[{"data":1,"prerenderedAt":1119},["ShallowReactive",2],{"layer:human-body-anatomy:understand":3},{"layer":4,"contentHash":1098,"dependencyHashes":1099,"approval":1112,"releaseId":1118},{"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":1093,"reviewStatus":1094,"authoring":1095},1,"human-body-anatomy","en","understand","How the body is put together","Tissues, bone, joints, muscle and the cavities that hold the organs","Go one level below the organs to the four tissue types they are built from, learn the direction words and the standard pose they are measured from, see why bone is a living composite, count the skeleton to 206, and place every major organ in its cavity with its mass.",[13,14,15,16,17],"Name the four tissue types, say what each does, and explain why bone, blood, cartilage and fat are all connective tissue.","Use the anatomical position and the full set of direction words to describe any point on the body without ambiguity.","Explain what bone is made of, why it is hollow, what marrow does, and why a baby has more bones than an adult.","Count the axial and appendicular skeletons, and describe the six joint types with the tissues that hold a synovial joint together.","Distinguish the three muscle types and explain how an antagonistic pair produces a controlled movement.",42,{"title":20,"rows":21},"L",[22,25,28,31,34,37,40],{"label":23,"value":24},"Depth","Understand",{"label":26,"value":27},"Reading time","About 42 minutes",{"label":29,"value":30},"Prior knowledge","The Discover layer of this topic",{"label":32,"value":33},"Chapters","12",{"label":35,"value":36},"Labs","2 sort games, 1 match game, 1 body explorer",{"label":38,"value":39},"Units used","mm, cm, m, m², g, kg, mL, %",{"label":41,"value":42},"Sensitivity","No photographs, nothing gruesome, no body comparisons",[44,48,51,57,60,66,87,152,157,160,189,194,197,230,235,238,267,282,287,355,360,363,366,371,399,403,421,424,429,464,467,472,475,510,534,539,542,557,562,594,598,601,628,633,638,675,678,736,741,744,747,773,778,791,797,802,805,878,882,896,901,904,908,911,916,920,925,1059,1063,1081],{"id":45,"type":46,"markdown":47},"u-intro","prose","In *Discover* you took the tour: what is in there and roughly where. This layer is the same body seen with sharper eyes. We will go down a level — to the **tissues** organs are built from — and up a level — to the **cavities** that hold the organs in place — and we will be precise about the words.\n\nPrecision matters here for a practical reason. \"Somewhere in the belly\" is not good enough for a doctor, a physiotherapist, a sports coach or a vet. Anatomy has a vocabulary that lets you name any point on any body without ambiguity, and it is not hard. It is about thirty words, and by the end of this layer you will own most of them.",{"id":49,"type":46,"markdown":50},"u-roadmap","The plan: four levels of organisation, the four tissue types, the direction words and the standard position they are measured from, bone as a living material, the skeleton counted bone by bone to **206**, the joints in detail, the three muscle types and how opposing pairs actually produce a movement, then the cavities and every major organ with its mass and position. We finish with skin, layer by layer.",{"id":52,"type":53,"title":54,"eyebrow":55,"navLabel":56},"u-ch-levels","chapter","Cells, tissues, organs, systems — sharply defined","Chapter 01","1 Organisation",{"id":58,"type":46,"markdown":59},"u-levels","The ladder from *Discover* holds up under close inspection, and each rung has a proper definition.\n\nA **cell** is the smallest unit that is alive: it takes in materials, uses energy, responds to signals and, in most cases, can make copies of itself. Your roughly **37.2 trillion cells** are not all alike. Estimates count more than two hundred different kinds, and each kind is **specialised**: its shape is its job.\n\nLook at the shapes and you can read the function straight off them. A red blood cell is a flat disc with a dip in the middle, which gives it the most surface for its size and lets it bend through the narrowest vessels. A nerve cell has a long, thin fibre — in your leg, some run most of a metre — because its job is to carry a signal a long way. A muscle cell is a long spindle packed with sliding filaments, because its job is to get shorter. A cell lining your intestine has a fuzzy brush of tiny projections on its free surface, because its job is to absorb.",{"id":61,"type":62,"variant":63,"title":64,"markdown":65},"u-def-specialisation","callout","definition","Specialisation, and the rule it follows","**Specialisation** means a cell is shaped and equipped for one particular job, instead of doing a little of everything.\n\nThe rule worth remembering: **structure fits function.