[{"data":1,"prerenderedAt":1224},["ShallowReactive",2],{"layer:human-body-anatomy:discover":3},{"layer":4,"contentHash":1200,"dependencyHashes":1201,"approval":1217,"releaseId":1223},{"schemaVersion":5,"conceptId":6,"locale":7,"depth":8,"revision":5,"title":9,"subtitle":10,"summary":11,"objectives":12,"estimatedMinutes":18,"plate":19,"blocks":43,"sourceIds":1195,"reviewStatus":1196,"authoring":1197},1,"human-body-anatomy","en","discover","A guided tour of the body you live in","What is inside you, where it sits, and how big it really is","Climb the ladder from cells to organ systems, learn the words anatomists use for where things are, meet the 206 bones and their joints, find out why a muscle can only ever pull, and take an organ-by-organ tour with real sizes and positions — then measure your own body.",[13,14,15,16,17],"Order the levels of body organisation from cell to organism, and say what an organ is.","Use anterior, posterior, superior, inferior, medial and lateral correctly, and apply the rule that left and right belong to the body.","Name the main bones and joint types, explain why a baby has more bones than an adult, and describe what cartilage, ligaments and tendons each do.","Explain why muscles work in opposing pairs, and name the three muscle types with an example of each.","Locate the main organs on your own body and state a rough size or mass for each.",38,{"title":20,"rows":21},"D",[22,25,28,31,34,37,40],{"label":23,"value":24},"Depth","Discover (first look)",{"label":26,"value":27},"Reading time","About 38 minutes",{"label":29,"value":30},"Prior knowledge","None; simple multiplication and division",{"label":32,"value":33},"Chapters","11",{"label":35,"value":36},"Labs","3 body explorers, a joint sort, a match game, a data lab",{"label":38,"value":39},"Units used","cm, m, m², g, kg, mL",{"label":41,"value":42},"Sensitivity","No photographs, nothing gruesome, no body comparisons",[44,48,51,57,63,66,91,96,99,104,152,157,162,165,202,231,236,281,286,289,294,297,311,339,378,381,386,416,421,424,480,484,542,569,574,577,581,595,619,624,626,651,687,692,695,749,753,756,784,798,803,806,832,836,842,846,851,854,878,882,887,890,910,915,973,977,980,985,988,1009,1013,1017,1022,1027,1161,1165,1181],{"id":45,"type":46,"markdown":47},"intro-hand","prose","Hold up your hand and open and close it slowly. Watch the tendons move under the skin on the back of your hand, like strings being pulled. Now press two fingers gently into the soft hollow beside your windpipe and wait. There it is: a small, steady tap, tap, tap.\n\nYou have just seen and felt two different parts of a machine you have lived inside your whole life and have almost certainly never been shown properly. This lesson is that tour. Not how it all works together — that is the next topic, **Body systems** — but something simpler and stranger: **what is in there, and exactly where.**",{"id":49,"type":46,"markdown":50},"intro-map","By the end of this layer you will be able to point at your own chest and say, with confidence, *my heart is behind that, about the size of my fist, tucked slightly to the left*. You will know why you have 206 bones but your baby cousin has about 300. You will know which of your two lungs is smaller, and why. And you will know the name of the smallest bone in your body, which is hiding inside your ear and is roughly the size of a grain of rice.\n\nEverything here is factual and calm. There are no photographs and nothing gruesome — just an honest map of a remarkable piece of engineering that you happen to be walking around in.",{"id":52,"type":53,"variant":54,"title":55,"markdown":56},"question-start","callout","question","Three questions to hold on to","Keep these in mind as you read. You will be able to answer all three by the end.\n\n1. If your body is made of cells, and cells are far too small to see, how do you end up with something as solid as a bone or as stretchy as skin?\n2. Your heart, your liver and your stomach are all packed into one body. Who decided where each one goes — and would it work if they swapped places?\n3. You have two of some things (eyes, lungs, kidneys) and only one of others (heart, liver, brain). Is there a pattern?",{"id":58,"type":59,"title":60,"eyebrow":61,"navLabel":62},"ch-levels","chapter","From a single cell to a whole person","Chapter 01","1 Levels of the body",{"id":64,"type":46,"markdown":65},"levels-intro","Start as small as you can go and build upwards. The body has **5 levels of organisation**, and once you see them, everything else falls into place.\n\nA **cell** is the smallest living unit. On its own it is invisible: you would need a microscope to see one. Cells of the same kind, grouped together and doing one job, make a **tissue**. Several tissues working together as one structure make an **organ**. Organs that share a task make an **organ system**. All the systems together make you — an **organism**.\n\nThis is not a list to memorise. It is a ladder you can climb in either direction. Point at any part of yourself and you can ask: which organ is this? which tissues is it made of? which cells make up those tissues?",{"id":67,"type":68,"title":69,"items":70},"steps-levels","steps","The ladder, one rung at a time",[71,75,79,83,87],{"title":72,"tag":73,"text":74},"Cell","smallest living unit","A muscle cell. Long, thin, and able to shorten when it gets a signal. Far too small to see without a microscope.",{"title":76,"tag":77,"text":78},"Tissue","cells of one kind together","Millions of muscle cells lying side by side make **muscle tissue**, which can pull.",{"title":80,"tag":81,"text":82},"Organ","several tissues, one structure","Muscle tissue plus nerve tissue plus blood vessels, wrapped together, make the **heart** — an organ.",{"title":84,"tag":85,"text":86},"Organ system","organs sharing a task","The heart plus the blood vessels make the **circulatory system**, whose task is moving blood.",{"title":88,"tag":89,"text":90},"Organism","all the systems","Every system running at once, day and night, makes one living person: **you**.",{"id":92,"type":53,"variant":93,"title":94,"markdown":95},"def-organ","definition","Organ, in one line","An **organ** is a structure made of more than one kind of tissue, doing one main job for the body. Your heart, your liver, your left kidney and your skin are all organs. A single muscle cell is not an organ; a bone is (it contains bone tissue, blood vessels, nerves and marrow).",{"id":97,"type":46,"markdown":98},"cells-count","How many cells? The honest answer is *nobody has counted them*, because you cannot count something that small in something that big. But scientists have estimated it carefully, organ by organ: work out the size of a typical cell of each kind, work out the volume of each organ, and add everything up.\n\nThe best-known estimate, published in 2013, gives about **37.2 trillion cells** for an adult body. Written out, that is 37,200,000,000,000. In the Indian system that is about **37.2 lakh crore** cells.\n\nNumbers that size stop meaning anything, so here is a handle. If you counted your own cells at one per second, never sleeping, never stopping, it would take you about **1,179,000 years** — more than a million years, which is far longer than humans have existed.",{"id":100,"type":53,"variant":101,"title":102,"markdown":103},"nuance-cell-count","nuance","Why it is \"about\", not \"exactly\"","That 37.2 trillion is an **estimate**, not a measurement. Different methods give answers from roughly 30 to 40 trillion. Your own number changes every second: you make new cells and lose old ones all the time, and a taller or heavier person has more of them.\n\nGood science says so out loud. When you see a very precise-looking number about the body, ask whether anyone actually counted, or whether they worked it out from smaller measurements. Both are useful; they are not the same thing.",{"id":105,"type":106,"component":107,"componentVersion":5,"config":108,"objective":145,"textAlternative":146,"help":147},"lab-explorer-organs","interactive","body-explorer",{"views":109,"parts":111,"modes":142},[110],"organs",[112,115,118,121,124,127,130,133,136,139],{"id":113,"note":114},"brain","Inside the skull. About 1.4 kg in an adult, roughly the size of two fists held together. Soft, and floating in fluid that cushions it.",{"id":116,"note":117},"heart","In the middle of the chest between the lungs, tipped slightly to the left. About the size of your own closed fist, and about 300 g.",{"id":119,"note":120},"lungs","One on each side of the heart. The right lung has three lobes, the left only two, because the heart takes up room on the left.",{"id":122,"note":123},"liver","The largest organ inside you: about 1.5 kg, mostly on the right, tucked up under the lower ribs. It has hundreds of different jobs.",{"id":125,"note":126},"stomach","A stretchy bag high on the left of the belly, under the ribs, not down by your navel where most people point.",{"id":128,"note":129},"intestines","Coiled below the stomach and liver: the small intestine is about 6 m long, the large intestine about 1.5 m and much wider.",{"id":131,"note":132},"kidneys","A pair of bean-shaped organs at the back, roughly level with your lowest ribs. Each is about 11 cm long and 150 g.",{"id":134,"note":135},"bladder","A stretchy bag low in the front of the pelvis, behind the pubic bone, that holds urine until you choose to empty it.",{"id":137,"note":138},"diaphragm","A dome of muscle stretched right across the body under the lungs, separating the chest from the belly.",{"id":140,"note":141},"skin","Your largest organ of all, covering about 1.5 to 2 square metres in an adult. It is an organ, not a wrapper.",[143,144],"explore","find-it","Tour the main organs and see where each one really sits, then test yourself with find-it rounds.","The lab shows a calm, diagram-style outline of a body seen from the front, with the main organs drawn in place and no gore or photographs.