Anatomy of the human bodyExtendabout 38 min

Beyond the syllabus: animals, projects, puzzles and careers

Other body plans, three things to build, puzzles worth reasoning through, and where this knowledge earns a living

Compare your body plan with a giraffe, a bird, a snake and a boneless octopus; build a working paper hand and a life-size organ map; solve puzzles spanning the whole topic; meet seven careers built on this knowledge; finish with open questions.

Start at chapter 1

In this part you’ll

Compare the human body plan with other vertebrates and one invertebrate, distinguishing "the same feature, rescaled" from "genuinely absent".
Build a simple physical model of a tendon-driven joint and explain what it gets right and what it leaves out.
Solve multi-step puzzles that combine bone counts, organ masses and body-part facts from across this topic.
Name several careers that depend on precise anatomical knowledge and describe what each one needs to know.
Identify at least one genuinely open question in this field and explain, in one’s own words, why it is still unanswered.

The first four layers stayed close to a typical human body and to the school syllabus. This layer does neither, deliberately: it goes wider, into other animals, real careers, projects you can actually build, and puzzles with no single obvious method. Some of it goes beyond what Classes 6 and 7 expect — that is the point of Extend, and it is flagged wherever it happens.

The plan: what your body plan shares with, and does not share with, other animals; three hands-on projects; a set of puzzles that reward careful reasoning rather than a memorised fact; a look at careers built on this knowledge; and a closing set of open questions that nobody has fully answered yet.

Chapter 01

The same basic plan, used very differently

Every animal with a backbone — every vertebrate — is built from a surprisingly similar parts list to yours: a skull, a spine, ribs, limbs (or their remnants), the same three or four muscle types. What differs enormously is how those shared parts are stretched, shrunk, fused or repurposed for a completely different life. Comparing your own body with a handful of others is one of the fastest ways to see which of your features are "just how a vertebrate is built" and which are unusual, human-specific solutions.

TableFive ways your own body plan shows up, stretched or removed, in other animals
Animal	What changed	What stayed the same
Giraffe	Each of its 7 neck bones is stretched to roughly 16.7× a human neck bone’s height	Still exactly 7 neck (cervical) vertebrae, the near-universal mammal number
Bird	Long bones are lighter, some almost hollow throughout with internal struts, built for minimum weight	Same basic compact-shell-and-marrow bone design, taken to a more extreme, flight-ready version
Snake	No limbs at all in almost every living species	Some snakes still carry tiny, non-functional hip and leg bones buried in their body — a leftover of walking ancestors
Octopus	No internal skeleton whatsoever: entirely soft-bodied, with only a small hard beak	Still has muscle and nervous tissue doing the jobs those tissues do in you, just with nothing rigid to attach to
Elephant	Legs built thick, straight and nearly pillar-like, positioned almost directly under the body	Same bone tissue, same joint types, same basic limb bone layout as a human leg

Lab

Decide, feature by feature, how closely an animal comparison actually matches your own body.

Sort each animal feature by whether it is essentially the same as yours, a stretched version of yours, or something you do not have at all.

6 cards, 3 bins. Tap a card, then tap its bin. You can also drag, or press a bin’s number key.

Text version of this activity

Six cards, three bins. This is a test of precise comparison rather than vague "animals are like us" or "animals are nothing like us" thinking: some features are genuinely identical in kind (joint types, bone count), some are the same basic thing wildly rescaled (a stretched vertebra, a lighter bone), and some are simply absent (an octopus has no bones to compare at all).

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Related to

Body systems and how they connect

Comparing body plans across animals also compares whole systems, not just parts — a mouse’s fast heartbeat and an elephant’s slow one are a circulatory-system story that topic explains.

Chapter 02

Senses across the animal kingdom: same job, different tool

Discover covered the five familiar human senses briefly. Other animals solve the same underlying problems — seeing in the dark, finding prey without light, sensing a predator behind them — with tools your own sense organs simply do not have.

