Quiz 17 — Bones, Muscles, and Skin

The Guide

Structure, movement, and the edge of the body

The three systems in this quiz are the ones you can see and feel. Bones hold the shape. Muscles change it. Skin ends it. They are taught together because they do one connected job: they give the body a form, they move that form around, and they mark where it stops and the world begins.

All three turn out to be busier than they look. Bone is living tissue that is torn down and rebuilt all your life. Skin is an organ with jobs in temperature, defense and sensation. And a muscle can do only one thing, which is the fact that most of this quiz turns on.

The skeleton, and what it is for

An adult has about 206 bones. A newborn has more — around 270 — because some of them fuse together during childhood. The skeleton does five jobs, and only the first two are the ones people name.

Support. A frame to hang everything on. Without it the body could not hold its shape.
Protection. The skull around the brain, the ribs around the heart and lungs, the spine around the spinal cord.
Movement. Bones give muscles something firm to pull against. A muscle attached to nothing rigid could not move you anywhere.
Making blood cells. Red marrow, in the middle of certain bones, produces red blood cells, white blood cells and platelets — the cargo from Quiz 15 and the defenders in Quiz 18. They are manufactured inside your bones.
Storing minerals. Bone holds most of the body's calcium, and releases it into the blood when the level there drops. This is a hormone loop of the kind Quiz 16 described, and it means the skeleton is also a warehouse the rest of the body draws on.

Bone itself is living tissue with cells, nerves and a blood supply. The hard part is a mineral framework, mostly calcium, laid down around fibers that keep it from being brittle — hard and slightly springy at once. Bone is dense on the outside and open and honeycombed on the inside, which is what makes a skeleton strong without being unliftable. And because bone is alive, it heals when it breaks and it changes with use: worked regularly, it is built up; left unloaded, it is quietly taken away again.

Joints, and the three tissues that hold everything together

A joint is where two bones meet. Some are built for a lot of movement, some for a little, and some for none at all.

Three different tissues do the connecting, and the test checks whether you can tell them apart. They are easy to confuse and easy to fix in place, because each joins a different pair of things.

	What it joins	What it does
Tendon	Muscle to bone	Passes the muscle's pull along to the bone
Ligament	Bone to bone	Holds a joint together and limits how far it can go
Cartilage	Covers bone ends; also in the ear and nose	Cushions the joint and lets the surfaces slide

A torn ligament and a torn tendon are different injuries with different consequences, and the difference follows from the table. Tear a tendon and the muscle still contracts, but the pull no longer reaches the bone, so the movement is lost. Tear a ligament and the muscles work fine, but the joint is loose and travels further than it should.

Muscles pull. That is the whole of it.

There are three kinds of muscle, and they differ in what they move and whether you command them.

	Where	Under your control?
Skeletal	Attached to bones	Yes — voluntary
Smooth	Walls of the stomach, intestines, blood vessels, bladder	No — involuntary
Cardiac	The heart, and nowhere else	No — involuntary, and it never rests for long

Now the fact this quiz turns on. A muscle contracts, which means it gets shorter and pulls the two things it is attached to toward each other. It cannot lengthen itself, and it cannot push. A contracted muscle relaxes and can be stretched out again, but nothing about that stretching is the muscle's own doing.

So a single muscle at a joint could move a bone one way and then be stuck. Every joint you can move back and forth is worked by at least two muscles pulling in opposite directions, and they take turns.

Feel it on yourself. Bend your arm and the biceps goes hard and short under your hand; the triceps at the back of your upper arm goes slack. Straighten the arm against something and they trade places. Neither one is pushing at any point. The forearm goes up because one muscle pulled it up, and it goes down because the other pulled it down.

Bones as levers. The joint is a pivot, the bone is a rigid bar, and the muscle supplies the force — usually attached very close to the joint. That is a poor arrangement for lifting heavy things, since the muscle has to pull hard to move a load far out at the hand. What it buys is speed and range: a shortening of an inch at the muscle swings the hand through a foot. So the body gives up strength in exchange for speed and reach, which suits a creature that throws, climbs and works with its hands.

Skin: the largest organ, and the busiest boundary

Skin is an organ, and by weight the largest one you have. It comes in layers, and each layer holds different equipment.

