body's core. It is tempting because they are flat and close to the ribs and the spine. The cure is the girdle rule: ask what the bone does. The scapula attaches the arm, and each hip bone attaches a leg, so both are appendicular.

2. Count the skull: 22 bones and seven neighbors

Tap your forehead, then your cheekbone, then your jaw. Each tap lands on a different skull bone. Some of those bones wrap the brain like a helmet; others frame your eyes, nose, and mouth.

The skull has 22 bones in two groups.

The cranial bones are the bones of the skull that surround and protect the brain.
The facial bones are the bones of the skull that frame the face.

Seven more bones are counted with the axial skeleton but not with the skull's 22.

The hyoid is a single bone in the neck counted with the axial skeleton as one of its seven associated bones.
The ossicles are the three tiny bones in each middle ear, six in all, counted with the axial skeleton as associated bones.

Worked example — Dataset F, the associated bones. You add the seven bones that sit beside the skull's count.

Hyoid: 1 bone.
Ossicles in the left ear: 3 bones.
Ossicles in the right ear: 3 bones.
Associated bones: 1 + 3 + 3 = 7 bones.

In words: the skull's 22 and its 7 neighbors are separate counts, and both belong to the axial 80.

Misconception check. Students often fold the ossicles into the skull's 22, because the ear sits inside the head. It is tempting because the ossicles are hidden in the skull's bone. The cure is to keep two piles: 22 skull bones in one, and 7 associated bones, the hyoid and six ossicles, in the other.

Honesty note: naming every skull bone is out of scope for this course. You need the count, 22, and the two groups, cranial and facial.

3. Stack the vertebral column: 7, 12, and 5

Run your fingers down the back of your neck. You feel a row of small bumps. Keep going between your shoulder blades and down to your lower back, and the bumps continue. Each bump is part of one vertebra, and the stack of them is your backbone.

The vertebral column is the stack of 24 vertebrae, 7 cervical, 12 thoracic, and 5 lumbar, plus the sacrum and the coccyx, with a disc between neighboring vertebral bodies. Read it from the top down.

The cervical vertebrae are the seven vertebrae of the neck, C1 to C7, at the top of the vertebral column.
The thoracic vertebrae are the twelve vertebrae of the chest region, T1 to T12, one for each pair of ribs.
The lumbar vertebrae are the five vertebrae of the lower back, L1 to L5, between the thoracic vertebrae and the sacrum.
The sacrum is the bone below the lumbar vertebrae formed from five fused vertebrae.
The coccyx is the small bone at the bottom of the vertebral column, usually formed from four fused vertebrae.

Two neck vertebrae have their own names. The atlas is C1, the first cervical vertebra, shaped as a ring. The axis is C2, the second cervical vertebra, which carries an upward peg called the dens. Between the bodies of two neighbors sits an intervertebral disc, a pad of fibrocartilage between the bodies of two neighboring vertebrae, forming a cartilaginous joint.

Worked example — Dataset F, the vertebral column. You count the column from the top down, then count it the way it started.

Vertebrae: 7 cervical + 12 thoracic + 5 lumbar = 24 vertebrae.
Vertebral column: 24 vertebrae + 1 sacrum + 1 coccyx = 26 bones.
Development check: 7 + 12 + 5 + 5 fused in the sacrum + 4 fused in the coccyx = 33 vertebrae at the start of development.

In words: the column starts as 33 separate pieces, and fusion at the bottom leaves 26 bones, 24 of them separate vertebrae.

Misconception check. Students often write "12 cervical and 7 thoracic," because both numbers are familiar and the two words blur. The cure is to say the order top down, matched to the body: 7 in the neck, 12 in the chest, one for each rib pair, and 5 in the lower back.

4. Close the cage: ribs, sternum, and the axial total

Put your hands on your sides and feel the curved bones under your skin. Follow one rib forward with your fingers, and it curves toward the flat bone in the middle of your chest.

The thoracic cage is the 12 pairs of ribs and the sternum, 25 bones that enclose the chest. The ribs come in two groups, with a smaller group inside the second.