Lab

Decide which human sense, if any, is doing the closest equivalent job to four animal senses.

Sort each sensory trick by which human sense organ does the closest equivalent job.

4 cards, 3 bins. Tap a card, then tap its bin. You can also drag, or press a bin’s number key.

Text version of this activity

Four sensory tricks, three bins. Two map fairly closely onto a human sense organ doing a more extreme version of the same job; two have no real human equivalent at all, which is the more important lesson — not every sense in nature has a human counterpart, even a weak one.

Need a different angle?

Try it

Notice that two of the sort-game's animal senses had no human equivalent at all: a pit viper's heat-sensing pit organs, which detect infrared warmth directly, and a shark's ability to sense the faint electric fields every living muscle produces. Comparative anatomy is not only about finding the human match for an animal feature — sometimes the honest, useful answer is that no human sense organ does anything like it, and that is worth knowing too.

Related to

Light

An owl’s eye and a human eye are both optical instruments obeying the same physics of light, lenses and images explained in the Light topic — built differently, but for the same underlying job.

Related to

Sound

Bat echolocation is the Sound topic’s echo-timing idea, run by a living animal instead of a measuring instrument.

Chapter 03

Measuring the body across history and cultures

Investigate showed that a hand span gives a different answer for every different-sized hand, which is why the centimetre eventually won out everywhere. But body-based units were not a passing mistake — they were the working standard for most of human history, in every civilisation, because a body is a ruler everyone always carries.

TableFive historic body-based units, roughly converted to modern measurement
Unit	Body part used	Rough modern length
Cubit	Elbow to fingertip	About 45–55 cm, varying by region and era
Hasta	Elbow to fingertip (the Indian cubit, used in classical texts)	About 45–50 cm
Angula	A finger’s width (a smaller unit built up into the hasta)	About 1.5–2 cm
Foot	A human foot’s length	About 30 cm, standardised much later
Fathom	Fingertip to fingertip with arms outstretched (like an arm span)	About 1.8 m

Aha

The Great Pyramid was measured in elbows

The ancient Egyptian royal cubit, based on the length of the Pharaoh's own forearm, was used to plan and build structures as enormous as the Great Pyramid of Giza — a body-based unit standing in for what is now one of the most measured structures on Earth. Classical Indian texts on architecture and sculpture (shilpa shastra) similarly specify proportions in hasta and the smaller angula, allowing a design to be scaled up or down consistently by anyone who knew the system, anywhere the same body-based convention was understood.

The same trade-off from Investigate applies here at civilisation scale: workable and universally available, but only as consistent as the bodies used to define it — which is exactly why every one of these units eventually needed a fixed, non-human reference to become a true international standard.

Worked example

0 / 2 steps shown

Converting angula to hasta

Classical Indian measurement used 24 angula (finger-widths) to make one hasta (cubit). A carved doorway is specified as 48 angula tall. How many hasta is that?

Need a different angle?

Lab

Connect five historic units of length to the body part each one was originally measured from.

Match each historic body-based unit to the body part it was measured from.

5 pairs are hiding in two mixed-up columns. Pick one from each side to join them.

Text version of this activity

Five unit cards, five body-part cards, shuffled. Every one of these units was eventually replaced by a fixed, non-human standard for exactly the reason explored in Investigate: bodies differ in size, and a unit needs to mean the same thing for everyone.

Need a different angle?

Reflect

The metre is now defined by the distance light travels in a tiny fraction of a second — about as far from "a body part" as a definition can get. Why do you think a modern standard unit needed to move away from the body entirely, rather than just picking one particular person's cubit as the permanent standard?

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Chapter 04

Repair and regeneration: what grows back, and what does not

Deepen showed that a child's broken bone heals faster than an adult's, because bone is living, rebuilding tissue. Push the same question further, across species, and the differences become dramatic.