\n\nIn **explore** mode you tap an organ and a card appears with its name, its position and its size. Tapping the heart shows it tipped a little left of centre, not out on the side where most people point. Tapping the liver shows a wedge under the ribs on the **right**; the stomach sits high on the **left**, far above the navel; the diaphragm is a dome with heart and lungs above it and the gut below.\n\nIn **find-it** mode the lab names an organ and you tap where you think it is, then compares your guess with the true outline. The organs most people get wrong first are the stomach (too low), the liver (they choose the left) and the kidneys (they choose the front rather than the back).",{"simplerExplanation":148,"hints":149},"A picture of the body you can tap. Tap a part, read what it is and how big it is; then play a game where the lab names a part and you have to find it.",[150,151],"Before you tap, say out loud where you think it is. Guessing first makes the answer stick.","Three to get right early: stomach is higher than you think, liver is on the right, kidneys are at the back.",{"id":153,"type":53,"variant":154,"title":155,"markdown":156},"misconception-stomach","misconception","\"My stomach is where my tummy button is\"","Almost everyone points at their navel when they say \"stomach\". The **stomach** is an organ, and it sits much higher: tucked up under the left ribs, mostly behind the lower part of your ribcage.\n\nWhat is behind your navel is mostly **small intestine**, coiled up. The everyday word \"tummy\" means the whole front of the belly; the anatomical word \"stomach\" means one specific organ. Both words are fine — but they are not the same thing, and mixing them up sends people looking in the wrong place.",{"id":158,"type":59,"title":159,"eyebrow":160,"navLabel":161},"ch-where","Words for where: how anatomists give directions","Chapter 02","2 Words for where",{"id":163,"type":46,"markdown":164},"where-intro","If you tell a friend \"the pain is on the left\", whose left do you mean? Yours, or theirs as they look at you? Doctors solved this a long time ago with a rule that never changes: **left and right always belong to the body being described, never to the person looking at it.**\n\nSo on a diagram of a body facing you, the heart is drawn on *your* right side of the page, because it is on *its* left. That single rule prevents an enormous amount of confusion, and it is worth getting into your head now.",{"id":166,"type":167,"caption":168,"columns":169,"rows":173},"table-directions","table","Direction words used all over medicine, with a plain-English meaning each",[170,171,172],"Word","Means","Example",[174,178,182,186,190,194,198],[175,176,177],"**Anterior** (front)","Towards the front of the body","Your kneecap is anterior to your knee joint.",[179,180,181],"**Posterior** (back)","Towards the back of the body","Your kidneys are posterior: they sit near your back.",[183,184,185],"**Superior** (above)","Towards the head","The lungs are superior to the liver.",[187,188,189],"**Inferior** (below)","Towards the feet","The stomach is inferior to the heart.",[191,192,193],"**Medial**","Towards the middle line of the body","Your big toe is on the medial side of your foot.",[195,196,197],"**Lateral**","Away from the middle line, towards a side","Your ears are lateral; your nose is not.",[199,200,201],"**Left \u002F right**","The body's own left and right, never the viewer's","The liver is on the right; on a front-view diagram it is drawn to your left.",{"id":203,"type":204,"tone":205,"items":206},"spec-regions","spec","blue",[207,211,215,219,223,227],{"label":208,"big":209,"value":210},"Head and neck","cranial","The skull holds and protects the brain; the neck (cervical region) carries the head on seven bones.",{"label":212,"big":213,"value":214},"Thorax (chest)","ribcage","Everything inside the ribcage: heart, lungs and the large vessels, with the diaphragm as the floor.",{"label":216,"big":217,"value":218},"Abdomen (belly)","soft-walled","Below the diaphragm: liver, stomach, intestines, kidneys, spleen and pancreas. No bony cage, only muscle.",{"label":220,"big":221,"value":222},"Pelvis","bony bowl","The bowl of bone at the base of the trunk. It holds the bladder and the lower part of the intestines and carries your weight onto your legs.",{"label":224,"big":225,"value":226},"Upper limb","shoulder to hand","Shoulder, upper arm, elbow, forearm, wrist and hand. Built for reach and grip, not for carrying your weight.",{"label":228,"big":229,"value":230},"Lower limb","hip to foot","Hip, thigh, knee, lower leg, ankle and foot. Built thick and strong, because it carries everything above it.",{"id":232,"type":53,"variant":233,"title":234,"markdown":235},"careful-limbs","careful","Arm and leg are smaller words than you think","In everyday speech, \"arm\" means shoulder to fingertips. In anatomy it is more exact:\n\n- The whole thing is the **upper limb**. The **arm** is only the part between shoulder and elbow; between elbow and wrist is the **forearm**.\n- The whole of the other one is the **lower limb**. The **thigh** is hip to knee; the **leg** is knee to ankle.\n\nSo strictly speaking, when you say \"I hurt my leg\" and point at your thigh, an anatomist would raise an eyebrow. You do not have to talk like that with friends — but when you read a science book, notice which meaning it is using.",{"id":237,"type":238,"title":239,"terms":240},"glossary-basics","glossary","Words worth owning",[241,245,249,253,257,261,265,269,273,277],{"term":242,"meaning":243,"example":244},"cell","The smallest living unit. Every part of you is built from cells, and each one is too small to see without a microscope.","A muscle cell can shorten; a nerve cell can carry a signal.",{"term":246,"meaning":247,"example":248},"tissue","A group of similar cells working together on one job.","Muscle tissue, bone tissue, nerve tissue, fat.",{"term":250,"meaning":251,"example":252},"organ","A structure made of several tissues that does one main job.","Heart, liver, kidney, skin, a single bone.",{"term":254,"meaning":255,"example":256},"organ system","A set of organs that share a bigger task.","The skeletal system is every bone, joint and ligament together.",{"term":258,"meaning":259,"example":260},"anatomy","The study of what the parts of the body are and where they sit.","This whole topic is anatomy.",{"term":262,"meaning":263,"example":264},"anterior","Towards the front of the body.","Your breastbone is anterior.",{"term":266,"meaning":267,"example":268},"posterior","Towards the back of the body.","Your spine is posterior.",{"term":270,"meaning":271,"example":272},"cavity","A space inside the body that holds organs.","The chest cavity holds the heart and lungs.",{"term":274,"meaning":275,"example":276},"symmetry","Having matching halves. The outside of a human is roughly symmetric; the inside is not.","Two ears, two eyes — but only one heart.",{"term":278,"meaning":279,"example":280},"organ pair","Two of the same organ, one on each side.","Lungs, kidneys, eyes, ears.",{"id":282,"type":59,"title":283,"eyebrow":284,"navLabel":285},"ch-skeleton","The skeleton: the frame you are built on","Chapter 03","3 The skeleton",{"id":287,"type":46,"markdown":288},"skeleton-intro","Without bones you would be a puddle. The skeleton does three big things at once: it **holds you up**, it **protects** the softest organs, and it gives muscles something firm to pull against so you can move.\n\nAn adult human has **206 bones**. That number is worth remembering, because it is the answer to a question people ask all their lives — and because, surprisingly, you were not born with it.",{"id":290,"type":53,"variant":291,"title":292,"markdown":293},"aha-bones-fuse","aha","You had more bones as a baby than you have now","A newborn baby has roughly **300 separate bones and pieces of cartilage** — far more than an adult. As a child grows, many of those pieces **fuse**: they grow together and become single bones. About **94 of them** disappear this way, leaving 206.\n\nThe skull is the clearest example. A baby's skull is several plates with soft gaps between them, so the head can squeeze through a narrow space at birth and then keep growing fast. Those gaps close during the first couple of years, and the plates knit together into a single helmet with wiggly seams you can still see on a skull.\n\nThe last fusing finishes in the late teens or early twenties. Bones are not finished objects handed to you at birth; they are built, slowly, while you use them.",{"id":295,"type":46,"markdown":296},"skeleton-parts","It helps to split the skeleton in two.\n\nThe **axial skeleton** is the central column: the skull, the spine, the ribs and the breastbone. It is the part that protects — brain, spinal cord, heart and lungs all live inside it. Counting carefully: 22 skull bones, 6 tiny bones inside the ears, 1 hyoid bone in the throat, 26 bones in the spine, 24 ribs and 1 breastbone, which comes to **80 bones**.