The true ribs are ribs 1 to 7, the first seven pairs.
The false ribs are ribs 8 to 12, the last five pairs; ribs 11 and 12 among them are also called floating ribs.
The floating ribs are ribs 11 and 12, the last two pairs, counted inside the false ribs.
The sternum is the breastbone at the front of the thoracic cage, in three parts from top to bottom: the manubrium, the body, and the xiphoid process.

Worked example — Dataset F, the thoracic cage and the axial total. You count the cage, check the rib groups, then add up the whole axial skeleton.

Ribs: 12 pairs × 2 = 24 ribs.
Thoracic cage: 24 ribs + 1 sternum = 25 bones.
Rib pairs by type: 7 true (ribs 1 to 7) + 5 false (ribs 8 to 12) = 12 pairs. Ribs 11 and 12, 2 pairs, are the floating ribs inside the false group.
Axial skeleton: 22 skull + 7 associated + 26 vertebral column + 25 thoracic cage = 80 bones.

In words: four regions add up to the axial 80, and the cage's 25 is 24 ribs plus one sternum.

Misconception check. Students often count three rib groups side by side, true plus false plus floating, and land on more than 12 pairs. It is tempting because the floating ribs have their own name. The cure is to nest them: the floating ribs are two of the five false pairs, so 7 + 5 = 12 pairs and no more.

5. Build the upper limb: the radius on the thumb side

Turn your palms forward, the way the anatomical position asks. Now look at your forearm. Two bones run from your elbow to your wrist, side by side. One of them ends on the thumb side of your wrist; the other ends on the little-finger side.

The arm hooks on through the pectoral girdle, the clavicle and the scapula on each side, the bones that attach an upper limb to the axial skeleton; they belong to the appendicular skeleton. Below the girdle, each upper limb has 30 bones.

The humerus is the single bone of the arm, between the shoulder and the elbow.
The radius is the forearm bone on the lateral, thumb side when the body is in the anatomical position.
The ulna is the forearm bone on the medial side, opposite the thumb, when the body is in the anatomical position.
The carpals are the eight bones of the wrist.
The metacarpals are the five bones of the palm.
The phalanges are the bones of the fingers or toes, 14 in each hand and 14 in each foot.

Worked example — Dataset F, the upper limb. You count one upper limb from the shoulder down.

Arm: 1 humerus.
Forearm: 2 forearm bones (radius, ulna).
Wrist: 8 carpals.
Palm: 5 metacarpals.
Fingers: 14 phalanges.
Upper limb: 1 humerus + 2 forearm bones (radius, ulna) + 8 carpals + 5 metacarpals + 14 phalanges = 30 bones.

In words: one arm bone, two forearm bones, and 27 in the wrist and hand make 30, and the girdle is counted on its own.

Misconception check. Students often put the ulna on the thumb side, because they picture their own forearm with the palm turned down. It is tempting because the forearm looks different once the hand is turned. The cure is to read from the anatomical position, palms forward. There the thumb is lateral, so the radius, the bone on the thumb side, is lateral too.

6. Build the lower limb: the tibia bears the weight

Stand on one foot for a moment. All of your weight runs down through one leg, from the hip to the foot. The bones of the leg carry that load on every step you take, and one bone of the lower leg carries the weight while its neighbor does not.

The leg hooks on through the pelvic girdle, the two hip bones, which attach the lower limbs to the axial skeleton; they belong to the appendicular skeleton. Below the girdle, each lower limb has 30 bones.

The femur is the thigh bone, the longest and strongest bone in the body, about a quarter of a person's height.
The patella is the kneecap, a sesamoid bone counted with the lower limb.
The tibia is the medial bone of the leg, which carries the body's weight.
The fibula is the lateral bone of the leg, which does not bear the body's weight.
The tarsals are the seven bones of the ankle.
The metatarsals are the five bones of the middle of the foot.

The toes carry 14 phalanges, the same count as the fingers.

Worked example — Dataset F, the lower limb and the girdle check. You count one lower limb, then use all four limbs to check the girdles.

Lower limb: 1 femur + 1 patella + 1 tibia + 1 fibula + 7 tarsals + 5 metatarsals + 14 phalanges = 30 bones.
Four limbs: 4 × 30 = 120 bones.
Girdle check: 126 appendicular − 120 limb bones = 6 girdle bones, and 2 clavicles + 2 scapulae + 2 hip bones = 6, so the girdles complete the appendicular count.