** If you are shown an unfamiliar cell and told its shape, you can usually make a decent guess at what it does — and if you are told what it does, you can often predict roughly what it must look like. This rule runs all the way up: it explains why a femur is thick and a rib is springy, why the left lung has a notch in it, and why your hand has 27 bones and your thigh has one.",{"id":67,"type":68,"title":69,"items":70},"u-steps-levels","steps","The four levels, with a worked path through one of them",[71,75,79,83],{"title":72,"tag":73,"text":74},"Cells","one kind, one job","Cardiac muscle cells: short, branched, joined end to end so an electrical signal spreads between them almost instantly.",{"title":76,"tag":77,"text":78},"Tissue","many cells of that kind","Millions of them make **cardiac muscle tissue**, a sheet that contracts as one.",{"title":80,"tag":81,"text":82},"Organ","several tissues, one structure","Cardiac muscle + connective tissue + nerve tissue + its own blood vessels + smooth inner lining = the **heart**.",{"title":84,"tag":85,"text":86},"Organ system","organs sharing a task","Heart + arteries + veins + capillaries = the **circulatory system**, whose task is transport.",{"id":88,"type":89,"component":90,"componentVersion":5,"config":91,"objective":145,"textAlternative":146,"help":147},"u-lab-sort-levels","interactive","sort-game",{"prompt":92,"bins":93,"items":103,"seconds":144},"Sort each item by its level of organisation: is it a cell, a tissue, an organ or an organ system?",[94,97,99,101],{"id":95,"label":96},"cell","Cell",{"id":98,"label":76},"tissue",{"id":100,"label":80},"organ",{"id":102,"label":84},"system",[104,108,112,116,120,124,128,132,136,140],{"id":105,"label":106,"bin":95,"why":107},"rbc","A red blood cell","One cell. Flat, dished, and specialised for carrying oxygen through narrow vessels.",{"id":109,"label":110,"bin":95,"why":111},"neuron","A nerve cell (neuron)","A single cell with a long fibre for carrying signals. Some run most of a metre.",{"id":113,"label":114,"bin":98,"why":115},"blood","Blood","Surprising but true: blood is a connective tissue, with cells suspended in a liquid instead of a solid.",{"id":117,"label":118,"bin":98,"why":119},"cartilage","Cartilage","A smooth, rubbery connective tissue that caps bone ends and shapes the nose and outer ear.",{"id":121,"label":122,"bin":98,"why":123},"epithelium","The lining of the small intestine","Epithelial tissue: a sheet of cells covering a surface, here specialised for absorbing.",{"id":125,"label":126,"bin":100,"why":127},"femur","The femur","A single bone contains bone tissue, cartilage, marrow, blood vessels and nerves — several tissues in one structure.",{"id":129,"label":130,"bin":100,"why":131},"heart-organ","The heart","Muscle, connective and nerve tissue plus its own vessels, built into one pump.",{"id":133,"label":134,"bin":100,"why":135},"skin-organ","The skin","Epidermis, dermis and the layer beneath: several tissues doing one job. The largest organ you have.",{"id":137,"label":138,"bin":102,"why":139},"skeletal-sys","The skeletal system","Every bone, joint, ligament and cartilage together — many organs sharing the task of support and protection.",{"id":141,"label":142,"bin":102,"why":143},"digestive-sys","The digestive system","Mouth, gullet, stomach, intestines, liver and pancreas working on one task.",0,"Place ten familiar body parts at the right level of organisation, including the three that catch people out.","Four bins are labelled cell, tissue, organ and organ system, and ten cards are dealt. Drag each card to a bin; a correct drop explains why, a wrong drop names the level you chose and the level it belongs to.\n\nThree cards are deliberately tricky. **Blood** goes in the tissue bin, which surprises almost everyone: it is a connective tissue whose cells happen to be suspended in a liquid rather than locked in a solid. **The femur** goes in the organ bin, not the tissue bin — a whole bone contains bone tissue, cartilage, marrow, vessels and nerves, which is exactly what makes something an organ. And **the skin** is an organ too, not a covering.\n\nA streak of three correct drops in a row doubles the score on the next card.",{"simplerExplanation":148,"hints":149},"Ask two questions of every card. Is it one cell, or many? Is it one structure, or a group of structures working together?",[150,151],"If it is made of more than one kind of tissue, it is at least an organ.","If several named organs are involved, it is a system.",{"id":153,"type":53,"title":154,"eyebrow":155,"navLabel":156},"u-ch-tissues","The four tissue types everything is built from","Chapter 02","2 Four tissues",{"id":158,"type":46,"markdown":159},"u-tissues-intro","Here is a genuinely surprising fact. Every organ in your body — the brain, the liver, a kidney, a toenail bed, the wall of your bladder — is built from just **4 kinds of tissue**, mixed in different proportions.\n\nFour building materials, two hundred cell types, dozens of organs. It is a little like discovering that every building in a city is made from brick, timber, glass and steel.",{"id":161,"type":162,"caption":163,"columns":164,"rows":168},"u-table-tissues","table","The four tissue types, what each does, and where you can point to one",[76,165,166,167],"What it does","Where it is","A clue to spotting it",[169,174,179,184],[170,171,172,173],"**Epithelial**","Covers and lines. Forms barriers, absorbs, secretes.","Outer layer of skin; lining of the mouth, gut, airways, blood vessels and bladder","Cells packed tightly in sheets, with almost no gaps and almost nothing between them.",[175,176,177,178],"**Connective**","Supports, binds, separates, stores and transports.","Bone, cartilage, tendons, ligaments, fat — and blood","Cells spread out in a lot of non-living material between them. That material is the point.",[180,181,182,183],"**Muscle**","Shortens, and so produces movement or squeezing.","Skeletal muscles, the heart wall, the walls of gut, vessels and bladder","Long cells full of sliding filaments. Two of the three types look striped.",[185,186,187,188],"**Nervous**","Senses, signals and coordinates.","Brain, spinal cord, and every nerve reaching out to the body","Cells with long fibres and branching ends, built for carrying messages fast.",{"id":190,"type":62,"variant":191,"title":192,"markdown":193},"u-aha-connective","aha","The strangest family in the body","Connective tissue is the odd one out, and understanding why makes the whole category click.