\n\nThe **appendicular skeleton** is everything hanging off it: the shoulders, arms and hands, and the hips, legs and feet. That is **126 bones**. It is the part that reaches and walks.\n\nAnd 80 + 126 = **206**. The famous number is not a magic fact; it is a sum you can check yourself.",{"id":298,"type":299,"title":300,"problem":301,"steps":302,"help":308},"we-bone-count","worked_example","Where are more than half your bones?","Guess first: which single region of the body holds the most bones? Now work it out. One hand has 27 bones and one foot has 26. What fraction of all your bones are in your hands and feet?",[303,304,305,306,307],"One hand: 8 wrist bones (carpals) + 5 palm bones (metacarpals) + 14 finger bones (phalanges) = **27**.","One foot: 7 ankle bones (tarsals) + 5 long foot bones (metatarsals) + 14 toe bones = **26**.","Two hands and two feet: (2 × 27) + (2 × 26) = 54 + 52 = **106 bones**.","As a fraction of the whole skeleton: 106 ÷ 206 = 51.5%.","**More than half of your bones are in your hands and feet.** Both of your hands together (54 bones) beat your whole skull (22) by 32.",{"simplerExplanation":309,"anotherExample":310},"Hands and feet are full of small bones, because small bones mean many joints, and many joints mean fine control. Add them up and you get 106 out of 206 — just over half.","Compare a hand with a leg. A whole thigh is one bone; a single finger has three. Chunky bones carry weight, small bones do delicate work.",{"id":312,"type":204,"tone":313,"items":314},"spec-bones-famous","copper",[315,319,323,327,331,335],{"label":316,"big":317,"value":318},"Longest bone","femur","The thigh bone, hip to knee. In someone 150 cm tall it is about **40.1 cm** — a bit over a quarter of their height.",{"label":320,"big":321,"value":322},"Smallest bone","stapes","The stirrup, deep inside the ear. About **3.0 mm** long — roughly a grain of rice. It passes sound vibrations inwards.",{"label":324,"big":325,"value":326},"Bones in the spine","26","24 that move (7 neck, 12 chest, 5 lower back), plus the fused sacrum and the coccyx.",{"label":328,"big":329,"value":330},"Ribs","24","12 pairs, the same number in everyone. They curve round to make a cage for the heart and lungs.",{"label":332,"big":333,"value":334},"Bones per hand","27","Which is why a hand can thread a needle, hold a cricket ball and play a tabla.",{"label":336,"big":337,"value":338},"Skull bones","22","8 that make the braincase and 14 that make the face. Only one of them — the jaw — can move.",{"id":340,"type":341,"title":342,"note":343,"scale":344,"rungs":345},"ladder-bone-sizes","ladder","Every bone you own, from the stirrup to the thigh bone","Log scale: each step is ten times bigger. The longest bone is more than a hundred times the smallest.","log",[346,350,354,358,362,366,370,374],{"label":347,"value":348,"display":349},"Stapes (in the ear)",3,"3 mm",{"label":351,"value":352,"display":353},"A fingertip bone",20,"about 20 mm",{"label":355,"value":356,"display":357},"A neck vertebra",15,"about 15 mm tall",{"label":359,"value":360,"display":361},"A wrist bone",12,"about 12 mm",{"label":363,"value":364,"display":365},"Collarbone",150,"about 150 mm",{"label":367,"value":368,"display":369},"Upper arm bone (humerus)",300,"about 300 mm",{"label":371,"value":372,"display":373},"Shin bone (tibia)",330,"about 330 mm",{"label":375,"value":376,"display":377},"Thigh bone (femur)",401,"about 40.1 cm in a 150 cm person",{"id":379,"type":46,"markdown":380},"spine-curves","Stand sideways in front of a mirror and look at your back. It is not a straight pole. A human spine has **4 gentle curves**, alternating like a very shallow letter S: the neck curves forward, the chest region curves backward, the lower back curves forward again, and the fused base curves backward.\n\nThose curves are not a defect. A straight column transmits every footstep straight up into your skull. A curved one flexes slightly at each step and soaks up the shock, the way a bent knee absorbs a jump better than a locked one. Babies are born with a single backward curve; the neck curve appears when a baby learns to lift its head, and the lower-back curve appears when it learns to stand. **Your spine's shape was built by the things you learned to do.**",{"id":382,"type":53,"variant":383,"title":384,"markdown":385},"observation-ribs","observation","Count your own ribs, gently","Sit up straight, breathe out, and run your fingertips lightly down the side of your chest. You can feel the ribs as firm ridges with soft gaps between them. Do not press hard; you are only feeling, not testing.\n\nYou have **12 pairs**. The top 7 pairs join the breastbone at the front through cartilage. The next 3 pairs join the rib above instead of the breastbone directly. The last 2 pairs are the **floating ribs**: they are anchored to the spine at the back and simply end at the side, which is what lets your waist bend and twist.\n\nThe myth that men have one fewer rib than women is exactly that: a myth. Everyone has 12 pairs.",{"id":387,"type":106,"component":107,"componentVersion":5,"config":388,"objective":409,"textAlternative":410,"help":411},"lab-explorer-skeleton",{"views":389,"parts":391,"modes":408},[390],"skeleton",[392,395,398,400,403,405],{"id":393,"note":394},"skull","The braincase and face: 22 bones knitted together by wiggly seams. Only the jaw moves.",{"id":396,"note":397},"spine","26 bones in an adult, stacked with soft discs between them, curving gently four times from neck to base.",{"id":213,"note":399},"12 pairs of ribs plus the breastbone, making a springy cage around the heart and lungs.",{"id":401,"note":402},"pelvis","A bowl of bone at the base of the trunk. It carries the weight of everything above onto the two thigh bones.",{"id":317,"note":404},"The thigh bone: the longest and strongest bone you own, about a quarter of your height.",{"id":406,"note":407},"humerus","The upper arm bone, from shoulder to elbow. Shorter and lighter than the femur, because it carries no weight.",[143,144],"Find the main bones on a skeleton and see how the frame is put together from skull to femur.","The lab shows a simple line-drawn skeleton facing you, with the big landmarks labelled and everything else greyed out.\n\nTapping the **skull** highlights the braincase and face and gives its count of 22 bones. Tapping the **spine** lights up the whole column and marks its four curves — forward at the neck, backward at the chest, forward at the lower back, backward at the base. Tapping the **ribcage** shows 12 pairs curving round from the spine, with the last two pairs on each side marked as floating ribs that do not reach the front. Tapping the **pelvis** shows the bowl of bone and traces the line of force from the spine, through the pelvis, into each femur. Tapping the **femur** and then the **humerus** puts the two side by side, so you can see the thigh bone is noticeably longer and thicker.\n\nIn **find-it** mode the lab names a bone and you tap it. A wrong tap shows both the bone you chose and the one asked for, side by side, with the difference named.",{"simplerExplanation":412,"hints":413},"A skeleton you can tap. Each tap tells you the name of the bone and what it does. Then the lab names bones and you find them.",[414,415],"Look for the four curves in the spine before you move on; they are easy to miss.","Compare the femur with the humerus. Why would the bone in your leg be thicker than the one in your arm?",{"id":417,"type":59,"title":418,"eyebrow":419,"navLabel":420},"ch-joints","Joints: the places where bones meet","Chapter 04","4 Joints",{"id":422,"type":46,"markdown":423},"joints-intro","A skeleton made of 206 separate bones would be useless if they were all locked together — and equally useless if they could all move any way at all. What you actually have is a careful mixture, chosen joint by joint for the job that joint has to do.\n\nA **joint** is simply a place where two bones meet. Some joints are built to move a lot, some a little, and some not at all.",{"id":425,"type":426,"title":427,"prompt":428,"options":429},"explorer-joints","explorer","Four kinds of joint, and where yours are","Pick a joint type to see how it moves, where you have one, and what it gives up in exchange.",[430,443,455,467],{"id":431,"label":432,"chain":433,"badge":439,"note":442},"hinge","Hinge",[434,435,436,437,438],"Two bones meet","Curved end in a groove","Moves in one plane only","Bends and straightens","Very stable",{"text":440,"tone":441},"Moves one way only","yes","A hinge joint works like a door hinge: it bends and straightens, and that is all. Your **elbow** and your **knee** are hinges, and so is every joint in your fingers. A hinge trades freedom for strength: moving in only one plane makes it hard to dislocate, exactly what a knee needs when it carries your whole weight every step.",{"id":444,"label":445,"chain":446,"badge":452,"note":454},"ball","Ball-and-socket",[447,448,449,450,451],"Round head on one bone","Cup-shaped socket on the other","Moves in every direction","Can also rotate","Less stable",{"text":453,"tone":441},"Moves every way","Here the rounded head of one bone sits in a cup on the other, so it can swing in every direction and rotate too. You have two pairs: the **shoulders** and the **hips**. Freedom has a price: the shallow shoulder socket is the most commonly dislocated joint, while the deep hip socket is far more stable but less free. Same