TableHow much repair different living things can manage
Living thing	What it can regrow
Starfish	A whole new arm from a stump, and in some species even a whole new body from one surviving arm and a fragment of the central disc
Some lizards	A new tail, though the replacement is usually shorter and built from cartilage rather than the original bone
Human liver	A remarkable amount of its own lost tissue can regrow after injury or surgery — one of the very few human organs able to do this
Human skin	Constantly replaces itself and heals over small wounds, but a large lost area needs medical treatment such as a graft
Human finger or limb	Does not regrow at all once lost, unlike a starfish arm or a lizard tail

Aha

One salamander can regrow a whole leg, bone and all

The axolotl, a salamander from Mexico, can regrow an entire lost limb — bone, muscle, nerve, blood vessels and skin, all correctly rebuilt and reconnected — repeatedly, throughout its life, with no scar left behind. It is one of the most capable regenerators among animals with backbones, studied closely by scientists precisely because it is a vertebrate, built from the same basic tissue types you are, that has kept an ability humans do not have.

This makes the earlier riddle sharper, not softer: an axolotl is not fundamentally different tissue from you at the level of cells and tissue types from Understand — bone, muscle, nerve, skin. Whatever lets its cells rebuild a whole limb from scratch is a difference in behaviour of very similar building blocks, which is exactly why axolotl regeneration is such an active area of research.

Predict first

Given that the human liver can regrow much of its own lost tissue, why do you think humans cannot regrow a whole lost finger the same way?

Regeneration ability is not the same across every organ or tissue in one body; it depends on the specific cell types involvedHumans never regenerate anything at allFingers are too small to regrowOnly cold-blooded animals can ever regenerate anything

Chapter 05

The body’s record book: a whole-topic recap

Before the last stretch of projects, puzzles and careers, a quick recap in record-book form — every figure below was properly introduced and sourced earlier in this topic; here they are gathered side by side.

Longest bone: femurAbout 40.1 cm in a 150 cm person — roughly a quarter of standing height.
Smallest bone: stapesAbout 3.0 mm, inside the ear — smaller than a grain of rice.
Most bones in one place: a hand27 bones in a single hand — more than 51.5% of the whole skeleton is in the hands and feet together.
Largest organ: skin1.5–2.0 m² — the only organ you can see and touch all of.
Largest organ inside the body: liverAbout 1.5 kg, and one of the only organs able to regrow much of itself.
Hardest-working muscle: the heartAbout 100,800 beats a day, without ever resting for a day off.
Biggest engineered force ratio: the biceps leverMust pull about 8.0 times harder than the load it lifts, from Deepen’s lever mathematics.
Longest fusing process: the skeletonFrom about 300 pieces at birth down to 206, finishing in the twenties.

Worked example

0 / 3 steps shown

Finding a record inside a data set you already have

Look back at the Investigate layer’s pulse data: fifteen pupils’ pulse rise after mild exercise ranged from 4 to 28 bpm. What was the range of that data set, and which pupil defines each end of it?

Need a different angle?

Chapter 06

Three projects you can actually build

Reading about tendons and levers is one thing; building a rough working model makes the idea impossible to forget. All three projects below use cheap household materials and no special tools.

Project 1: a working paper hand

Step 01Trace and cutpaper, pen
Trace your own hand onto stiff paper or thin card and cut it out, including gaps between the fingers.
Step 02Mark the jointspen
Draw a short line across each finger at every knuckle — these are where the "joints" will bend.
Step 03Add tendonsdrinking straws, string
Tape a short length of drinking straw along the back of each finger as a sleeve, then thread a piece of string through each straw so it runs to the fingertip and out past the wrist.
Step 04Pull and watchnothing extra
Pull each string from the wrist end. The finger curls, exactly the way pulling a tendon curls your own finger — and letting go straightens it only if you built in a light elastic pulling the other way, just like an antagonistic pair.