\n\nIn epithelial tissue the cells are everything: packed shoulder to shoulder with nothing much between them. In connective tissue it is the opposite — the cells are scattered, and the **stuff between them** does the work.\n\nChange that in-between material and you change what the tissue is:\n\n- Soak it with hard mineral crystals and you get **bone**.\n- Make it firm but rubbery and you get **cartilage**.\n- Make it into parallel ropes of a tough protein called collagen and you get **tendons and ligaments**.\n- Fill the cells with stored fat and you get **fat tissue**.\n- Make the in-between material a **liquid**, and you get **blood**.\n\nBone and blood look like nothing at all alike. They are cousins.",{"id":195,"type":46,"markdown":196},"u-collagen","One protein deserves a name check: **collagen**. It is the body's rope. It is the main protein in tendons, ligaments, cartilage, skin and the flexible part of bone, and it is the most abundant protein in your whole body — very roughly a third of all the protein you contain.\n\nCollagen is what makes things **tough but bendy**. It resists being pulled apart without being brittle. Notice where that matters: a tendon must not snap when a muscle yanks it; a ligament must not tear when a joint twists; skin must not split when you stretch. All three are collagen doing the same job in three places.\n\nBone uses collagen too, which brings us to what bone actually is.",{"id":198,"type":89,"component":199,"componentVersion":5,"config":200,"objective":224,"textAlternative":225,"help":226},"u-lab-match-tissues","match-pairs",{"prompt":201,"mode":202,"pairs":203},"Match each body part to the tissue type it is mostly made of.","connect",[204,207,210,212,215,218,221],{"a":205,"b":206},"A tendon","Connective tissue (collagen ropes)",{"a":208,"b":209},"The lining of your mouth","Epithelial tissue",{"a":114,"b":211},"Connective tissue (liquid in between)",{"a":213,"b":214},"The wall of the stomach","Smooth muscle tissue",{"a":216,"b":217},"The optic nerve","Nervous tissue",{"a":219,"b":220},"The tip of your nose","Cartilage — a connective tissue",{"a":222,"b":223},"The wall of the heart","Cardiac muscle tissue","Connect seven familiar body parts to the tissue they are chiefly built from, including the four that are all connective.","Seven body-part cards on the left, seven tissue cards on the right, shuffled. Draw a line between each pair; correct links lock in with a short reason, wrong links simply bounce back.\n\nThe point of the round is the connective tissue group. **Four** of the seven answers are connective tissue — a tendon, blood and cartilage in the nose, plus (if you look it up) bone itself. They share one feature: scattered cells with a lot of material between them, and that in-between material is what makes each one different.\n\nOne answer is epithelial (mouth lining), one is nervous (optic nerve), and one is muscle.",{"simplerExplanation":227,"hints":228},"Join each part to its tissue. When in doubt, ask: is this mostly cells, or mostly the stuff between the cells? Mostly in-between stuff means connective.",[229],"Four of the seven are connective tissue. Find them first and the rest gets easier.",{"id":231,"type":53,"title":232,"eyebrow":233,"navLabel":234},"u-ch-position","Anatomical position: the pose all the words are measured from","Chapter 03","3 Position words",{"id":236,"type":46,"markdown":237},"u-position","Every direction word in anatomy is defined from one standard pose called the **anatomical position**: standing upright, feet together and flat, arms by the sides, **palms facing forward**, head and eyes looking straight ahead.\n\nThe palms-forward part looks odd, and it is the whole reason the pose exists. Turn your palm to face backwards and the two bones of your forearm cross over each other; now \"the bone on the thumb side\" has moved. Fix the pose and every word means the same thing every time, for every body, whatever position that body happens to be in at the moment. A patient lying flat on a scanner is still described as if standing in anatomical position.",{"id":239,"type":240,"tone":241,"items":242},"u-spec-directions","spec","blue",[243,247,251,255,259,263],{"label":244,"big":245,"value":246},"Anterior \u002F posterior","front \u002F back","Anterior is towards the front. The breastbone is anterior to the heart; the spine is posterior to it.",{"label":248,"big":249,"value":250},"Superior \u002F inferior","up \u002F down","Superior is towards the head. The diaphragm is inferior to the lungs and superior to the liver.",{"label":252,"big":253,"value":254},"Medial \u002F lateral","in \u002F out","Medial is towards the midline. Your nose