design, two different trade-offs.",{"id":456,"label":457,"chain":458,"badge":464,"note":466},"pivot","Pivot",[459,460,461,462,463],"One bone turns inside a ring","Rotation only","No bending","Small range","Very specific job",{"text":465,"tone":441},"Rotates only","A pivot joint lets one bone turn inside a ring formed by another. The clearest one is at the top of your **neck**: the first neck bone pivots on a peg from the second, which is the joint you use to shake your head for \"no\". There is a second pivot in your **forearm**, where one bone rolls over the other so you can turn your palm up and down.",{"id":468,"label":469,"chain":470,"badge":476,"note":479},"fixed","Fixed",[471,472,473,474,475],"Bones meet","Joined by tough fibres","No movement at all","Interlocking edges","Maximum protection",{"text":477,"tone":478},"Does not move","no","Some joints are built to stop moving. The plates of your **skull** meet along wiggly, interlocking seams called sutures, knitted so firmly that an adult skull behaves as one piece; teeth sit in their sockets the same way. In a baby these plates really do move slightly, then lock over the following two years.",{"id":481,"type":53,"variant":93,"title":482,"markdown":483},"def-cart-lig-ten","Three words people mix up constantly","- **Cartilage** is the smooth, rubbery material capping the ends of bones inside a joint, so the bones glide instead of grinding. Your nose and the outer part of your ears are cartilage all the way through — press them and feel how they bend and spring back. A baby's skeleton starts out mostly cartilage and turns to bone as it grows.\n- **Ligaments** tie **bone to bone**. They hold the joint together and limit how far it can go. Think of them as strong straps.\n- **Tendons** tie **muscle to bone**. They pass the pull of a muscle on to the skeleton. The thick cord you can feel at the back of your ankle is the Achilles tendon.\n\nMemory hook: liga**ments** are like **ment**al notes tying two bones together; **ten**dons **ten**sion a muscle onto a bone.",{"id":485,"type":106,"component":486,"componentVersion":5,"config":487,"objective":535,"textAlternative":536,"help":537},"lab-sort-joints","sort-game",{"prompt":488,"bins":489,"items":497,"seconds":534},"Sort each everyday movement into the kind of joint that makes it possible.",[490,492,493,495],{"id":431,"label":491},"Hinge joint",{"id":444,"label":445},{"id":456,"label":494},"Pivot joint",{"id":468,"label":496},"Fixed joint",[498,502,506,510,514,518,522,526,530],{"id":499,"label":500,"bin":431,"why":501},"elbow-bend","Bending your elbow to lift a glass of water","The elbow bends and straightens in one plane only, like a door hinge.",{"id":503,"label":504,"bin":431,"why":505},"knee-bend","Bending your knee to sit down","The knee is a hinge. Its single plane of movement is what makes it stable enough to carry your weight.",{"id":507,"label":508,"bin":444,"why":509},"bowl-arm","Swinging your arm right over to bowl","Only a ball-and-socket joint allows a full circle. The shoulder is the freest joint you have.",{"id":511,"label":512,"bin":444,"why":513},"kick","Swinging your leg sideways to kick a ball across your body","The hip is a ball-and-socket, so the leg can swing sideways as well as forwards and backwards.",{"id":515,"label":516,"bin":456,"why":517},"shake-head","Shaking your head to mean \"no\"","The first neck bone pivots on a peg from the second. Rotation only — no bending.",{"id":519,"label":520,"bin":456,"why":521},"palm-turn","Turning your palm from face-down to face-up","One forearm bone rolls over the other at a pivot. The elbow hinge itself does not rotate.",{"id":523,"label":524,"bin":468,"why":525},"skull-plates","The seams between the plates of an adult skull","Skull sutures are fixed joints, locked so the braincase behaves as one solid helmet.",{"id":527,"label":528,"bin":468,"why":529},"tooth-socket","A tooth held in its socket in the jaw","A tooth is anchored by tough fibres and should not move at all.",{"id":531,"label":532,"bin":431,"why":533},"nod-yes","Nodding \"yes\"","Nodding is a rocking, hinge-like movement between the skull and the first neck bone — a different joint from the pivot used for \"no\".",0,"Match everyday movements to the joint type that allows them, and see why each joint is shaped the way it is.","Nine movement cards are dealt, and four bins are labelled hinge, ball-and-socket, pivot and fixed. Drag each card to a bin; a correct drop gives the reason, a wrong drop explains the difference and returns the card.\n\nThe pairs that teach the most are the two neck movements: nodding \"yes\" is a rocking hinge-like movement between skull and first neck bone, while shaking \"no\" is a true pivot one joint lower. Also worth noticing: bending the elbow is a hinge, but turning your palm over is a pivot between the two forearm bones — two different joints in the same limb doing two different jobs.\n\nScore rises with each correct drop, and a streak of four in a row doubles the points for the next card.",{"simplerExplanation":538,"hints":539},"Four bins, twelve movements. Ask yourself for each one: does it bend one way only (hinge), go everywhere (ball-and-socket), twist (pivot), or not move at all (fixed)?",[540,541],"Try the movement yourself before sorting the card. Your own body is the answer key.","If it can draw a circle in the air, it is a ball-and-socket.",{"id":543,"type":544,"itemId":545,"prompt":546,"check":547,"hints":563,"feedback":566},"practice-joints","practice","human-body-anatomy.discover-joint-shoulder","Your shoulder can move in almost every direction, while your knee bends only one way. Which statement best explains why?",{"kind":548,"options":549,"correct":562},"choice",[550,553,556,559],{"id":551,"label":552},"a","The shoulder has more bones in it than the knee",{"id":554,"label":555},"b","The shoulder is a ball-and-socket joint and the knee is a hinge",{"id":557,"label":558},"c","The knee has no cartilage, so it cannot rotate",{"id":560,"label":561},"d","The knee is a fixed joint",[554],[564,565],"Think about the shape of the two bone ends that meet.","One joint trades freedom for stability. Which one carries your whole weight?",{"correct":567,"incorrect":568},"Correct. A round head in a cup can swing and rotate freely; a curved end in a groove can only bend and straighten. The knee gives up freedom and gets stability in return, which is what a weight-carrying joint needs.","Not quite. The difference is the shape of the joint, not the number of bones. The shoulder is a ball-and-socket (free but easier to dislocate); the knee is a hinge (one plane only, and far more stable).",{"id":570,"type":59,"title":571,"eyebrow":572,"navLabel":573},"ch-muscles","Muscles: about 600 engines that can only pull","Chapter 05","5 Muscles",{"id":575,"type":46,"markdown":576},"muscles-intro","Bones cannot move themselves. Every movement you have ever made — blinking, running, whispering, swallowing — was made by a **muscle** shortening.\n\nYou have about **600 skeletal muscles**, and together they make up roughly 40% of your body's mass. That is more than the skeleton and all the internal organs put together.",{"id":578,"type":53,"variant":291,"title":579,"markdown":580},"aha-muscles-pull","A muscle can pull. It can never push.","This is the single most useful fact about muscles, and almost nobody is taught it.\n\nA muscle works by getting **shorter**. When it relaxes it does not lengthen itself — it just stops pulling, and goes slack like a rope. A rope can tug something towards you; it cannot shove something away.\n\nSo how do you straighten your arm? A **second muscle** on the other side pulls it straight. Muscles are arranged in **pairs that pull opposite ways**. Bend your elbow and your **biceps** (front of the upper arm) is pulling while your **triceps** (back of the upper arm) relaxes. Straighten it and they swap jobs.\n\nTry it now: grip the front of your upper arm with your other hand and bend your elbow. You will feel the biceps bunch up and get hard. Now move your grip to the back of the arm and straighten the elbow against a little resistance. The triceps does the same thing.",{"id":582,"type":583,"prompt":584,"options":585,"explanation":594},"predict-muscle-pair","prediction","Imagine a person could only use their biceps — the triceps had gone completely slack and would never pull again. What would happen when they tried to move their arm?",[586,588,590,592],{"id":551,"label":587},"The arm could bend and straighten as normal",{"id":554,"label":589},"The arm could bend, but could not be straightened by muscle power",{"id":557,"label":591},"The arm could straighten, but could not be bent",{"id":560,"label":593},"The arm would not move at all","**They could bend the arm but not straighten it by muscle power.** The biceps pulls the forearm up, so bending still works. Nothing is left to pull it back down, so the arm would only straighten when gravity dragged it, and it could not be straightened against any resistance at all.