Project 2: a life-size organ map

Step 01Trace an outlinea paper roll, a partner
Lie down on a long sheet of paper and have a partner trace around you, or draw a simple body outline at your own height.
Step 02Mark the landmarksruler
Using your own measurements, mark the level of your lowest ribs, your navel, and the top of your hip bones.
Step 03Draw organs to scalethe sizes from this topic
Draw a fist-sized heart tipped slightly left of centre in the chest; a liver wedge under the right ribs about 1.5 kg’s worth of space; a stomach high on the left; kidneys at the back near the lowest ribs.
Step 04Check yourselfearlier layers’ tables
Compare your finished map against the organ tour table. Which organ did you draw in the wrong place before checking?

Project 3: track your own growth. Measure your height once a month for a school term and plot it on a simple graph, one dot per measurement. A single measurement tells you almost nothing about growth; a series of them, spaced out over time, shows you a real growth curve — the same idea a doctor uses at a check-up, but built entirely from your own data.

Lab

See what a real growth-tracking project produces: not one number, but a rising sequence of them.

Example: one pupil’s height over a school year (cm) (cm)

Round 1 / 1★ 0 ptsBest: 0

Challenge 1This pupil grew about 3.8 cm over the whole year. Adjust the first or last value so the range becomes exactly 4 cm.

Target: range = 4. Right now the range is 3.8. Add or remove dots below — it checks as you go.

Tap the number line to add a value; tap a dot to remove it. Dashed long line = mean (●), dotted line = median (▲).

The values (9)

148
148.5
149
149.4
150
150.3
150.9
151.2
151.8

Mean (share it out equally)149.9 cm

sum ÷ count = 1349.1 ÷ 9 = 149.9

Median (the middle value)150 cm

148148.5149149.4150150.3150.9151.2151.8

9 values (odd), so the middle one — number 5 in order — is the median.

Mode (most common)No mode

Every value appears only once. The usual convention: when nothing repeats, we say there is no mode.

Range (spread)3.8 cm

max − min = 151.8 − 148 = 3.8

Text version of this activity

The lab plots nine monthly height measurements for one example pupil, rising steadily from 148 cm to nearly 152 cm across a school year — never falling, and rising by a different amount each month, which is exactly what real growth data looks like: a trend, not a straight line.

Drag any point and see how the described "3.8 cm over the year" range changes. This is the project from the steps above, already filled in with one term’s worth of realistic example data, so you have a model to compare your own measurements against.

Need a different angle?

Chapter 07

Puzzles that reward careful reasoning

None of the puzzles below can be solved by remembering a single fact. Each one needs you to combine two or three things you already know.

Worked example

0 / 3 steps shown

A riddle: the bone that touches no other bone

I am a small, U-shaped bone in your throat. I anchor your tongue and help you swallow and speak. Unlike every other bone in your body, I do not touch any other bone at all. What am I?

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Worked example

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Rank five body parts from lightest to heaviest

Using figures from this topic, rank these from lightest to heaviest for a 60 kg adult: the heart, the brain, the liver, the skin, the skeleton.

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Worked example

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A chained puzzle: from femur to lever ratio

A femur is measured at 42.7 cm, belonging to a person estimated to be 160 cm tall. If that same person’s biceps attaches 4 cm from the elbow and their hand sits 32 cm from the elbow, how many times harder than a held weight must their biceps pull — and does the femur length used to estimate their height affect this answer at all?

Need a different angle?

Try it

Lab

Solve five riddle-style clues drawn from facts across this whole topic.

Match each riddle-style clue to the body part it describes.

5 pairs are hiding in two mixed-up columns. Pick one from each side to join them.

Text version of this activity

Five clues, five answers, shuffled. Every clue reuses a fact already taught somewhere in this topic, so a correct match is a genuine test of whether the fact stuck, not a guess.

Need a different angle?

Chapter 08

Careers built on knowing the body

"What is in there, and where" is not only a school topic. Whole careers exist because someone needs to know it precisely, every working day.