is medial to your eyes; your ears are the most lateral part of your head.",{"label":256,"big":257,"value":258},"Proximal \u002F distal","near \u002F far","Used for limbs, measured from where the limb joins the body. Your elbow is proximal to your wrist; your fingertips are the most distal part of your arm.",{"label":260,"big":261,"value":262},"Superficial \u002F deep","near skin \u002F far in","Skin is superficial; bone is deep. A scraped knee is a superficial injury.",{"label":264,"big":265,"value":266},"Left \u002F right","the body's own","Always the body being described, never the viewer. On a front-view diagram, the liver appears on the left of the page because it is on the body's right.",{"id":268,"type":269,"title":270,"problem":271,"steps":272,"help":280},"u-we-describe","worked_example","Describe one point on the body with no ambiguity at all","Using only the direction words, describe the position of the **right kidney** so precisely that someone who has never seen a body could find it.",[273,274,275,276,277,278,279],"Start with the region: it is in the **abdomen**, below the diaphragm.","Front or back? It lies against the back wall of the abdomen, so it is **posterior**.","Middle or side? It is beside the spine, not on it, so it is **lateral** to the vertebral column — but not far out, so it is also fairly medial within the abdomen.","Up or down? It sits roughly level with the lowest ribs, so it is **inferior** to the diaphragm and liver.","Which side? The **right**, meaning the body's own right.","Put it together: *the right kidney lies in the posterior abdomen, lateral to the vertebral column, inferior to the liver, at about the level of the lowest ribs.* Every word in that sentence means exactly one thing.","A useful extra: the right kidney sits **lower than the left** by roughly the height of half a vertebra, because the liver occupies the space above it. The liver is about 1.5 kg and has to go somewhere.",{"simplerExplanation":281},"Answer five questions in order — which region, front or back, middle or side, high or low, left or right — and you have described any point in the body exactly.",{"id":283,"type":62,"variant":284,"title":285,"markdown":286},"u-careful-left-right","careful","The mistake that keeps being made","On any diagram, scan or X-ray of a body facing you, **the body's right is on your left**. Radiologists put a small letter R or L on every film precisely because getting this wrong has real consequences: operating on the correct side is one of the things hospitals check and re-check before every procedure.\n\nWhen you look at a body-outline diagram in this topic, run the same check. Find the liver. If it is drawn towards the left of the page, you are looking at a body that faces you, and everything else follows.",{"id":288,"type":289,"title":290,"terms":291},"u-glossary-terms","glossary","The anatomy vocabulary, gathered",[292,296,300,304,308,312,316,320,324,328,332,336,340,344,347,351],{"term":293,"meaning":294,"example":295},"anatomical position","The standard pose all direction words are measured from: standing, feet flat, arms down, palms facing forward.","A patient lying in a scanner is still described as if standing in it.",{"term":297,"meaning":298,"example":299},"anterior","Towards the front of the body. Also called ventral.","The kneecap is anterior to the knee joint.",{"term":301,"meaning":302,"example":303},"posterior","Towards the back of the body. Also called dorsal.","The kidneys are posterior organs.",{"term":305,"meaning":306,"example":307},"superior","Towards the head.","The lungs are superior to the diaphragm.",{"term":309,"meaning":310,"example":311},"inferior","Towards the feet.","The bladder is inferior to the intestines.",{"term":313,"meaning":314,"example":315},"medial","Towards the midline of the body.","Your big toe is on the medial side of your foot.",{"term":317,"meaning":318,"example":319},"lateral","Away from the midline, towards one side.","Your ears are lateral.",{"term":321,"meaning":322,"example":323},"epithelial tissue","Sheets of tightly packed cells that cover and line surfaces.","The lining of your mouth and gut.",{"term":325,"meaning":326,"example":327},"connective tissue","Tissue with scattered cells and a lot of material between them; that material decides what it is.","Bone, cartilage, tendon, fat and blood.",{"term":329,"meaning":330,"example":331},"collagen","The tough, rope-like protein in tendons, ligaments, cartilage, skin and bone. The most abundant protein in the body.","It is what makes a tendon strong without being brittle.",{"term":333,"meaning":334,"example":335},"marrow","The soft tissue inside bones. Red marrow makes blood cells; yellow marrow stores fat.","Red marrow is busiest in the breastbone, ribs, pelvis and the ends of long bones.",{"term":337,"meaning":338,"example":339},"ligament","A band of tough tissue tying bone to bone across a joint.","Ligaments inside the knee stop it sliding too far.",{"term":341,"meaning":342,"example":343},"tendon","A cord of tough tissue tying muscle to bone, so the pull reaches the skeleton.","The Achilles tendon at the back of the ankle.",{"term":117,"meaning":345,"example":346},"Smooth, rubbery connective tissue that caps bone ends, cushions joints and shapes the nose and outer ear.","A baby skeleton starts out mostly cartilage.",{"term":348,"meaning":349,"example":350},"antagonistic pair","Two muscles on opposite sides of a joint, each pulling the opposite way.","Biceps bends the elbow; triceps straightens it.",{"term":352,"meaning":353,"example":354},"cavity","A space inside the body that holds organs, lined with a smooth membrane.","The chest cavity and the abdominal cavity, separated by the diaphragm.",{"id":356,"type":53,"title":357,"eyebrow":358,"navLabel":359},"u-ch-bone","Bone is alive, and it is two materials at once","Chapter 04","4 What bone is",{"id":361,"type":46,"markdown":362},"u-bone-living","The skeletons in museums give a badly wrong impression. A dry museum bone is to a living bone roughly what a dried leaf is to a living plant.