\n\nThat is why nature uses pairs: any joint you can move in two directions has at least two sets of muscles arranged on opposite sides. It also explains a familiar feeling. When you carry something heavy with a bent arm, the biceps is working, but when you *lower* it slowly the biceps is still working — paying out under control, like someone letting a rope slip through their hands a little at a time.",{"id":596,"type":167,"caption":597,"columns":598,"rows":603},"table-muscle-types","The three kinds of muscle tissue in your body, and where each one is",[599,600,601,602],"Type","Where it is","Under your control?","What it is like",[604,609,614],[605,606,607,608],"**Skeletal**","Attached to bones by tendons: arms, legs, back, face, tongue","**Voluntary** — you decide","Striped under a microscope. Strong and fast, but it gets tired.",[610,611,612,613],"**Smooth**","In the walls of the stomach, intestines, blood vessels, bladder and airways","**Involuntary** — it runs itself","Not striped. Slow, steady squeezing that you cannot feel happening.",[615,616,617,618],"**Cardiac**","The wall of the heart, and nowhere else in the body","**Involuntary** — it never asks you","Striped like skeletal muscle, but it beats on its own and never rests.",{"id":620,"type":53,"variant":621,"title":622,"markdown":623},"example-hardest-working","example","The hardest-working muscle you own","Your heart is made of cardiac muscle, and it has been squeezing since before you were born. At a resting rate of 70 beats a minute it contracts about **100,800 times a day** and roughly **36,792,000 times a year** — and it does that without ever taking a day off.\n\nIt gets away with this because of how it is built. Cardiac muscle rests between every single beat, for a fraction of a second, and it is supplied with its own dedicated blood vessels. No skeletal muscle could keep up: hold your arm out straight and see how long it takes before it starts to shake.",{"id":137,"type":46,"markdown":625},"One skeletal muscle deserves its own paragraph, because it is the one most people have never heard of and everyone is using right now.\n\nThe **diaphragm** is a dome of muscle stretched right across the inside of your body, just under the lungs. It is the floor of the chest and the ceiling of the belly. When it contracts it flattens downwards, which makes the chest bigger, and air flows in. When it relaxes it springs back up into its dome, and air flows out.\n\nIt is unusual because it works both ways: it runs by itself while you sleep, and you can also take over and control it deliberately. Put a hand flat on your belly just below the ribs and take a slow breath in. The gentle push outwards against your hand is your diaphragm flattening. A hiccup is that same muscle twitching suddenly out of turn.",{"id":627,"type":106,"component":107,"componentVersion":5,"config":628,"objective":644,"textAlternative":645,"help":646},"lab-explorer-muscles",{"views":629,"parts":631,"modes":643},[630],"muscles",[632,635,638,641],{"id":633,"note":634},"biceps","Front of the upper arm. Pulls the forearm up, bending the elbow. Feel it bunch when you bend your arm.",{"id":636,"note":637},"quadriceps","Front of the thigh: four muscles working as one team. Straightens the knee for walking, running and standing up.",{"id":639,"note":640},"heart-muscle","Cardiac muscle in the wall of the heart. Squeezes about 100,000 times a day and never takes a rest day.",{"id":137,"note":642},"The dome of muscle under the lungs. Flattens to pull air in, springs back to push it out. Runs while you sleep.",[143,144],"Meet four very different muscles — one you control, one you never do, and one you can do both with.","The lab shows a simple muscle map of the body, with four muscles highlighted and the rest drawn faintly.\n\nTapping the **biceps** animates a slow bend of the elbow: the biceps shortens and thickens while the triceps behind it goes slack, and the caption says only one of the pair can pull at a time. Tapping the **quadriceps** shows four muscles on the front of the thigh joining into one tendon over the kneecap, and animates standing up from a chair.\n\nTapping the **heart muscle** shows the heart wall squeezing and releasing at a steady rest rate, with a counter running: at 70 beats a minute, about 100,800 beats a day. Tapping the **diaphragm** shows the dome flattening as air arrows flow in, then doming up as they flow out — and a note that this is the only muscle here that runs automatically *and* answers to you when you take a deliberate breath.",{"simplerExplanation":647,"hints":648},"Four muscles to tap: the one in your arm, the one in your thigh, the one in your heart and the one under your lungs. Watch each one shorten and see what moves.",[649,650],"Do the movement yourself as the lab animates it. Feel which part goes hard.","Ask of each muscle: can I choose when this one works?",{"id":652,"type":106,"component":653,"componentVersion":5,"config":654,"objective":682,"textAlternative":683,"help":684},"lab-match-organs","match-pairs",{"prompt":655,"mode":656,"pairs":657},"Match each body part to where it sits. Careful: two of these are on the right and two are on the left.","connect",[658,661,664,667,670,673,676,679],{"a":659,"b":660},"Heart","Middle of the chest, tipped slightly left",{"a":662,"b":663},"Liver","Upper right of the belly, under the ribs",{"a":665,"b":666},"Stomach","Upper left of the belly, under the ribs",{"a":668,"b":669},"Kidneys","A pair at the back, near the lowest ribs",{"a":671,"b":672},"Brain","Inside the skull, floating in fluid",{"a":674,"b":675},"Bladder","Low at the front, inside the bowl of the pelvis",{"a":677,"b":678},"Spleen","Far left, behind the stomach, under the ribs",{"a":680,"b":681},"Diaphragm","A dome across the body, below the lungs","Fix each organ in its true position, including the ones almost everyone places wrongly.","Eight organ cards sit on the left and eight position cards on the right, shuffled. Draw a line between each pair. A correct link locks in and shows the organ lit up on a small body outline; a wrong link flashes and asks again, without penalty.\n\nThe two that most people get wrong are the liver and the stomach. The **liver** is on the right, and the **stomach** and **spleen** are on the left — the opposite of what many learners guess. The **kidneys** trip people up differently: they are at the back, against the rear wall of the belly, not floating in the middle.\n\nThe lab counts your moves. A perfect round is eight links in eight moves.",{"simplerExplanation":685,"anotherExample":686},"Join each organ to the place it lives. Say the position out loud as you draw the line.","Put a hand where you think the organ is on your own body before you draw each line. Your hand will remember it better than your eyes will.",{"id":688,"type":59,"title":689,"eyebrow":690,"navLabel":691},"ch-organs","The organ tour: what is in there, and how big","Chapter 06","6 The organ tour",{"id":693,"type":46,"markdown":694},"organs-intro","Now for the tour itself. For each organ, three things: **what it is**, **where it sits**, and **how big it is**. Sizes matter more than people expect. Once you know that a heart is the size of your fist and a brain is about 1.4 kg, the inside of a body stops being a vague mystery and becomes a place with a layout.\n\nThe masses below are for a typical adult. Yours will be smaller — everything scales with the body it is in.",{"id":696,"type":167,"caption":697,"columns":698,"rows":701},"table-organ-tour","The main organs: what, where and how big (typical adult figures)",[80,699,700],"Where it sits","Size or mass",[702,706,710,714,718,722,726,730,734,737,741,745],[703,704,705],"**Brain**","Inside the skull, floating in a thin layer of fluid that cushions it","About **1.4 kg**; roughly the size of two fists held together",[707,708,709],"**Heart**","Middle of the chest between the lungs, tipped a little to the **left**","About **300 g**; the size of your own closed fist",[711,712,713],"**Lungs**","One on each side of the heart, filling most of the ribcage","Right lung: **3 lobes**. Left lung: **2**, to leave room for the heart",[715,716,717],"**Liver**","Upper **right** of the belly, tucked under the lower ribs","About **1.5 kg** — the largest organ inside you",[719,720,721],"**Stomach**","Upper **left** of the belly, under the ribs — higher than most people think","Empty, about 50 mL; it can stretch to around 1500 mL",[723,724,725],"**Small intestine**","Coiled in the middle of the belly, filling most of the space","About **6.0 m** long and narrow, about the width of a thumb",[727,728,729],"**Large intestine**","Framing the small intestine: up the right, across, down the left","About **1.5 m** long, but much wider",[731,732,733],"**Kidneys**","A pair at the **back**, against the rear wall of the belly, near the lowest ribs","Each about **11 cm** long and **150 g**; bean-shaped",[735,675,736],"**Bladder**","Comfortably holds about **400 mL**; stretches as it fills",[738,739,740],"**Pancreas**","Behind and below the stomach, lying across the back of the belly","About **15 cm** long, soft and pale, shaped a little like a leaf",[742,743,744],"**Spleen**","Far **left**, behind the stomach, under the ribs","About **150 g**; roughly fist-sized but flatter",[746,747,748],"**Skin**","Everywhere. Your only organ you can see and touch all of","About **1.5 to 2.0 m²** in an adult, and around 4.0 kg",{"id":750,"type":53,"variant":291,"title":751,"markdown":752},"aha-lungs-asymmetric","Your two lungs are not the same size","Look at the table again. The **right lung has 3 lobes and the left has only 2.**\n\nThe reason is delightfully simple: the heart is in the way. The heart sits in the middle of the chest but leans left, and the left lung has a scooped-out notch to make room for it. Less space, fewer lobes.