In words: four limbs of 30 give 120, and the 6 girdle bones bring the appendicular skeleton to 126. The subtraction is the girdle rule written as arithmetic.

Misconception check. Students often name the fibula as the weight-bearing bone, because it runs the full length of the leg beside the tibia. The cure is the medial/lateral pair. The tibia is the medial bone and carries the body's weight; the fibula is lateral and does not bear weight.

7. Classify a joint: by what holds it and by how it moves

Press gently along the top of your head. The bones there do not budge. Now wiggle your fingers. Those joints move freely. Between those two extremes sit joints that allow only limited movement.

A joint is a place where two or more bones meet; joints are sorted by how the bones are held together and by how much they move. That gives two separate sorts.

By structure, what holds the bones together:

A fibrous joint is a joint in which fibrous connective tissue holds the bones together, with no joint cavity, such as a skull suture.
A suture is an immovable fibrous joint between skull bones.
A cartilaginous joint is a joint in which hyaline cartilage or fibrocartilage holds the bones together, with no joint cavity, such as an intervertebral disc or the pubic symphysis.
The pubic symphysis is a cartilaginous joint in the pelvis, held together by cartilage and with no joint cavity.
A synovial joint is a joint in which the bone ends meet inside a fluid-filled joint cavity; every one of them moves freely.

By function, how much the joint moves:

A synarthrosis is a joint that is immovable or nearly so.
An amphiarthrosis is a joint that allows limited movement.
A diarthrosis is a freely movable joint; every synovial joint belongs to this functional class.

The one rule that links the two sorts: every synovial joint is a diarthrosis.

Worked example — Dataset F, three joints read two ways. You read each joint from its structure to its function.

Joint 1, a skull suture: fibrous by structure → synarthrosis (immovable) by function; no cavity, so not synovial.
Joint 2, an intervertebral disc: cartilaginous by structure, built with fibrocartilage; no cavity, so not synovial.
Joint 3, the elbow: synovial by structure → diarthrosis by function → hinge type; movement pair: flexion and extension.

In words: a suture is held by fibers and does not move, a disc is held by cartilage, and the elbow meets inside a cavity and moves freely.

Misconception check. Students often call a skull suture synovial, because every joint they can name seems to be one. The cure is the cavity. A suture has fibrous connective tissue between the bones and no joint cavity, so it is fibrous and immovable; a synovial joint always has a fluid-filled cavity.

8. Open a synovial joint: five parts and one fluid

Think of a door hinge that never needs oil. The two leaves have smooth faces, a sleeve wraps the hinge, and a lining inside the sleeve keeps the faces wet. A synovial joint works the same way, and it has five parts.

Read the joint as a labeled figure, from the bone ends outward.

Articular cartilage, a thin layer of hyaline cartilage with no blood vessels of its own, caps each bone end.
The joint cavity is the fluid-filled space between the bone ends of a synovial joint.
The articular capsule is the sleeve of fibrous connective tissue that wraps a synovial joint and encloses its cavity.
The synovial membrane lines the capsule and secretes synovial fluid, the fluid secreted by the synovial membrane into a joint cavity, which lubricates the joint and nourishes the articular cartilage.
A ligament is a band of dense regular connective tissue that holds the bones of a joint together and steadies the joint.

Read it as structure to function. The cartilage has no blood vessels, so the fluid around it is how it is fed. The smooth cartilage and the wet cavity let the bone ends glide. The capsule and the ligaments keep the bones lined up while they move.

Misconception check. Students often say "the cartilage makes the synovial fluid," because the cartilage is the part the fluid touches. The cure is to follow the lining. The synovial membrane lines the capsule and secretes the fluid; the cartilage is fed by that fluid.

Pause and think, then choose one. (a) Explain to a friend why articular cartilage depends on synovial fluid. (b) Match each part of the door-hinge picture to one of the five structures above.

9. Match the six synovial types to their joints

Try four small moves. Turn your head to say no. Bend your elbow. Wave your hand side to side at the wrist. Touch your thumb to your little finger. Each move ran on a differently shaped synovial joint.