\n\n**Bone is living tissue.** It contains cells that build it, cells that dissolve it, blood vessels that feed it and nerves that report on it. It heals when it breaks — no other structural material you know does that. It gets thicker where it is loaded and thinner where it is not, which is why the bones of a tennis player's racket arm are measurably denser than their other arm. It is being quietly rebuilt all the time: over several years, most of the material in your skeleton is replaced.",{"id":364,"type":46,"markdown":365},"u-bone-material","Bone is a **composite**: two materials combined so that the mixture beats either one alone.\n\n- About **35% of dry bone is organic**, chiefly collagen — flexible, tough, resists being pulled apart.\n- About **65% is mineral**, mostly a calcium phosphate crystal — hard, stiff, resists being squashed.\n\nTake one away and you can see what each contributes. Soak a chicken bone in vinegar for a few days and the acid dissolves the mineral out: what remains is floppy enough to tie in a knot, because only the collagen is left. Bake a bone dry in an oven and the collagen is destroyed: what remains is hard but crumbles like chalk under a squeeze.\n\nTogether they make something both stiff and tough, which is a rare and valuable combination. Engineers copy the idea: reinforced concrete is a hard, brittle material (concrete) with flexible ropes (steel bars) running through it, for exactly the same reason.",{"id":367,"type":62,"variant":368,"title":369,"markdown":370},"u-try-bone","try_it","The bendy-bone experiment","This one is safe, cheap and genuinely surprising.\n\n1. Take a clean, dry chicken drumstick bone left over from a meal. Wash it well with soap and hot water and let it dry.\n2. Try to bend it. It will not bend.\n3. Put it in a jar, cover it completely with white vinegar, and screw the lid on. Leave it somewhere cool for four to seven days, changing the vinegar once halfway through.\n4. Rinse it, and try bending it again.\n\nIt will now bend, and a thin one can often be tied in a loose knot. The vinegar is a weak acid; it has dissolved out the mineral and left only the collagen. What you are holding is the flexible half of the composite, without its stiff partner.\n\nWash your hands afterwards, and do not put the bone anywhere near your mouth.",{"id":372,"type":240,"tone":373,"items":374},"u-spec-bone-parts","copper",[375,379,383,387,391,395],{"label":376,"big":377,"value":378},"Compact bone","outer shell","Dense, smooth and strong; the layer you see on a museum bone. It carries most of the load.",{"label":380,"big":381,"value":382},"Spongy bone","inner lattice","A honeycomb of struts near the ends. Almost as strong as solid bone for a fraction of the mass, because the struts line up along the directions of force.",{"label":384,"big":385,"value":386},"Marrow","in the middle","Soft tissue in the hollow centre. **Red marrow makes blood cells**; yellow marrow stores fat. In an adult, red marrow is mostly in the breastbone, ribs, pelvis, skull and the ends of the long bones.",{"label":388,"big":389,"value":390},"Periosteum","living skin","A thin, tough, richly supplied membrane wrapped round the outside. It feels pain, feeds the bone and is where new bone is laid down for growth in width.",{"label":392,"big":393,"value":394},"Growth plate","near each end","A band of cartilage where new bone is added, making the bone longer. It closes in the late teens or early twenties, and that is when you stop growing taller.",{"label":396,"big":397,"value":398},"Joint cartilage","at the ends","A smooth, slippery cap where two bones meet, so they glide instead of grinding. It has no blood supply of its own, which is why it heals slowly.",{"id":400,"type":62,"variant":191,"title":401,"markdown":402},"u-aha-marrow","Your bones are a blood factory","This is the fact that most changes how people think about a skeleton. Inside your bones, **red marrow is making blood cells right now** — several million every second, continuously, for your whole life.\n\nSo the skeleton is not just scaffolding that happens to be hollow. The hollow is the point: it saves mass *and* houses a factory. A bone is simultaneously a strut, a store of calcium the rest of the body can draw on, and a production line.