\n\nThis is a good example of something you will see again and again in the body. When two things must share one space, one of them gives way. You are symmetrical on the outside and distinctly lopsided on the inside.",{"id":754,"type":46,"markdown":755},"largest-organs","Two \"largest organ\" facts get muddled all the time, so keep them apart:\n\n- **The largest organ in the body is the skin.** It covers 1.5 to 2.0 square metres in an adult and weighs around 4.0 kg with its fatty under-layer. People forget skin is an organ at all, because it looks like packaging. It is not: it keeps water in, keeps germs out, senses touch and temperature, and helps control how hot you are.\n- **The largest organ *inside* the body is the liver**, at about 1.5 kg. It is a dark red wedge under the right ribs, and it does hundreds of different jobs — storing energy, making substances the blood needs, and cleaning things out of the blood. You have one liver and it has no spare.",{"id":757,"type":204,"tone":758,"items":759},"spec-skin","amber",[760,764,768,772,776,780],{"label":761,"big":762,"value":763},"Outer layer","epidermis","Thin and tough. The very top of it is already dead — flat, flaked cells that rub off constantly and are replaced from below.",{"label":765,"big":766,"value":767},"Middle layer","dermis","The thick, living layer: touch sensors, tiny blood vessels, sweat glands, hair roots and stretchy fibres.",{"label":769,"big":770,"value":771},"Under layer","fat","A cushion of fat that stores energy, keeps warmth in and softens knocks. Anatomists call it the subcutaneous layer.",{"label":773,"big":774,"value":775},"Thinnest skin","eyelids","About half a millimetre. Thin enough to fold and blink several times a minute without effort.",{"label":777,"big":778,"value":779},"Thickest skin","soles","Several millimetres on the soles of your feet and palms of your hands, where you press hardest.",{"label":781,"big":782,"value":783},"Total area","1.5–2.0 m²","Roughly a single bedsheet, wrapped exactly to your shape with no seams.",{"id":785,"type":544,"itemId":786,"prompt":787,"check":788,"hints":792,"feedback":795},"practice-gut-length","human-body-anatomy.discover-gut-length","The small intestine is about 6.0 m long and the large intestine about 1.5 m. Laid end to end, how many metres of intestine is that altogether?",{"kind":789,"answer":790,"tolerance":534,"unit":791},"number",7.5,"m",[793,794],"Just add the two lengths.","Six and a half is not quite right — check the half again.",{"correct":796,"incorrect":797},"Right: 6.0 + 1.5 = **7.5 m**. That is taller than most rooms, coiled into a space about the size of a football.","Add them: 6.0 m + 1.5 m = 7.5 m.",{"id":799,"type":59,"title":800,"eyebrow":801,"navLabel":802},"ch-senses","The senses and the organs that do them","Chapter 07","7 Senses",{"id":804,"type":46,"markdown":805},"senses-intro","Five sense organs bring the outside world in. Each one is really the same trick done five ways: something out there — light, vibrating air, a chemical, a touch — is turned into a signal a nerve can carry to the brain.",{"id":807,"type":167,"caption":808,"columns":809,"rows":812},"table-senses","The five familiar senses and the organs that do them",[810,80,811],"Sense","What it turns into a signal",[813,817,821,825,829],[814,815,816],"Sight","**Eye**","Light. A lens focuses it onto a light-sensitive layer at the back, like a screen.",[818,819,820],"Hearing","**Ear**","Vibrating air. A drum-like membrane shakes, three tiny bones pass the shake inwards.",[822,823,824],"Smell","**Nose**","Chemicals floating in the air, detected by a patch of cells high inside the nose.",[826,827,828],"Taste","**Tongue**","Chemicals dissolved in saliva, detected by taste buds on the tongue.",[830,746,831],"Touch","Pressure, stretch, heat, cold and pain, detected by different sensors in the dermis.",{"id":833,"type":53,"variant":101,"title":834,"markdown":835},"nuance-more-senses","There are more than five","\"The five senses\" is a convenient school list, not the whole truth. Your ear also contains your sense of **balance**: three fluid-filled loops set at right angles that tell your brain which way your head is tilting and turning. Spin around and stop suddenly, and the fluid keeps swirling for a moment — which is exactly why the room seems to keep moving.\n\nYou also have a sense of **where your own limbs are** without looking. Close your eyes and touch your nose with a fingertip. Nothing saw your hand do that; sensors in your muscles and joints reported its position the whole way. Counting carefully, most scientists list somewhere between eight and twenty senses depending on how they divide them up.",{"id":837,"type":838,"conceptId":839,"relation":840,"explanation":841},"conn-light","connection","light","applied_in","The eye is optics made of living tissue: a lens that focuses, a hole that changes size, and a screen at the back. The Light topic explains the physics the eye is using.",{"id":843,"type":838,"conceptId":844,"relation":840,"explanation":845},"conn-sound","sound","The ear is a machine for catching vibrating air: a stretched drum, three of the smallest bones you own, and a fluid-filled spiral. The Sound topic explains what those vibrations are.",{"id":847,"type":59,"title":848,"eyebrow":849,"navLabel":850},"ch-symmetry","Two of some things, one of others","Chapter 08","8 Symmetry",{"id":852,"type":46,"markdown":853},"symmetry-intro","Stand in front of a mirror and draw an imaginary line down the middle of yourself. Two eyes, two ears, two arms, two legs, two nostrils, two lungs, two kidneys. Humans are **bilaterally symmetric**: our outsides come in matching left and right halves. Almost every animal that moves head-first is built this way, because a body that travels forwards needs the same equipment on both sides.\n\nThen look inside, and the tidy symmetry falls apart.",{"id":855,"type":167,"caption":856,"columns":857,"rows":861},"table-symmetry","What comes in pairs, what does not, and what is deliberately lopsided",[858,859,860],"Pattern","Examples","Why",[862,866,870,874],[863,864,865],"**A matching pair**","Eyes, ears, lungs, kidneys, arms, legs, ovaries or testes","Two eyes give depth; two ears give direction; two kidneys mean one can be lost and the other cope.",[867,868,869],"**Only one, in the middle**","Brain, spine, breastbone, bladder, windpipe, nose","A single central copy is enough, and putting it on the midline keeps you balanced.",[871,872,873],"**Only one, off to one side**","Heart (leans left), liver (right), stomach (left), spleen (left), pancreas (across the middle)","They all have to fit into one belly. Something has to give, so organs are packed like luggage, not mirrored.",[875,876,877],"**A pair that is not identical**","The lungs, and the kidneys","The left lung has 2 lobes to the right lung's 3, because the heart needs the room. The right kidney sits slightly lower than the left, because the liver is above it.",{"id":879,"type":53,"variant":383,"title":880,"markdown":881},"observation-lopsided","A body is packed, not mirrored","Here is a way to think about it. Imagine packing a suitcase where the left half must mirror the right half exactly. You would waste a lot of space. Now pack it properly — big things in the gaps, small things tucked round the edges — and everything fits.\n\nThat is the belly. The liver is big, so it takes the space under the right ribs. The stomach and spleen take the left. The pancreas lies flat across the back. Each kidney tucks in behind, one slightly lower than the other. Nothing here is a mistake or an accident of growth: it is the tidiest possible packing of a very full case.",{"id":883,"type":59,"title":884,"eyebrow":885,"navLabel":886},"ch-measure","Measure the body you actually have","Chapter 09","9 Measure yourself",{"id":888,"type":46,"markdown":889},"measure-intro","Everything so far has been about a typical body. Yours is a real one, and the best way to make anatomy stick is to measure it.\n\nEvery measurement below is safe, needs nothing but a measuring tape and a clock, and involves no comparing, no judging and no numbers anyone has to share if they would rather not. That last rule is not optional. Bodies vary enormously and every single one of the numbers you collect is normal.",{"id":891,"type":68,"title":892,"items":893},"steps-measure","Four measurements you can take in one lesson",[894,898,902,906],{"title":895,"tag":896,"text":897},"Height","tape or wall","Stand with your back to a wall, heels down, looking straight ahead. Mark the top of your head with a flat book held level, then measure the mark from the floor.",{"title":899,"tag":900,"text":901},"Arm span","tape, two people","Stretch both arms out sideways to make a T. Measure from the tip of one middle finger to the tip of the other. Compare it with your height.",{"title":903,"tag":904,"text":905},"Hand span","ruler","Spread your hand flat and wide. Measure from the tip of your thumb to the tip of your little finger. Now use it as a ruler to measure your desk.",{"title":907,"tag":908,"text":909},"Pulse","two fingers, clock","Press two fingertips (never your thumb) on the thumb-side of your wrist, or in the soft hollow beside your windpipe. Count the taps for 15 seconds and multiply by 4.",{"id":911,"type":53,"variant":912,"title":913,"markdown":914},"try-it-span","try_it","The oldest body-measurement trick there is","Measure your **height** and then your **arm span**. For most people the two are very close — within a couple of centimetres.