There are six synovial types, each with its example joint:

A pivot joint is a synovial joint in which one bone turns around another, as at the atlantoaxial joint between C1 and C2.
A hinge joint is a synovial joint that bends and straightens in one direction, such as the elbow.
A condyloid joint is the synovial type found at the wrist.
A saddle joint is the synovial type found at the base of the thumb.
A plane joint is a synovial joint between flat bone surfaces that slide past one another, as between carpals or between tarsals.
A ball-and-socket joint is a synovial joint in which a rounded bone end sits in a cup; the hip and the shoulder are the only two, and they have the widest range of motion.

Worked example — Dataset F, the shoulder read in full. You read the shoulder from its structure to its type.

Joint 4, the shoulder: synovial by structure → diarthrosis by function → ball-and-socket type; movement pair: abduction and adduction.
Count the types: pivot + hinge + condyloid + saddle + plane + ball-and-socket = 6 types.

In words: the shoulder is one of only two ball-and-socket joints, and the six types cover every synovial joint you will meet in this course.

Misconception check. Students often call the elbow a ball-and-socket joint, because it is large and moves a lot. The cure is the pinned list: the hip and the shoulder are the only ball-and-socket joints. The elbow bends and straightens in one direction, so it is a hinge.

10. Name the move: the movement pairs

In a dance class, the teacher calls out moves: lift the leg to the side, bring it back, point the toes, flex the foot, turn the palms up. Anatomy has a precise word for each of those moves, and most of them come in pairs that undo each other.

Flexion is a movement that bends a joint, decreasing the angle between the bones.
Extension is a movement that straightens a joint, increasing the angle between the bones.
Abduction is a movement of a limb away from the midline of the body.
Adduction is a movement of a limb toward or across the midline of the body.
Rotation is a movement that turns a bone around its own long axis, as the head turns to say no.
Circumduction is a movement of a limb around in a cone-shaped circle.
Plantar flexion is a movement that points the toes down and raises the heel.
Dorsiflexion is a movement that lifts the front of the foot toward the shin.
Pronation is a movement that turns the palm to face backward.
Supination is a movement that turns the palm to face forward, back to the anatomical position.

Worked example — name the dance-class moves. You name each move the teacher calls out.

Lift the leg out to the side: away from the midline, so abduction at the hip.
Bring the leg back in: toward the midline, so adduction.
Point the toes: plantar flexion at the ankle.
Pull the toes up toward the shin: dorsiflexion.
Turn the palms from facing back to facing forward: supination.
Draw a circle in the air with a straight arm: circumduction at the shoulder.

Misconception check. Students often mix up flexion and abduction, because both seem to "lift" a limb. The cure is to ask what changed. Flexion changes the angle between two bones at a joint; abduction moves a limb away from the midline. Lifting your arm straight out to the side is abduction; bending your elbow is flexion.

Pause and think, then choose one. (a) Describe one move from a sport or a dance you know, using two movement terms. (b) Describe an everyday task, such as reaching for a high shelf, using two movement terms.

11. Weigh the trade: mobility versus stability

Hold a tennis ball on a saucer, then hold it in a deep cup. On the saucer it rolls in every direction, and a small bump knocks it off. In the cup it barely moves, and it is hard to knock out. Your ball-and-socket joints, the hips and the shoulders, make that same trade.

The shoulder's shallow socket lets the arm swing freely, so it moves through a wide range and dislocates easily. The hip's deep socket holds the femur firmly and carries the body's weight. That is the mobility-versus-stability trade-off: the more freely a joint moves, the less firmly it holds.

Run the three-step read from Week 8 on the shoulder.

Name it: the shoulder, a synovial ball-and-socket joint with a shallow socket.
State its function: it lets the arm swing through a wide range of motion.
Name the clinical connection: a dislocation is an injury in which a bone is forced out of its normal place in a joint, as when the humerus slips out of the shoulder's shallow socket.

You name the structure and you stop there. How a dislocation is treated, and any care for it, are not part of this course.

Misconception check. Students often assume the joint that moves the most is the strongest joint, because range of motion sounds like a strength. The cure is the trade. The shoulder's shallow socket buys range and gives up hold, so it dislocates easily; the hip's deep socket gives up range and holds the body's weight.