TableEight careers, and the anatomy knowledge each one leans on
Career	What they need to know precisely
Radiographer	Where every organ sits, so an X-ray, ultrasound, CT or MRI image can be read correctly
Physiotherapist	Which muscles move which joints, and how bone and muscle respond to load and rest
Orthopaedic surgeon	The exact structure of bones and joints, to repair a fracture or replace a worn joint
Forensic anthropologist	How bone measurements relate to a person’s height, age and other features
Prosthetics engineer	How a lever at a real joint behaves, to design a replacement that moves the same way
Sports scientist	Muscle types, antagonistic pairs and lever mechanics, to train movement safely and effectively
Medical illustrator	Exact organ shape, size and position, to draw accurate diagrams for textbooks and doctors
Museum conservator	How real bone, cartilage and tissue age and decay, to preserve skeletons and specimens correctly

Worked example

0 / 3 steps shown

A prosthetics engineer’s lever problem

A prosthetics engineer is designing a replacement forearm and hand. The natural biceps attachment point this device replaces sat 4 cm from the elbow, with the hand 32 cm out — a lever ratio of 8.0. If the engineer moves the artificial "tendon" attachment to 8 cm from the elbow instead, what trade-off are they choosing, using the lever reasoning from Deepen?

Need a different angle?

Reflect

Pick one career from the table that you had not thought about before. What is one thing from this whole topic — Discover, Understand, Investigate, Deepen or Extend — that person would use on an ordinary working day?

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Chapter 09

Open questions nobody has fully answered

Think about it

Open questions for curious learners

Nobody has fully answered these yet. Pick one and dig in.

Could a bone be grown or 3D-printed to exactly replace a piece lost to injury, matching its real internal structure rather than just its outer shape?
A starfish can regrow a lost arm; a human liver can partly regrow itself, but a human cannot regrow a lost finger. Why do some living things regenerate far more than others, and could that ever be learned from and applied?
If bone genuinely rebuilds itself around the load it carries, could astronauts on very long space missions be given exercise routines precise enough to prevent almost all bone loss?
Elephants need proportionally thicker legs than mice because of scaling laws. Is there an absolute upper limit to how large a land animal could ever be built, using bone as the material?
Would it be possible to design a prosthetic joint that outperforms a human joint's own trade-off between range of movement and stability, rather than just imitating it?

Words to know

All maths vocabulary →

A few last words worth owning

vertebrate: An animal built around an internal backbone.; Example: Humans, giraffes, birds and snakes are all vertebrates.
invertebrate: An animal with no internal backbone or skeleton.; Example: An octopus is an invertebrate.
comparative anatomy: The scientific study of similarities and differences between the body plans of different species.; Example: Comparing a giraffe’s neck bones with a human’s is comparative anatomy.
regeneration: The regrowth of lost or damaged body parts.; Example: A starfish regrowing a lost arm; a human liver regrowing much of its own lost tissue.
body-based unit: A unit of length defined by a part of the human body.; Example: The cubit, the hasta, the foot and the hand span are all body-based units.

Chapter 10

Check yourself

Reflect

This layer went beyond the school syllabus in several places — animal scaling, forensic estimation, open research questions. Pick one idea from anywhere in this topic (any of the five layers) that you would like to research further on your own, and write down one specific question you would try to answer first.

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Keep this

Cheat sheet

Same plan, different scale. A giraffe has exactly 7 neck vertebrae, like you — each one about 16.7× as tall. Bird bones push the hollow-tube idea further for lightness. An octopus has no bones at all: an invertebrate, unlike you, a vertebrate.
Projects: a paper hand with straw-and-string tendons; a life-size organ map using this topic’s real sizes; tracking your own height monthly to build a real growth curve.
Puzzles reward combining facts. The hyoid bone touches no other bone. A 60 kg skeleton (about 9.0 kg) outweighs the heart, brain and liver put together.
Careers from radiographer to forensic anthropologist to prosthetics engineer all depend on precisely knowing what is in the body and where, or on the mechanics of levers and bone.
This layer goes beyond the syllabus on purpose — Deepen and Extend are allowed to, and this is flagged rather than hidden.