Epidermis: The outer layer. New cells are made at its base and pushed up as older ones die, so the surface of you is a layer of dead cells being shed and replaced continuously. This is also where melanin sits, the pigment that gives skin its color.
Dermis: The working layer underneath, and where nearly everything happens: blood vessels, nerve endings, sweat glands, oil glands, and the roots of hairs. A cut that bleeds and hurts has reached the dermis. A scrape that does neither has not.
Fat layer: Beneath the dermis. Insulates against cold, cushions what is under it, and stores energy.

Four jobs are worth knowing by name. The skin is a barrier, keeping germs out and water in — which is why a large burn is so dangerous, since the loss of fluid through unprotected tissue can kill before any infection does. It provides sensation, through the nerve endings in the dermis that Quiz 16 called receptors. It controls temperature: sweat glands release water that carries heat away as it evaporates, and the blood vessels in the dermis widen to let heat escape or narrow to hold it in, which is what makes you flush when hot and go pale when cold. And with sunlight on it, the skin makes vitamin D, which the body needs to use calcium — so the skin and the skeleton are connected more directly than they look.

Melanin, plainly. Melanin absorbs ultraviolet light from the sun before it can damage the cells beneath. Skin that has more of it is better protected and burns less readily; skin that has less burns sooner. That is the whole of the difference in this respect, and it is a difference of degree rather than of kind — more melanin is real protection, not immunity, and ultraviolet damage adds up over years in everyone. The same pigment also blocks some of the light the skin uses to make vitamin D, so darker skin makes less of it from the same time outdoors, which matters in northern places with long winters.

The short version

Bone is living tissue that supports, protects, makes blood cells and stores calcium, and it is built up or taken away according to the load put on it. Tendons join muscle to bone, ligaments join bone to bone. A muscle can only pull, so muscles work in opposing pairs. Skin is a barrier that also senses, regulates heat, and makes vitamin D, and melanin in its outer layer absorbs ultraviolet light.

Worked Examples

Two questions, worked through

Example 1. Why does the elbow need two muscles when one would seem to be enough?

Ask what one muscle could accomplish. The biceps contracts and pulls the forearm up. Now the arm is bent, and to straighten it the forearm has to travel back down. The biceps cannot do that job: relaxing lets it be stretched, but relaxing is not pulling, and a muscle has no way to push the bone away from itself.

Something has to pull the forearm the other way, and it has to be attached on the other side of the joint. That is the triceps.

Because muscles pull and cannot push, so a second muscle is needed to reverse the movement. Whenever a question asks why a body part has a pair of muscles, this is the answer, and it holds at the jaw, the fingers and the eye as much as at the elbow.

Example 2. A worker steps off a curb badly. The ankle swells, and the doctor says a ligament was torn. What follows about what the ankle can and cannot do?

Go back to what a ligament joins: bone to bone. Its work is holding the joint together and stopping it from traveling too far. It is not part of the pulling chain at all.

So the muscles of the lower leg still contract, and the tendons still carry that pull to the foot bones. The signal, the muscle and the connection to the bone are all intact. What has been lost is the restraint on the joint — it is now loose and can be forced further than it should go, which is why the ankle gives way and why it will do so again more easily.

The muscles can still move the foot; the joint is no longer held steady. Sort any question of this shape by asking what the torn tissue was joining. Muscle to bone means the movement is lost. Bone to bone means the movement remains and the joint is unstable.

The Quiz

Ten questions

Answer all ten, then press the button at the bottom. Nothing is timed.

Diagram — questions 1 to 4

Use the arm diagram from the Guide above: the upper panel shows the arm bending, the lower panel shows it straightening, with the biceps drawn above the upper arm bone and the triceps below it.

1.According to the diagram, what is the triceps doing while the arm bends?

AContracting, to help lift the forearm
BPushing the forearm upward from below
CRelaxing, and being stretched long and thin
DHolding the joint together like a ligament

2.Why can the biceps not straighten the arm by itself?

AA muscle can only pull, and the biceps pulls the forearm up.
BThe biceps is not attached to the forearm.
CThe biceps is an involuntary muscle.
DThe elbow is a ball-and-socket joint.

3.What structure carries the biceps's pull to the bone of the forearm?