Caveat: the saucer-and-cup picture is a classroom model of the trade, not a description of every joint's shape.

Key terms

Axial skeleton — The division of the skeleton along the body's central axis: the skull, the hyoid and the ear ossicles, the vertebral column, and the thoracic cage, 80 bones in an adult. (first used: 1. Sort the skeleton: two divisions and the girdle rule)
Appendicular skeleton — The division of the skeleton made of the upper and lower limbs and the girdles that attach them to the central axis, 126 bones in an adult. (first used: 1. Sort the skeleton: two divisions and the girdle rule)
Cranial bones — The bones of the skull that surround and protect the brain. (first used: 2. Count the skull: 22 bones and seven neighbors)
Facial bones — The bones of the skull that frame the face. (first used: 2. Count the skull: 22 bones and seven neighbors)
Hyoid — A single bone in the neck counted with the axial skeleton as one of its seven associated bones. (first used: 2. Count the skull: 22 bones and seven neighbors)
Ossicles — The three tiny bones in each middle ear, six in all, counted with the axial skeleton as associated bones. (first used: 2. Count the skull: 22 bones and seven neighbors)
Vertebral column — The stack of 24 vertebrae, 7 cervical, 12 thoracic, and 5 lumbar, plus the sacrum and the coccyx, with a disc between neighboring vertebral bodies. (first used: 3. Stack the vertebral column: 7, 12, and 5)
Cervical vertebrae — The seven vertebrae of the neck, C1 to C7, at the top of the vertebral column. (first used: 3. Stack the vertebral column: 7, 12, and 5)
Thoracic vertebrae — The twelve vertebrae of the chest region, T1 to T12, one for each pair of ribs. (first used: 3. Stack the vertebral column: 7, 12, and 5)
Lumbar vertebrae — The five vertebrae of the lower back, L1 to L5, between the thoracic vertebrae and the sacrum. (first used: 3. Stack the vertebral column: 7, 12, and 5)
Sacrum — The bone below the lumbar vertebrae formed from five fused vertebrae. (first used: 3. Stack the vertebral column: 7, 12, and 5)
Coccyx — The small bone at the bottom of the vertebral column, usually formed from four fused vertebrae. (first used: 3. Stack the vertebral column: 7, 12, and 5)
Atlas — C1, the first cervical vertebra, shaped as a ring. (first used: 3. Stack the vertebral column: 7, 12, and 5)
Axis — C2, the second cervical vertebra, which carries an upward peg called the dens. (first used: 3. Stack the vertebral column: 7, 12, and 5)
Intervertebral disc — A pad of fibrocartilage between the bodies of two neighboring vertebrae, forming a cartilaginous joint. (first used: 3. Stack the vertebral column: 7, 12, and 5)
Thoracic cage — The 12 pairs of ribs and the sternum, 25 bones that enclose the chest. (first used: 4. Close the cage: ribs, sternum, and the axial total)
True ribs — Ribs 1 to 7, the first seven pairs. (first used: 4. Close the cage: ribs, sternum, and the axial total)
False ribs — Ribs 8 to 12, the last five pairs; ribs 11 and 12 among them are also called floating ribs. (first used: 4. Close the cage: ribs, sternum, and the axial total)
Floating ribs — Ribs 11 and 12, the last two pairs, counted inside the false ribs. (first used: 4. Close the cage: ribs, sternum, and the axial total)
Sternum — The breastbone at the front of the thoracic cage, in three parts from top to bottom: the manubrium, the body, and the xiphoid process. (first used: 4. Close the cage: ribs, sternum, and the axial total)
Pectoral girdle — The clavicle and the scapula on each side, the bones that attach an upper limb to the axial skeleton; they belong to the appendicular skeleton. (first used: 5. Build the upper limb: the radius on the thumb side)
Humerus — The single bone of the arm, between the shoulder and the elbow. (first used: 5. Build the upper limb: the radius on the thumb side)
Radius — The forearm bone on the lateral, thumb side when the body is in the anatomical position. (first used: 5. Build the upper limb: the radius on the thumb side)
Ulna — The forearm bone on the medial side, opposite the thumb, when the body is in the anatomical position. (first used: 5. Build the upper limb: the radius on the thumb side)
Carpals — The eight bones of the wrist. (first used: 5. Build the upper limb: the radius on the thumb side)
Metacarpals — The five bones of the palm. (first used: 5. Build the upper limb: the radius on the thumb side)
Phalanges — The bones of the fingers or toes, 14 in each hand and 14 in each foot. (first used: 5. Build the upper limb: the radius on the thumb side)
Pelvic girdle — The two hip bones, which attach the lower limbs to the axial skeleton; they belong to the appendicular skeleton. (first used: 6. Build the lower limb: the tibia bears the weight)
Femur — The thigh bone, the longest and strongest bone in the body, about a quarter of a person's height. (first used: 6. Build the lower limb: the tibia bears the weight)
Patella — The kneecap, a sesamoid bone counted with the lower limb. (first used: 6. Build the lower limb: the tibia bears the weight)
Tibia — The medial bone of the leg, which carries the body's weight. (first used: 6. Build the lower limb: the tibia bears the weight)
