--- title: "Bones and Muscles" book: "Primary & Middle School Biology" subject: biology language: en chapter: 17 exercises: 10 source: https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles --- # Chapter 17 — Bones and Muscles Jump, and land: something inside you just worked like a perfect machine. Rigid rods held your shape, hinges let your [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) fold and spring, and hundreds of little motors pulled exactly when needed. The rods are your [bones](#def-g3-bones-and-muscles-skeleton), the hinges your [joints](#prop-g3-bones-and-muscles-joints), the motors your [muscles](#def-g3-bones-and-muscles-muscle) — the body’s equipment for every movement it will ever make. ## 17.1 The skeleton **Definition 17.1 (Skeleton and bones).** The *skeleton* is the frame of about $200$ *bones* inside the body. Its main regions: - the *skull* , a solid helmet of bone around the brain; - the *spine* (backbone), a bendable column of small bones stacked from neck to hips, holding the trunk upright; - the *ribs* , curved bones forming a cage around heart and lungs; - the *[limb](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) bones* : long bones in [arms](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) and [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) — the thigh bone is the longest of the body. ![The skeleton’s plan: helmet, column, cage, and long limb bones meeting at joints.](https://one-course.com/images/onecourse/chapters/biology-1/g3-bones-and-muscles/fig-981d6bd5d646.svg) *The [skeleton](#def-g3-bones-and-muscles-skeleton)’s plan: helmet, column, cage, and long [limb](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) [bones](#def-g3-bones-and-muscles-skeleton) meeting at [joints](#prop-g3-bones-and-muscles-joints).* **Example 17.2 (Feel your frame).** No X-ray needed: press gently and the [skeleton](#def-g3-bones-and-muscles-skeleton) reports. Knuckles; the ridge of the shin; the chain of small bumps down the middle of your back — the [spine](#def-g3-bones-and-muscles-skeleton), one small [bone](#def-g3-bones-and-muscles-skeleton) after another; the [ribs](#def-g3-bones-and-muscles-skeleton) you can count under your fingers. The frame is right under the [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense). **Remark 17.3 (Hard outside, alive inside).** A [bone](#def-g3-bones-and-muscles-skeleton) is not dead material like a chalk stick: it is living, fed by blood, growing while you grow — and able to repair itself. A broken [arm](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) in a cast mends because the [bone](#def-g3-bones-and-muscles-skeleton) itself slowly rebuilds the break. Chalk sticks never do that. ![Inside the decorated cast, held still, a living bone is quietly rebuilding itself.](https://one-course.com/images/onecourse/chapters/biology-1/g3-bones-and-muscles/img-eccb44af4ca3.jpg) *Inside the decorated cast, held still, a living [bone](#def-g3-bones-and-muscles-skeleton) is quietly rebuilding itself.* ## 17.2 Joints, where the frame bends **Proposition 17.4 (Rigid rods, bending places).** [Bones](#def-g3-bones-and-muscles-skeleton) themselves never bend — a bent [bone](#def-g3-bones-and-muscles-skeleton) is a broken [bone](#def-g3-bones-and-muscles-skeleton). All the body’s bending happens at the *joints*, where two [bones](#def-g3-bones-and-muscles-skeleton) meet: [shoulder](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-joint), [elbow](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-joint), [wrist](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-joint), hip, [knee](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-joint), [ankle](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-joint), and the many small [joints](#prop-g3-bones-and-muscles-joints) of the [spine](#def-g3-bones-and-muscles-skeleton), whose little bends add up to a supple back. **Proof.** *Admitted at this level.* ∎ **Example 17.5 (The spine’s trick).** No single [bone](#def-g3-bones-and-muscles-skeleton) in your back can curl — yet you can roll forward almost into a ball. The [spine](#def-g3-bones-and-muscles-skeleton) is a column of more than $20$ small [bones](#def-g3-bones-and-muscles-skeleton), each turning a few degrees on its neighbors: many small bends making one big one. That is also why you should bend [knees](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-joint), not back, to lift something heavy — [knees](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-joint) are built for big folds, the [spine](#def-g3-bones-and-muscles-skeleton) for gentle ones. ## 17.3 Muscles, the movers **Definition 17.6 (Muscle).