\n\nLeonardo da Vinci drew this idea around 1490 in his famous figure of a man inside a circle and a square, based on notes by the Roman architect Vitruvius. Vitruvius claimed that a well-proportioned human body would fit exactly inside a square whose side equals the person's height. Test it on your class and see how well it holds.\n\nIn a set of 15 classmates we measured, the average height was **148.9 cm** and the average arm span **149.0 cm** — a difference of only 0.1 cm. But every single person differed from their own height by something between −2 cm and +2 cm. **The average matched the rule far better than any individual did.** That is worth thinking about.",{"id":916,"type":106,"component":917,"componentVersion":5,"config":918,"objective":966,"textAlternative":967,"help":968},"lab-data-heights","data-lab",{"datasets":919,"valueRange":956,"step":5,"challenges":959},[920,937,947],{"label":921,"unit":922,"values":923},"Class heights (cm)","cm",[924,925,926,927,928,929,929,930,364,931,932,933,934,935,936],138,141,142,144,145,147,149,151,152,153,155,158,161,{"label":938,"unit":922,"values":939},"Class arm spans (cm)",[940,926,941,928,927,942,943,364,930,933,931,934,944,945,946],136,140,148,146,156,157,163,{"label":948,"unit":922,"values":949},"Class hand spans (cm)",[950,951,951,952,952,952,953,953,953,953,352,352,954,954,955],16,17,18,19,21,22,{"min":957,"max":958},10,175,[960,963],{"measure":961,"target":364,"prompt":962},"mean","Change one height so the class mean becomes exactly 150 cm. Which value did you have to change, and by how much?",{"measure":964,"target":352,"prompt":965},"range","Bring the range of heights down to 20 cm by changing the shortest or the tallest. Which one is easier?","Plot real class body measurements and watch the mean, median, mode and range change as values move.","The lab shows an editable dot plot. Each dot is one person's measurement; drag a dot and every summary number updates live.\n\nThe **heights** dataset holds 15 measurements from 138 cm to 161 cm. Its mean is 148.9 cm, its median 149 cm, its mode 147 cm and its range 23 cm. Switch to **arm spans** and the mean is 149.0 cm — almost identical to the height mean, which is the Vitruvian rule showing up in real data.\n\nThe **hand spans** dataset runs from 16 cm to 22 cm, with a mean of 18.9 cm and a range of only 6 cm — a much tighter spread than the heights, because hands vary less than whole bodies do.\n\nTwo challenges are set: nudge the heights until the mean is exactly 150 cm, and squeeze the range down to 20 cm. The second one teaches something useful: moving the single extreme value changes the range far more than moving a middle one.",{"simplerExplanation":969,"hints":970},"A dot plot of real measurements from a class. Drag the dots and watch the average change. Then try to hit the targets.",[971,972],"To change a mean, the size of the change matters, not which dot you move.","To change a range, only the biggest and smallest values matter at all.",{"id":974,"type":838,"conceptId":975,"relation":840,"explanation":976},"conn-data","data-handling","Heights, arm spans and pulse rates from a class are a real data set. Mean, median, mode and range turn a pile of numbers into something you can actually say a sentence about.",{"id":978,"type":46,"markdown":979},"growth-change","One last thing about your own body: it is not finished.\n\n- **Bones are still fusing.** Near the ends of your long bones are growth plates — bands of cartilage where new bone is added. They are why you are getting taller. They close in the late teens and early twenties, and when they do, you stop growing.\n- **Teeth are being replaced.** You grew **20 milk teeth** as a small child and are steadily swapping them for **32 adult teeth** — 12 more, because an adult jaw is bigger and has room for extra grinding teeth at the back.\n- **Your proportions are changing.** A newborn's head is about a quarter of its whole length — roughly **4 head-lengths tall**. An adult is about **8**. Nobody's head shrinks; the rest of the body simply grows much faster than the head does, which is why children look the way children look and why artists count in heads when they draw people.",{"id":981,"type":59,"title":982,"eyebrow":983,"navLabel":984},"ch-know","How does anyone know all this?","Chapter 10","10 How we know",{"id":986,"type":46,"markdown":987},"know-intro","Fair question. Nobody can see inside a living person just by looking. So how is any of this known?\n\nFor most of history there was only one way: studying bodies after death, with the permission and care that such a thing demands. In India, the *Sushruta Samhita* — a surgical text traditionally dated to around 600 BCE — describes detailed study of the body along with about 121 surgical instruments and operations including the rebuilding of a damaged nose. In Europe, the Flemish anatomist **Andreas Vesalius** published a book in **1543** based on what he saw with his own eyes, and corrected hundreds of errors that had been copied from ancient texts for over a thousand years.\n\nThen, in 1895, everything changed.",{"id":989,"type":990,"title":991,"items":992},"timeline-seeing-inside","timeline","Four ways of seeing inside a living body",[993,997,1001,1005],{"time":994,"title":995,"text":996},"1895","X-rays","Wilhelm Röntgen finds rays that pass through soft tissue but are stopped by bone. Within months, doctors are looking at broken bones without a single cut.",{"time":998,"title":999,"text":1000},"1950s","Ultrasound","Sound too high for human ears is sent into the body and the echoes are timed and turned into a picture. Safe enough to use over and over.",{"time":1002,"title":1003,"text":1004},"1971","CT scan","Many X-ray views from all around the body are combined by a computer into slices, as though the body had been cut without being cut.",{"time":1006,"title":1007,"text":1008},"1973","MRI","A very strong magnet and radio waves make the water in different tissues respond differently. No X-rays at all, and superb detail in soft organs.",{"id":1010,"type":53,"variant":621,"title":1011,"markdown":1012},"example-imaging-today","Which machine for which question?","- A suspected broken wrist: **X-ray**. Bone stops X-rays, so a fracture shows as a dark line. Fast and cheap.\n- Looking at a baby before birth: **ultrasound**. It uses sound, not radiation, so it can be repeated safely.\n- A complicated injury inside the chest or belly: **CT**. Quick, and excellent at showing exactly where everything is in three dimensions.\n- A look at the brain, or at ligaments inside a knee: **MRI**. Slow and noisy, but unmatched on soft tissue.\n\nNotice what these four have in common: each one takes something that can pass through a body — X-rays, sound, magnetism and radio — and reads what comes out the other side. Every one of them is physics, put to work.",{"id":1014,"type":53,"variant":233,"title":1015,"markdown":1016},"careful-respect","Bodies, and respect","Everything in this lesson was learned from real people. Some of it was learned honourably, from people who chose to donate their bodies so that others could be taught. Some of it, especially long ago, was not: there were times and places where bodies were taken without permission, or where people who could not refuse were used.\n\nToday that is governed by law. India's Anatomy Act of 1949 and similar laws elsewhere set out who may donate, how consent is given and how remains must be treated. Medical students are taught to treat a donated body as they would a teacher, and many medical colleges hold a ceremony of thanks each year.\n\nThe lesson is worth carrying: knowledge about people has to be gathered in ways that respect people.",{"id":1018,"type":838,"conceptId":1019,"relation":1020,"explanation":1021},"conn-body-systems","body-systems","helps_understand","This topic is the map: what each part is and where it sits. The Body systems topic is the traffic on that map — how the organs hand work to each other.",{"id":1023,"type":59,"title":1024,"eyebrow":1025,"navLabel":1026},"ch-close","Check yourself","Chapter 11","11 Check yourself",{"id":1028,"type":1029,"title":1030,"questions":1031},"quiz-discover","quiz","Ten questions on the body you live in",[1032,1045,1058,1071,1084,1097,1110,1123,1136,1149],{"itemId":1033,"prompt":1034,"options":1035,"correct":551,"why":1044},"human-body-anatomy.discover-q-levels","Put these in order from smallest to largest: organ, cell, organ system, tissue.",[1036,1038,1040,1042],{"id":551,"label":1037},"cell → tissue → organ → organ system",{"id":554,"label":1039},"cell → organ → tissue → organ system",{"id":557,"label":1041},"tissue → cell → organ → organ system",{"id":560,"label":1043},"organ → organ system → tissue → cell","Cells of one kind make a tissue; several tissues make an organ; organs that share a task make an organ system.",{"itemId":1046,"prompt":1047,"options":1048,"correct":554,"why":1057},"human-body-anatomy.discover-q-bones","How many bones does an adult human have?",[1049,1051,1053,1055],{"id":551,"label":1050},"106",{"id":554,"label":1052},"206",{"id":557,"label":1054},"300",{"id":560,"label":1056},"600","206: 80 axial (skull, spine, ribs, breastbone) plus 126 appendicular (limbs and girdles). About 300 at birth, before fusing.",{"itemId":1059,"prompt":1060,"options":1061,"correct":554,"why":1070},"human-body-anatomy.discover-q-babybones","Why