Fibula — The lateral bone of the leg, which does not bear the body's weight. (first used: 6. Build the lower limb: the tibia bears the weight)
Tarsals — The seven bones of the ankle. (first used: 6. Build the lower limb: the tibia bears the weight)
Metatarsals — The five bones of the middle of the foot. (first used: 6. Build the lower limb: the tibia bears the weight)
Joint — A place where two or more bones meet; joints are sorted by how the bones are held together and by how much they move. (first used: 7. Classify a joint: by what holds it and by how it moves)
Fibrous joint — A joint in which fibrous connective tissue holds the bones together, with no joint cavity, such as a skull suture. (first used: 7. Classify a joint: by what holds it and by how it moves)
Suture — An immovable fibrous joint between skull bones. (first used: 7. Classify a joint: by what holds it and by how it moves)
Cartilaginous joint — A joint in which hyaline cartilage or fibrocartilage holds the bones together, with no joint cavity, such as an intervertebral disc or the pubic symphysis. (first used: 7. Classify a joint: by what holds it and by how it moves)
Pubic symphysis — A cartilaginous joint in the pelvis, held together by cartilage and with no joint cavity. (first used: 7. Classify a joint: by what holds it and by how it moves)
Synovial joint — A joint in which the bone ends meet inside a fluid-filled joint cavity; every one of them moves freely. (first used: 7. Classify a joint: by what holds it and by how it moves)
Synarthrosis — A joint that is immovable or nearly so. (first used: 7. Classify a joint: by what holds it and by how it moves)
Amphiarthrosis — A joint that allows limited movement. (first used: 7. Classify a joint: by what holds it and by how it moves)
Diarthrosis — A freely movable joint; every synovial joint belongs to this functional class. (first used: 7. Classify a joint: by what holds it and by how it moves)
Joint cavity — The fluid-filled space between the bone ends of a synovial joint. (first used: 8. Open a synovial joint: five parts and one fluid)
Articular capsule — The sleeve of fibrous connective tissue that wraps a synovial joint and encloses its cavity. (first used: 8. Open a synovial joint: five parts and one fluid)
Synovial fluid — The fluid secreted by the synovial membrane into a joint cavity, which lubricates the joint and nourishes the articular cartilage. (first used: 8. Open a synovial joint: five parts and one fluid)
Ligament — A band of dense regular connective tissue that holds the bones of a joint together and steadies the joint. (first used: 8. Open a synovial joint: five parts and one fluid)
Pivot joint — A synovial joint in which one bone turns around another, as at the atlantoaxial joint between C1 and C2. (first used: 9. Match the six synovial types to their joints)
Hinge joint — A synovial joint that bends and straightens in one direction, such as the elbow. (first used: 9. Match the six synovial types to their joints)
Condyloid joint — The synovial type found at the wrist. (first used: 9. Match the six synovial types to their joints)
Saddle joint — The synovial type found at the base of the thumb. (first used: 9. Match the six synovial types to their joints)
Plane joint — A synovial joint between flat bone surfaces that slide past one another, as between carpals or between tarsals. (first used: 9. Match the six synovial types to their joints)
Ball-and-socket joint — A synovial joint in which a rounded bone end sits in a cup; the hip and the shoulder are the only two, and they have the widest range of motion. (first used: 9. Match the six synovial types to their joints)
Flexion — A movement that bends a joint, decreasing the angle between the bones. (first used: 10. Name the move: the movement pairs)
Extension — A movement that straightens a joint, increasing the angle between the bones. (first used: 10. Name the move: the movement pairs)
Abduction — A movement of a limb away from the midline of the body. (first used: 10. Name the move: the movement pairs)
Adduction — A movement of a limb toward or across the midline of the body. (first used: 10. Name the move: the movement pairs)
Rotation — A movement that turns a bone around its own long axis, as the head turns to say no. (first used: 10. Name the move: the movement pairs)
Circumduction — A movement of a limb around in a cone-shaped circle. (first used: 10. Name the move: the movement pairs)
Plantar flexion — A movement that points the toes down and raises the heel. (first used: 10. Name the move: the movement pairs)
Dorsiflexion — A movement that lifts the front of the foot toward the shin. (first used: 10. Name the move: the movement pairs)
Pronation — A movement that turns the palm to face backward. (first used: 10. Name the move: the movement pairs)
Supination — A movement that turns the palm to face forward, back to the anatomical position. (first used: 10. Name the move: the movement pairs)
Dislocation — An injury in which a bone is forced out of its normal place in a joint, as when the humerus slips out of the shoulder's shallow socket. (first used: 11. Weigh the trade: mobility versus stability)