\n\nIt also explains something you may have heard: why a doctor investigating certain blood problems takes a small sample from inside a bone. They are going to the factory floor.",{"id":404,"type":405,"prompt":406,"options":407,"explanation":420},"u-predict-hollow","prediction","Bird bones and human long bones are both hollow in the middle rather than solid. Compared with a solid rod of the same mass of the same material, a hollow tube of the same mass is:",[408,411,414,417],{"id":409,"label":410},"a","Much weaker, but lighter",{"id":412,"label":413},"b","About the same strength",{"id":415,"label":416},"c","Stiffer and stronger against bending",{"id":418,"label":419},"d","Only stronger if it is filled with marrow","**Stiffer and stronger against bending.** Bending stresses the outer material most; the centre line barely does anything. Move that lazy central material outwards into a wider ring and you get far more bending strength for the same mass — why scaffolding poles, bicycle frames and aircraft spars are tubes, and why your femur is one too.\n\nThe marrow in the middle is not there for strength — it is there because a safe, sheltered hollow is a fine place to put a blood factory. **The same design solves two problems at once.**",{"id":422,"type":46,"markdown":423},"u-bone-fusing","Now back to the count. A newborn has roughly **300 separate bones and cartilages**; an adult has **206**. About **94** pieces have merged. Where did they go?\n\n- **The skull.** A baby's braincase is several plates separated by soft membrane, including the well-known soft spot on the top. Separate plates let the head compress slightly during birth and then expand quickly as the brain grows. The plates knit together over the first couple of years, and the seams keep tightening into adulthood.\n- **The sacrum.** 5 separate vertebrae at the base of the spine fuse into one solid triangular block, which is what the pelvis hangs on.\n- **The coccyx.** About 4 tiny vertebrae at the very bottom fuse into one small piece.\n- **The hip bone.** Each hip starts as three separate bones that fuse into one during the teenage years.\n- **Long bones.** Each has separate end-caps that fuse to the shaft when the growth plate closes.\n\nCounting a baby's vertebrae the original way gives **33**; in an adult the fused groups count as one each, giving **26**.",{"id":425,"type":53,"title":426,"eyebrow":427,"navLabel":428},"u-ch-axial","The skeleton, counted bone by bone to 206","Chapter 05","5 Counting to 206",{"id":430,"type":162,"caption":431,"columns":432,"rows":436},"u-table-axial","The axial skeleton, counted bone by bone to 80",[433,434,435],"Part","Bones","What it protects or does",[437,441,445,449,453,457,460],[438,439,440],"Skull","22","8 cranial bones make the braincase; 14 facial bones make the face. Only the lower jaw can move.",[442,443,444],"Ear ossicles","6","3 in each middle ear — hammer, anvil and stirrup — passing sound vibration inwards. They include the smallest bone you have.",[446,447,448],"Hyoid","1","A small U-shaped bone in the throat that anchors the tongue. Remarkably, it touches no other bone.",[450,451,452],"Vertebral column","26","7 neck + 12 chest + 5 lower back = 24 movable, plus the fused sacrum and coccyx. It houses the spinal cord.",[454,455,456],"Ribs","24","12 pairs curving from spine to front, caging the heart and lungs while still allowing the chest to expand.",[458,447,459],"Sternum","The breastbone, at the front centre of the ribcage. Full of red marrow, which is why it is sometimes sampled by doctors.",[461,462,463],"**Total**","**80**","22 + 6 + 1 + 26 + 24 + 1 = 80",{"id":465,"type":46,"markdown":466},"u-spine-detail","The spine repays a closer look, because it solves two opposite problems at once: it must be **rigid enough to hold you up** and **flexible enough to let you bend and twist**.\n\nThe solution is to make it out of many short blocks rather than one long pole. Each vertebra can only shift a small amount against its neighbour, but 24 small shifts add up to a spine that can curl right over. Between each pair is an **intervertebral disc**: a tough ring with a soft, water-rich centre that acts as a cushion and a pivot.\n\nNow look at the sizes. The 7 **cervical** (neck) vertebrae are small and light, because they carry only a head. The 12 **thoracic** (chest) vertebrae are bigger and each has a pair of ribs attached. The 5 **lumbar** (lower back) vertebrae are the biggest and thickest of all, because they carry the weight of everything above them. **Size follows load, all the way down the column.**",{"id":468,"type":62,"variant":469,"title":470,"markdown":471},"u-nuance-curves","nuance","Why 4 curves and not a straight line","A newborn's spine has essentially one long backward curve. Two more appear as a baby develops: the **neck curve** when it starts lifting its head while lying on its front, and the **lower-back curve** when it starts standing and walking. The adult spine therefore has 4 curves, alternating forward and backward.\n\nTwo reasons this matters:\n\n- **Shock absorbing.** A straight column passes the jolt of every footstep straight up to the skull. A gently curved one flexes slightly and softens it, the same way a bent knee lands a jump better than a locked one.\n- **Balance.** The curves stack your head directly over your hips, so standing upright costs very little muscular effort. Try standing with your head pushed forward for two minutes and you will feel what the curve is saving you.\n\nNote the order: **the curves formed because of what the baby learned to do.