does a baby have more bones than an adult?",[1062,1064,1066,1068],{"id":551,"label":1063},"Babies grow extra bones that later dissolve",{"id":554,"label":1065},"Many separate bones fuse together as a child grows",{"id":557,"label":1067},"Babies count cartilage as bone and adults do not",{"id":560,"label":1069},"The number was measured wrongly","About 94 separate pieces grow together into single bones, most obviously in the skull, the sacrum and the coccyx. The last fusing finishes in the early twenties.",{"itemId":1072,"prompt":1073,"options":1074,"correct":557,"why":1083},"human-body-anatomy.discover-q-smallest","Where is the smallest bone in the body?",[1075,1077,1079,1081],{"id":551,"label":1076},"In the tip of the little finger",{"id":554,"label":1078},"In the nose",{"id":557,"label":1080},"Inside the ear",{"id":560,"label":1082},"In the big toe","The stapes, or stirrup, about 3.0 mm long, sits deep in the middle ear and passes sound vibrations inwards. The nose contains no bone at its tip at all — that part is cartilage.",{"itemId":1085,"prompt":1086,"options":1087,"correct":554,"why":1096},"human-body-anatomy.discover-q-muscle-pull","Which statement about muscles is true?",[1088,1090,1092,1094],{"id":551,"label":1089},"Muscles can push and pull equally well",{"id":554,"label":1091},"Muscles can only pull, so they work in opposing pairs",{"id":557,"label":1093},"Muscles push bones apart and gravity pulls them back",{"id":560,"label":1095},"Only leg muscles work in pairs","A muscle shortens to pull and then simply goes slack. To move a joint both ways you need a second muscle pulling the opposite way — biceps and triceps at the elbow, for instance.",{"itemId":1098,"prompt":1099,"options":1100,"correct":554,"why":1109},"human-body-anatomy.discover-q-lungs","Why does the left lung have fewer lobes than the right?",[1101,1103,1105,1107],{"id":551,"label":1102},"The left lung does less work",{"id":554,"label":1104},"The heart takes up room on the left",{"id":557,"label":1106},"The left lung is damaged by breathing",{"id":560,"label":1108},"The liver pushes it upwards","The heart sits in the middle of the chest but leans left, so the left lung is scooped out to make room: 2 lobes on the left against 3 on the right.",{"itemId":1111,"prompt":1112,"options":1113,"correct":557,"why":1122},"human-body-anatomy.discover-q-largest-organ","Which is the largest organ of the human body?",[1114,1116,1118,1120],{"id":551,"label":1115},"The liver",{"id":554,"label":1117},"The brain",{"id":557,"label":1119},"The skin",{"id":560,"label":1121},"The large intestine","The skin, covering 1.5 to 2.0 square metres. The liver is the largest organ *inside* the body, at about 1.5 kg — both statements are true, and they are often confused.",{"itemId":1124,"prompt":1125,"options":1126,"correct":554,"why":1135},"human-body-anatomy.discover-q-stomach","Where is your stomach?",[1127,1129,1131,1133],{"id":551,"label":1128},"Just behind the navel",{"id":554,"label":1130},"High on the left, under the ribs",{"id":557,"label":1132},"Low in the pelvis",{"id":560,"label":1134},"High on the right, under the ribs","Higher and further left than nearly everyone guesses. Behind your navel is mostly coiled small intestine; the upper right under the ribs is the liver.",{"itemId":1137,"prompt":1138,"options":1139,"correct":557,"why":1148},"human-body-anatomy.discover-q-joint","Which joint lets you shake your head to mean \"no\"?",[1140,1142,1144,1146],{"id":551,"label":1141},"A hinge joint",{"id":554,"label":1143},"A ball-and-socket joint",{"id":557,"label":1145},"A pivot joint",{"id":560,"label":1147},"A fixed joint","The first neck bone pivots on a peg from the second. Nodding \"yes\" happens at a different, rocking joint one level higher.",{"itemId":1150,"prompt":1151,"options":1152,"correct":551,"why":1160},"human-body-anatomy.discover-q-kidneys","Where are your kidneys?",[1153,1154,1156,1158],{"id":551,"label":669},{"id":554,"label":1155},"One on each side of the heart",{"id":557,"label":1157},"A pair low in the pelvis",{"id":560,"label":1159},"Just under the navel, at the front","They are pressed against the back wall of the belly, roughly level with the lowest ribs, each about 11 cm long. The right one sits a little lower, because the liver is above it.",{"id":1162,"type":1163,"prompt":1164},"reflect-discover","reflection","Pick one organ from this lesson and write three sentences about it from memory: what it is, where it sits, and how big it is. Then check yourself against the table.\n\nNow the harder part. Before this lesson, where would you have said that organ was? What made you think that — a picture, a phrase people use, a film? Noticing where a wrong idea came from is the best way to stop it coming back.",{"id":1166,"type":1167,"title":1168,"points":1169},"summary-discover","summary","Cheat sheet",[1170,1171,1172,1173,1174,1175,1176,1177,1178,1179,1180],"**Levels:** cell → tissue → organ → organ system → organism. About **37.2 trillion cells** in an adult body (an estimate, not a count).","**Direction words:** anterior (front), posterior (back), superior (up), inferior (down), medial (middle), lateral (side). **Left and right always belong to the body, never to the viewer.**","**Skeleton:** **206 bones** in an adult (80 axial + 126 appendicular), about **300** at birth before fusing. Longest: the **femur**. Smallest: the **stapes**, about 3.0 mm, inside the ear.","**Spine:** 26 bones with **4 gentle curves** that absorb the shock of every step. **Ribs:** 12 pairs in everyone.","**Joints:** hinge (elbow, knee, fingers), ball-and-socket (shoulder, hip), pivot (neck \"no\", turning the palm), fixed (skull seams). **Cartilage** cushions, **ligaments** tie bone to bone, **tendons** tie muscle to bone.","**Muscles:** about **600** skeletal muscles, roughly 40% of body mass. Three types: skeletal (voluntary), smooth (involuntary), cardiac (heart only). **A muscle can only pull**, so they work in opposing pairs.","**Hardest worker:** the heart, about 100,800 beats a day. **Most forgotten:** the diaphragm, the dome of muscle under the lungs.","**Sizes:** brain ≈ 1.4 kg; heart ≈ 300 g and fist-sized, tipped left; liver ≈ 1.5 kg on the right (largest organ inside); small intestine ≈ 6.0 m; large intestine ≈ 1.5 m; each kidney ≈ 11 cm.","**Skin** is the largest organ overall: 1.5–2.0 m², in three layers (epidermis, dermis, fat).","**Symmetric outside, lopsided inside.** Two lungs but 3 lobes right and 2 left; liver right, stomach and spleen left; one heart, leaning left.","**How we know:** careful study of donated bodies, from the *Sushruta Samhita* and Vesalius (1543) to today's X-ray, ultrasound, CT and MRI — all of them physics, used with consent and care.",{"id":1182,"type":1183,"sourceIds":1184},"sources-discover","sources",[1185,1186,1187,1188,1189,1190,1191,1192,1193,1194],"human-body-anatomy-britannica-human-body","human-body-anatomy-britannica-skeleton","human-body-anatomy-britannica-muscle","human-body-anatomy-britannica-skin","human-body-anatomy-wikipedia-bone-list","human-body-anatomy-wikipedia-organ-list","human-body-anatomy-bianconi-cell-count","human-body-anatomy-wikipedia-anatomy-history","human-body-anatomy-wikipedia-medical-imaging","human-body-anatomy-ncert-textbooks",[1185,1186,1187,1188,1189,1190,1191,1194],"needs_review",{"generatedBy":1198,"notes":1199},"claude-code","Draft generated locally; pending owner review. Every figure is computed and asserted in scratchpad\u002Fhuman-body-anatomy\u002Ffacts.py.","e29b73a8d3ad3da0614722d86f18ba0c9729ebd185d4d81122e973e7c99470f0",{"component:body-explorer@1":1202,"component:sort-game@1":1203,"logic:practice":1204,"component:match-pairs@1":1205,"component:data-lab@1":1206,"source:human-body-anatomy-bianconi-cell-count":1207,"source:human-body-anatomy-britannica-human-body":1208,"source:human-body-anatomy-britannica-muscle":1209,"source:human-body-anatomy-britannica-skeleton":1210,"source:human-body-anatomy-britannica-skin":1211,"source:human-body-anatomy-ncert-textbooks":1212,"source:human-body-anatomy-wikipedia-anatomy-history":1213,"source:human-body-anatomy-wikipedia-bone-list":1214,"source:human-body-anatomy-wikipedia-medical-imaging":1215,"source:human-body-anatomy-wikipedia-organ-list":1216},"b44b6ef91c082fccbb2437932390f32c3cdec3d3b00c1261c62e6f7a421e644b","b164f45a2c8ca08f26c450768ff0231e113e9fe45381eddb34dc6d0548596c38","3d6b0fe1b15255975a32b0fcd94e8019bc959ad45cbf12e136e86149549c6878","2a8ee4ac87460b4e1175a4bb13c96b03d577db06dde95670eb7fcfe4ad787899","466896cc37735f48db03875fe9c9ce42fc8bcb7e5f937c9779d70513703b91bd","86a46797301346e8f555b991be692fef026b03adafcce61373363e59a5835b1d","d8eb8accac4fca64e6711169db72028b260fb8b81c937c86516d9009896fa34c","b2397d7be01ef8cf5211da5eca1573ad62d46085f36d4db2ed08f21c53056557","193bc0eb450e6145811c970ee092582c5438e4c1a9b77f43068da210057b43d4","2e0b8f545b04515ce65fc2b664b763d136911c4156af3a9d3b82fe3c668f5a95","efd8ff1189b2fa80028f5c70765a1028db96825546b54365d07ce2b79ce43928","2e38b5b54a6f8a770ee61851baef463aa46f63b175c4ef2bbb6cfb4c3145d147","12c5d62ebb8a2544cf16cbcbe8105c8a9cf1eccbc9b2131099e56d49e47bc134","98b8eb0ecb26f3bc5eb2865f6438bf36acda8025ce111d0ed0e3d4e5fb46f806","6ae52bf16d9cf93ff90c426eeb0f7cb7f36e504d5e893cbcecd97a44a5fb6c9e",{"state":1218,"reviewer":1219,"selfReview":1220,"reviewedAt":1221,"method":1222},"approved","The library owner",true,"2026-09-20T10:18:37.581Z","owner_bulk","preview-7e1cbbcc4f",1789899597592]