AA ligament
BCartilage
CA nerve
DA tendon

4.The elbow is drawn as a hinge. Which movement would that rule out?

ABending the arm toward the shoulder
BSwinging the forearm out sideways at the elbow
CStraightening the arm fully
DHolding the arm still halfway

Table — questions 5 to 7

Bone density measured in three groups of adults at the start of a study and again eight weeks later. Each group's starting value is set at 100 so the groups can be compared.

Group	At the start	After 8 weeks
Walking and lifting most days	100	102
No regular exercise	100	99
Confined to bed after surgery	100	93

5.Which group's bone density changed the most over the eight weeks?

AWalking and lifting most days
BNo regular exercise
CConfined to bed after surgery
DAll three changed by the same amount.

6.What do these results best support about bone?

ABone is a fixed mineral structure that does not change once an adult stops growing.
BBone is living tissue that is built up or taken away according to the load placed on it.
CBone density depends on how much calcium is eaten and nothing else.
DBone density falls in everyone at the same steady rate.

7.A reader concludes from this table that doctors should never confine a patient to bed. How should that be judged?

AIt is well supported, since the bed-rest group did worst.
BIt is well supported, because a loss of seven points is a large one.
CIt cannot be judged, because bone density is impossible to measure reliably.
DIt goes further than the evidence, which shows one effect of bed rest and nothing about the reasons it was ordered.

Passage — questions 8 to 10

A four-person crew spends July on an outdoor job. Dave burns within about twenty minutes in full sun. Mari, whose skin is much darker, says she has never burned in her life and does not need sunscreen or a hat.

Melanin, the pigment in the outer layer of skin, absorbs ultraviolet light from the sun before it reaches the cells below. Skin with more melanin absorbs more of it and burns far less readily. The protection is real but partial: ultraviolet damage still reaches the cells, and it accumulates over years of exposure. Skin cancers are less common in people with darker skin, and they are more often found at a late stage, when they are harder to treat.

8.What accounts for the difference in how quickly the two of them burn?

AMelanin in Mari's epidermis absorbs more of the ultraviolet light.
BMari's skin has no nerve endings in the dermis.
CDave's skin is thinner in every layer.
DUltraviolet light does not reach darker skin at all.

9.How should Mari's conclusion that she needs no protection be judged?

ASound, because not burning shows the sun is doing no harm.
BUnsound, because the passage says melanin gives no protection.
CUnsound, because the protection is partial and damage still adds up over years.
DSound, because skin cancers are less common in people with darker skin.

10.Which conclusion does the passage best support?

AOnly people who burn easily need to watch their skin.
BBoth workers benefit from protection and from noticing changes in their skin, though their risks are not the same.
CSunscreen is useful only for people with light skin.
DSkin cancer is equally common in everyone.

Send this line to your teacher

The line records which questions you missed and which answer you chose. That is more useful to your teacher than the score, because it shows where a question went wrong. If a question felt unclear even though you got it right, add its number with a question mark — for example 5? — before you send it.

Score ______ / 10 Missed — write the question number and the letter you chose:
______________________________________________________________

The Key

Answers, and the trap in each one

1. C — relaxing, and being stretched long and thin. The upper panel draws it that way, and the drawing follows from the rule: when one muscle of a pair contracts, the other has to give way or the bone could not move. A has both muscles contracting at once, which would lock the joint rather than bend it — and locking is in fact what happens when you tense your arm hard without moving it. B is the deeper error and it is the one this quiz exists to correct: muscles do not push.

2. A — a muscle can only pull. The biceps is attached above the elbow and below it, so contracting brings those two points together and the forearm comes up. To send the forearm back down, something has to pull from the other side. B is simply false, and the diagram shows the attachment. This question and the last one are the same fact asked twice from opposite ends, because it is the fact that makes the rest of muscle anatomy make sense.

3. D — a tendon. Tendons join muscle to bone. A muscle's contraction is useless unless the pull reaches something rigid, and the tendon is what carries it there. A is the answer people give when the two words have not been separated. Ligaments join bone to bone. Say the two pairings once each — muscle to bone, bone to bone — and this question and the ankle example both come out right.