** A *muscle* is a fleshy motor attached to the [bones](#def-g3-bones-and-muscles-skeleton). A muscle can *contract* — shorten and thicken — and in contracting it *pulls* the [bone](#def-g3-bones-and-muscles-skeleton) it is attached to. Muscles only pull; no muscle can push. **Proposition 17.7 (Working in pairs).** Since [muscles](#def-g3-bones-and-muscles-muscle) only pull, moving a [joint](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-joint) both ways takes a *pair*: one [muscle](#def-g3-bones-and-muscles-muscle) in front of the [arm](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) bends the [elbow](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-joint), its partner behind straightens it; each relaxes while the other pulls. Nearly every [joint](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-joint) of the body is moved by such pairs. **Proof.** *Admitted at this level.* ∎ ![Bending the elbow: the front muscle of the pair bulges as it shortens and pulls the forearm up, while its partner behind rests. To straighten, the two swap roles.](https://one-course.com/images/onecourse/chapters/biology-1/g3-bones-and-muscles/fig-ee96386cc3b6.svg) *Bending the [elbow](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-joint): the front [muscle](#def-g3-bones-and-muscles-muscle) of the pair bulges as it shortens and pulls the forearm up, while its partner behind rests. To straighten, the two swap roles.* **Example 17.8 (Feel the pair at work).** Grip your upper [arm](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) with the other hand and slowly bend the [elbow](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-joint): the [muscle](#def-g3-bones-and-muscles-muscle) in front swells and hardens — it is contracting. Now straighten the [arm](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts): the front softens, and behind the [arm](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) its partner tightens instead. You have just felt [Proposition 17.7](#prop-g3-bones-and-muscles-pairs) happen. **Method 17.9 (Looking after bones and muscles).** 1. Move every day — [bones](#def-g3-bones-and-muscles-skeleton) grow stronger, [muscles](#def-g3-bones-and-muscles-muscle) too, with use; 2. eat the builders’ foods: dairy for [bones](#def-g3-bones-and-muscles-skeleton) , and varied meals ( [Chapter 12](https://one-course.com/books/biology/1/en/chapter/12-teeth-and-eating-well#ch-g2-teeth-and-eating-well) ); 3. warm up before hard exercise, so [muscles](#def-g3-bones-and-muscles-muscle) stretch instead of tearing; 4. lift heavy things with bent [knees](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-joint) and a straight back; 5. helmets and pads where falls are likely: [bones](#def-g3-bones-and-muscles-skeleton) repair, but slowly. ## 17.4 Exercises **Exercise 17.1 ★.** Name the four regions of the [skeleton](#def-g3-bones-and-muscles-skeleton) in [Definition 17.1](#def-g3-bones-and-muscles-skeleton), and what each protects or holds up. **Solution of Exercise 17.1.** The [skull](#def-g3-bones-and-muscles-skeleton) protects the brain; the [spine](#def-g3-bones-and-muscles-skeleton) holds the trunk upright (a bendable column); the [ribs](#def-g3-bones-and-muscles-skeleton) form a protective cage around heart and lungs; the [limb](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) [bones](#def-g3-bones-and-muscles-skeleton) carry [arms](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) and [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts). **Exercise 17.2 ★.** About how many [bones](#def-g3-bones-and-muscles-skeleton) has the [skeleton](#def-g3-bones-and-muscles-skeleton)? Which is the longest? **Solution of Exercise 17.2.** About $200$. The longest is the thigh [bone](#def-g3-bones-and-muscles-skeleton). **Exercise 17.3 ★.** Can a [bone](#def-g3-bones-and-muscles-skeleton) bend? Where does all the body’s bending happen? **Solution of Exercise 17.3.** No — a bent [bone](#def-g3-bones-and-muscles-skeleton) is a broken [bone](#def-g3-bones-and-muscles-skeleton). All bending happens at the [joints](#prop-g3-bones-and-muscles-joints), where two [bones](#def-g3-bones-and-muscles-skeleton) meet. **Exercise 17.4 ★.