Summary

206 bones: 80 axial and 126 appendicular.
If a bone attaches a limb, it is appendicular.
The skull has 22 bones; 7 associated bones sit beside it.
Vertebrae run 7 cervical, 12 thoracic, 5 lumbar.
The thoracic cage is 12 pairs of ribs plus the sternum.
Each limb has 30 bones.
The radius is on the thumb side; the tibia bears the weight.
Joints sort by structure and by function.
Every synovial joint is a diarthrosis.
The synovial membrane makes the synovial fluid.
Only the hip and the shoulder are ball-and-socket joints.
Flexion changes an angle; abduction leaves the midline.
The shoulder trades stability for mobility; the hip trades mobility for stability.

Check your understanding

Work these before looking at the answers — they are ungraded, and the answers (with reasoning) are at the bottom of the page.

A made-up flash card sorts four bones this way: "clavicle, axial; sternum, axial; hip bone, axial; humerus, appendicular." Find each error and give the correct division.
A made-up model hand on a classroom desk has 8 carpals, 5 metacarpals, and 12 phalanges. How many bones is it short of a complete wrist and hand, and which group is missing bones?
A made-up description reads: "At this joint the two bones are held by fibrous connective tissue, there is no cavity, and the bones do not move." Classify the joint by structure and by function, and say whether it could be synovial.
A classmate says, "When you lift your arm straight out to the side, that is flexion at the shoulder, and the fibula is the bone that bears your weight when you stand." Say what is wrong with each half and correct it.

Answers (worked reasoning — read after attempting)

The clavicle is a girdle bone that attaches the arm, so it is appendicular, not axial. The sternum is part of the thoracic cage, so axial is correct. The hip bone attaches the leg, so it is appendicular, not axial. The humerus is in the arm, so appendicular is correct. Count the errors: 1 + 1 = 2 wrong out of 4, both of them the girdle trap.
A complete wrist and hand is 8 carpals + 5 metacarpals + 14 phalanges = 27 bones. The model has 8 + 5 + 12 = 25 bones. 27 − 25 = 2, so it is 2 bones short, and the missing bones are phalanges, since the carpals and metacarpals are complete.
Fibrous connective tissue with no cavity makes it a fibrous joint. Bones that do not move make it a synarthrosis. It cannot be synovial, because every synovial joint has a fluid-filled cavity and moves freely. This is the description of a skull suture.
Lifting the arm out to the side moves it away from the midline, so it is abduction, not flexion; flexion would bend a joint and decrease the angle between bones. The tibia, not the fibula, bears the body's weight; the tibia is the medial bone of the leg, and the fibula is lateral and does not bear weight.

Next up in this module: the lecture, the slides, the Readings & Resources page, the tutorial, the practice set, and the quiz. Next week the muscles that pull on these levers get their own model: the sliding-filament model of contraction.

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