** Structure followed function, in a single lifetime.",{"id":473,"type":46,"markdown":474},"u-ribs-pelvis","The **ribcage** is a compromise of a different kind. A solid box of bone would protect the heart and lungs perfectly and make breathing impossible. Instead you get 12 pairs of springy curved bones, each joined to the spine behind by a small movable joint and to the breastbone in front by flexible cartilage. When you breathe in, the whole cage swings up and out; when you breathe out, it drops back. Protection and movement, both.\n\nThe top 7 pairs reach the breastbone through their own cartilage. The next 3 pairs join the cartilage of the rib above rather than the breastbone itself. The last 2 pairs — the **floating ribs** — attach only at the spine and end free at the side, which is precisely what allows your waist to twist and bend.\n\nThe **pelvis** is the opposite design: heavy, rigid and fused. It has to be. It is the junction where the weight of your entire upper body transfers from a single central column, the spine, into two separate legs. It also forms a bowl that supports the bladder and the lower intestines from below.",{"id":476,"type":89,"component":477,"componentVersion":5,"config":478,"objective":503,"textAlternative":504,"help":505},"u-lab-explorer-frame","body-explorer",{"views":479,"parts":482,"modes":500},[480,481],"skeleton","muscles",[483,486,489,491,494,497],{"id":484,"note":485},"skull","22 bones: 8 cranial forming the braincase, 14 facial. Only the lower jaw moves. Sutures are fixed joints.",{"id":487,"note":488},"spine","26 bones in an adult: 7 cervical, 12 thoracic, 5 lumbar, plus fused sacrum and coccyx. Four alternating curves.",{"id":125,"note":490},"The longest and strongest bone: a hollow tube of compact bone with spongy bone at each end and marrow inside.",{"id":492,"note":493},"biceps","Crosses the front of the elbow. Contracts to bend the elbow and to turn the palm upwards.",{"id":495,"note":496},"quadriceps","Four muscles on the front of the thigh sharing one tendon over the kneecap. Straightens the knee.",{"id":498,"note":499},"diaphragm","The muscular floor of the chest cavity and ceiling of the abdominal cavity. Flattens to draw air in.",[501,502],"explore","find-it","Switch between the skeleton and the muscles over the same body, and see which muscle crosses which joint.","This lab has two views of the same body outline, and a switch between them.\n\nIn the **skeleton** view, tapping a bone gives its count, its design and its job: the skull with 22 bones and fixed sutures, the spine with its four curves, the ribcage with seven true, three false and two floating pairs, the fused pelvic bowl, and the two long-bone tubes.\n\nSwitching to the **muscles** view draws the muscles over the same skeleton, faintly visible beneath. Now tapping the **biceps** shows it crossing the front of the elbow joint — and the key idea appears: a muscle moves the joint it crosses. The **quadriceps** crosses the front of the knee, so it straightens the knee. The **diaphragm** crosses no joint at all, which is why it moves no bone; it changes the size of a cavity instead.\n\nIn **find-it** mode the lab asks for a bone or a muscle at random, so you have to keep both maps in your head at once.",{"simplerExplanation":506,"hints":507},"Two pictures of the same body: bones, then muscles laid over the bones. Look at where each muscle starts and finishes, and which joint it crosses in between.",[508,509],"A muscle can only move a joint it crosses. Check this for the biceps and the quadriceps.","The diaphragm crosses no joint. What does it move instead?",{"id":511,"type":162,"caption":512,"columns":513,"rows":515},"u-table-append","The appendicular skeleton, counted to 126",[433,434,514],"Detail",[516,520,524,528,531],[517,518,519],"Pectoral girdle","4","Two collarbones and two shoulder blades. The shoulder blade floats in muscle and is not locked to the spine, which is why your shoulder has such a huge range.",[521,522,523],"Upper limbs","2 × 30 = 60","Each: humerus, radius, ulna, 8 carpals, 5 metacarpals, 14 phalanges = 30. The hand alone is 27.",[525,526,527],"Pelvic girdle","2","Two hip bones, each fused from three. Locked firmly to the sacrum, because this girdle carries weight rather than reaching.",[529,522,530],"Lower limbs","Each: femur, patella, tibia, fibula, 7 tarsals, 5 metatarsals, 14 phalanges = 30. The foot alone is 26.",[461,532,533],"**126**","4 + 60 + 2 + 60 = 126. With 80 axial bones that makes **206**.",{"id":535,"type":62,"variant":536,"title":537,"markdown":538},"u-example-girdles","example","Two girdles, two opposite design decisions","Compare the shoulder with the hip and you are looking at the same engineering problem solved twice, with opposite priorities.\n\n**The shoulder girdle is built for reach.** The shoulder blade is not bolted to the spine at all; it glides on a bed of muscle across the back of the ribcage. The only bony link to the rest of the skeleton is the slim collarbone, which acts as a strut holding the shoulder out sideways. The socket is shallow. Result: enormous range, and the most commonly dislocated joint in the body. Collarbones break comparatively easily, and that, in a way, is the price of the design.\n\n**The pelvic girdle is built for load.