** The [spine](#def-g3-bones-and-muscles-skeleton) can curl although none of its [bones](#def-g3-bones-and-muscles-skeleton) can. Explain its trick. **Solution of Exercise 17.4.** It is a column of more than $20$ small [bones](#def-g3-bones-and-muscles-skeleton), each able to turn a few degrees on its neighbors; the many small bends add up to one big supple curl. **Exercise 17.5 ★.** What does a [muscle](#def-g3-bones-and-muscles-muscle) do when it [contracts](#def-g3-bones-and-muscles-muscle)? What can no [muscle](#def-g3-bones-and-muscles-muscle) ever do? **Solution of Exercise 17.5.** It shortens and thickens, and so pulls the [bone](#def-g3-bones-and-muscles-skeleton) it is attached to. No [muscle](#def-g3-bones-and-muscles-muscle) can push. **Exercise 17.6 ★.** Why does bending and straightening the [elbow](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-joint) need two [muscles](#def-g3-bones-and-muscles-muscle)? Tell what each does at each moment. **Solution of Exercise 17.6.** Because a [muscle](#def-g3-bones-and-muscles-muscle) only pulls, one [muscle](#def-g3-bones-and-muscles-muscle) can move the [joint](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-joint) only one way. Bending: the front [muscle](#def-g3-bones-and-muscles-muscle) [contracts](#def-g3-bones-and-muscles-muscle) and pulls the forearm up while the back one relaxes. Straightening: the back [muscle](#def-g3-bones-and-muscles-muscle) [contracts](#def-g3-bones-and-muscles-muscle) while the front one relaxes. **Exercise 17.7 ★.** How does a broken [arm](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) in a cast mend? What does this show about [bone](#def-g3-bones-and-muscles-skeleton), by [Remark 17.3](#rem-g3-bones-and-muscles-alive)? **Solution of Exercise 17.7.** The living [bone](#def-g3-bones-and-muscles-skeleton) slowly rebuilds the break itself; the cast only holds the pieces still while it works. It shows [bone](#def-g3-bones-and-muscles-skeleton) is living material, fed by blood — not dead like chalk. **Exercise 17.8 ★.** Try it: do the grip test of [Example 17.8](#ex-g3-bones-and-muscles-pair) on your own [arm](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts). Describe what the front of the [arm](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) does when you bend, and when you straighten. **Solution of Exercise 17.8.** Bending: the front of the [arm](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) swells and hardens (that [muscle](#def-g3-bones-and-muscles-muscle) is contracting). Straightening: the front softens while the back of the [arm](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) tightens — the pair swapping roles. **Exercise 17.9 ★★.** Why do grown-ups say “lift with your [knees](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-joint), not with your back”? Answer with [Example 17.5](#ex-g3-bones-and-muscles-spinetrick). **Solution of Exercise 17.9.** [Knees](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-joint) are [joints](#prop-g3-bones-and-muscles-joints) built for big folds, moved by the strongest [muscles](#def-g3-bones-and-muscles-muscle) of the body; the [spine](#def-g3-bones-and-muscles-skeleton) is built for gentle bends spread over many small [bones](#def-g3-bones-and-muscles-skeleton). Lifting with bent [knees](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-joint) and a straight back gives the heavy work to the [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) and spares the [spine](#def-g3-bones-and-muscles-skeleton). **Exercise 17.10 ★★.** A puppet on strings can be made to wave: each string can only *pull* its [arm](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) one way. How is this exactly the [muscles](#def-g3-bones-and-muscles-muscle)’ situation, and what is the puppet-maker’s answer to it? Compare with [Proposition 17.7](#prop-g3-bones-and-muscles-pairs). **Solution of Exercise 17.10.** Each string, like each [muscle](#def-g3-bones-and-muscles-muscle), can only pull. The puppet-maker’s answer is the biologist’s: two strings per movement — one to pull the [arm](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) up, another to pull it back down — working in turns, exactly like the body’s [muscle](#def-g3-bones-and-muscles-muscle) pairs.