4. B — swinging the forearm out sideways. A hinge moves in one plane, like a door. Bending and straightening are both within that plane; sideways is not. D describes holding still, which no joint design forbids. Hinge and ball-and-socket questions are about the number of directions available, and the two everyday examples — a door and a shoulder — settle most of them without any anatomy.

5. C — the group confined to bed. Read the sizes of change, not the direction: two points up, one point down, seven points down. Seven is the largest. A gained rather than lost, and a question about the largest change includes gains. If the question had asked which group lost the most, the answer would be the same — but that is luck, and reading exactly what was asked is the habit worth keeping.

6. B — bone is living tissue that responds to the load on it. All three rows point the same way. Load it and it is built up; leave it idle and it is taken away; remove the load altogether and the loss is fast. A is what most people picture when they think of a skeleton, and this table is the evidence against it. C names something that does matter, calcium, and claims it is the only thing — a claim the table cannot support, since every group ate as they normally would.

7. D — it goes further than the evidence. The table shows one consequence of bed rest. It says nothing about why anyone was confined to bed, what would have happened otherwise, or what the surgery required. A single measured effect does not settle a decision that has other consequences on both sides. A and B are both true statements about the table used to support a conclusion the table does not reach. That combination — accurate reading, overreaching conclusion — is the most common wrong answer on this kind of item across the whole test. C throws out a measurement that the study plainly made.

8. A — melanin absorbs more of the ultraviolet light. The passage says it directly, and the Guide puts melanin in the epidermis, the outer layer, which is where light arrives first. D turns real protection into total protection, which the passage denies in the very next sentence. Watch for answers that take a difference of degree and make it a difference of kind; they are usually a stretched version of something the passage actually said.

9. C — unsound, because the protection is partial and damage adds up. Both halves of the passage's point have to be held at once. Mari is right that she does not burn and right that her skin is better protected. She is wrong to conclude from that that the sun is doing nothing, because burning is not the only kind of damage and the rest accumulates quietly over years. B overcorrects in the other direction and misquotes the passage, which says the protection is real. This is worth carrying off the page: not burning is not the same as not being harmed, and a new or changing spot on the skin is worth showing to a doctor whatever a person's complexion. The passage's last line is the reason — cancers found late are the hard ones, and they are found late partly because nobody was looking.

10. B — both benefit, though their risks are not the same. It is the only choice that holds everything the passage says: different amounts of melanin, different speeds of burning, real but partial protection, and a risk that exists for both while being greater for one. A and C both use Mari's lower risk to conclude she has none, which is the same error as her own. D goes the other way and flattens a difference the passage states. When a passage gives you two things that are both true and pull in different directions, the answer is nearly always the one that keeps both.

Your Score

What the number means

8 to 10	Solid. Check topic 17 on your map, and go on to Quiz 18.
6 or 7	Close. Read the whole key, then take this again in a few days before you check the box.
5 or fewer	Worth another pass. Bend and straighten your own arm with a hand on the muscle, and say out loud which one is pulling. Then write the three connectors and what each joins: tendon, ligament, cartilage.

Misses on 1 to 4 are almost always the one rule — muscles pull and never push — or the tendon and ligament pair, and both are fixed by saying them once in your own words. Misses on 5 to 7 are the reading habit rather than the biology: find the size of each change, then check whether the conclusion offered stays inside what was measured. Misses on 8 to 10 are about holding two true things at the same time, which is what the hardest questions on the real test ask you to do.

The twitching leg

In Bologna in the 1780s, Luigi Galvani and his wife and collaborator Lucia Galeazzi Galvani were dissecting frogs when a leg on the bench jerked as a scalpel touched it. Nothing was alive to command it. They spent years chasing the effect, and Galvani concluded that animals carry an electricity of their own, stored in the muscle and released by the nerve.

Alessandro Volta disagreed. He argued the electricity came from the two different metals touching the wet tissue, and that the frog was only a detector. To prove it he built a stack of alternating metal discs separated by soaked cardboard, and it produced a steady current with no animal in it at all. That stack was the first battery.

Volta was right about the metals, and Galvani was not wrong about the body. Nerve signals really are electrical, as Quiz 16 described, though it took another century to show it properly. A disagreement about a twitching leg produced both a corrected idea and a device that changed the world, which is roughly how the process is supposed to work.