Where this comes from

Sources

Human body (opens another website) — Encyclopaedia Britannicaawaiting check
Supports the levels of organisation (cells, tissues, organs, organ systems), the four main tissue types, the naming of body regions and cavities, and the general description of the organs and their positions.
Human skeleton (opens another website) — Encyclopaedia Britannicaawaiting check
Supports 206 bones in the adult skeleton, the axial and appendicular division, the skull, vertebral column and its curves, the ribcage and pelvis, joint types, cartilage, ligaments and tendons, and bone as living tissue with marrow.
Human muscle system (opens another website) — Encyclopaedia Britannicaawaiting check
Supports the three muscle types (skeletal, smooth, cardiac), voluntary and involuntary control, antagonistic pairs such as biceps and triceps, muscles pulling rather than pushing, the diaphragm, and the approximate count of skeletal muscles.
List of organs of the human body (opens another website) — Wikipediaawaiting check
Supports the inventory of organs named in this topic, which system each belongs to, and which organs come in pairs.
Giraffe (opens another website) — Encyclopaedia Britannicaawaiting check
Supports the animal comparison that a giraffe has seven neck (cervical) vertebrae, the same count as a human, but each one is far longer.
Octopus (opens another website) — Encyclopaedia Britannicaawaiting check
Supports the animal comparison that an octopus has no internal skeleton at all, used to explain the term "invertebrate" against the human "vertebrate" body plan.
Cubit (opens another website) — Wikipediaawaiting check
Supports the history of body-based measurement units across cultures, including the cubit (elbow to fingertip) and hasta, used before standard metric units were agreed.
Axolotl (opens another website) — Encyclopaedia Britannicaawaiting check
Supports the fact that the axolotl, a salamander, can regrow entire lost limbs, used as a contrast with human and starfish regeneration ability.
Owl (opens another website) — Encyclopaedia Britannicaawaiting check
Supports the fact that an owl’s eyes are fixed, tube-shaped and cannot rotate in their sockets, which is why an owl turns its whole head to look around.
Bat (opens another website) — Encyclopaedia Britannicaawaiting check
Supports the fact that most bats navigate and hunt using echolocation: emitting high-pitched calls and reading the returning echoes.
NCERT textbooks (Curiosity, Science for Classes 6 and 7) (opens another website) — National Council of Educational Research and Training, Indiaawaiting check
Syllabus alignment for Classes 6 and 7: body organisation, the skeleton and joints, muscles and movement, the sense organs, and measuring the body in class.

End of Extend

What you just read

Compare the human body plan with other vertebrates and one invertebrate, distinguishing "the same feature, rescaled" from "genuinely absent".
Build a simple physical model of a tendon-driven joint and explain what it gets right and what it leaves out.
Solve multi-step puzzles that combine bone counts, organ masses and body-part facts from across this topic.
Name several careers that depend on precise anatomical knowledge and describe what each one needs to know.
Identify at least one genuinely open question in this field and explain, in one’s own words, why it is still unanswered.

The web

Explore a connection

Usesanother area
Light
The eye is a lens, a screen and a shutter — optics built out of living tissue.
Usesanother area
Sound
The ear turns shaking air into signals a nerve can carry: a drum, three tiny bones and a spiral of fluid.
Helps you understand
Body systems and how they connect
Once you know where each organ sits, you can follow how they pass work to each other.

Want to save topics or ask for new ones? Invited families can connect a learning device. Everything here stays free to read without signing in.

Revision 1 · release preview-7e1cbbcc4f · accepted 20/09/2026

Converting angula to hasta

Finding a record inside a data set you already have

Project 1: a working paper hand

Project 2: a life-size organ map

The values (9)

A riddle: the bone that touches no other bone

Rank five body parts from lightest to heaviest

A chained puzzle: from femur to lever ratio

A prosthetics engineer’s lever problem

A few last words worth owning

Twelve questions across animals, history, projects, puzzles and careers

Cheat sheet

Sources

What you just read

Explore a connection