** The two hip bones are fused solid to the sacrum. The socket is deep and grips the head of the femur closely. Result: a joint that can be stood on all day, with a far smaller range and far fewer dislocations.\n\nReach or bear weight — you cannot have the best of both, so the body has one of each.",{"id":540,"type":46,"markdown":541},"u-hands-feet","More than half of your bones — **106 of 206**, or 51.5% — are in your hands and feet. Two hands together hold 54 bones, which is 32 more than your entire skull.\n\nWhy so many small bones? Because **bones are only useful in combination with the joints between them**, and more bones mean more joints mean finer control. A hand with one bone would be a club. A hand with 27 can hold a pen, a cricket ball, a chapati and a violin bow, each with a different grip.\n\nFeet have almost as many (26) for a related but different reason: a foot must be a springy, adjustable platform that reshapes itself to uneven ground at every step, and it does that by having many small bones lashed together with ligaments into a pair of arches.",{"id":543,"type":544,"itemId":545,"prompt":546,"check":547,"hints":551,"feedback":554},"u-practice-count","practice","human-body-anatomy.understand-axial-count","Count the axial skeleton yourself: 22 skull bones, 6 ear ossicles, 1 hyoid, 26 in the vertebral column, 24 ribs and 1 sternum. How many bones is that altogether?",{"kind":548,"answer":549,"tolerance":144,"unit":550},"number",80,"bones",[552,553],"Add them in the order given; do not try to do it in one jump.","22 + 6 = 28. Keep going.",{"correct":555,"incorrect":556},"Correct: 22 + 6 + 1 + 26 + 24 + 1 = **80**. Add the 126 appendicular bones and you get the famous 206.","Add carefully: 22 + 6 + 1 + 26 + 24 + 1 = 80 bones.",{"id":558,"type":53,"title":559,"eyebrow":560,"navLabel":561},"u-ch-joints","Joints, and the three tissues that hold them together","Chapter 06","6 Joints in detail",{"id":563,"type":162,"caption":564,"columns":565,"rows":569},"u-table-joints","Six joint types, with the movement each allows and a place to find one",[566,567,568],"Joint","Movement it allows","Where",[570,574,578,582,586,590],[571,572,573],"**Hinge**","Bends and straightens, in one plane only","Elbow, knee, finger and toe joints",[575,576,577],"**Ball-and-socket**","Every direction, plus rotation","Shoulder (shallow socket, free), hip (deep socket, stable)",[579,580,581],"**Pivot**","Rotation around one axis and nothing else","Top of the neck (shaking \"no\"); between the two forearm bones (turning the palm)",[583,584,585],"**Gliding**","Small sliding movements in several directions","Between the wrist bones, between the ankle bones, between adjoining vertebrae",[587,588,589],"**Saddle**","Two directions plus a little rotation, like a rider on a saddle","The base of your thumb — the joint that lets a thumb face the fingers",[591,592,593],"**Fixed**","None at all","The seams (sutures) between adult skull plates; a tooth in its socket",{"id":595,"type":62,"variant":191,"title":596,"markdown":597},"u-aha-thumb","The one joint that separates you from most animals","Look at the base of your thumb, where it meets the wrist. That **saddle joint** lets the thumb swing across the palm and press its tip against the tip of any finger. Anatomists call it an **opposable thumb**, and it is the reason a human hand can grip a pen, thread a needle, turn a key and grasp a bat.\n\nTry to imagine doing any of those with a thumb that only bent forward and back like a finger. Then try it for real: tuck your thumb flat against your palm and attempt to tie a shoelace or pick a single coin off a table.\n\nOne small joint, an enormous consequence. Tool-making, writing and most of technology rest partly on it.",{"id":599,"type":46,"markdown":600},"u-synovial","Most of the joints you actually move are built to the same plan, called a **synovial joint**, and it is a beautiful piece of design:\n\n- The ends of both bones are capped with smooth, slippery **cartilage**.\n- The whole joint is sealed inside a tough **capsule**.\n- Inside the capsule is a small amount of **synovial fluid**, a slippery liquid roughly the consistency of egg white, which lubricates the surfaces and feeds the cartilage.\n- Outside the capsule, **ligaments** run from bone to bone, limiting how far the joint can travel.\n- Crossing the joint, **tendons** carry in the pull of muscles that live some distance away.\n\nThe friction inside a healthy synovial joint is astonishingly low — lower than ice sliding on ice. Your knee joints have been doing this, thousands of times a day, for as long as you have been walking.",{"id":602,"type":240,"tone":603,"items":604},"u-spec-three-tissues","neutral",[605,608,612,616,620,624],{"label":118,"big":606,"value":607},"cushion","Smooth and rubbery, capping the bone ends so they glide. No blood supply of its own, which is why damage to it heals slowly and badly.",{"label":609,"big":610,"value":611},"Ligament","bone to bone","Tough collagen straps holding the joint together and limiting its range. A sprain is a stretched or torn ligament.",{"label":613,"big":614,"value":615},"Tendon","muscle to bone","Collagen cords that carry a muscle's pull to the skeleton. They let the bulky muscle sit far from the joint it moves.",{"label":617,"big":618,"value":619},"Synovial fluid","oil","A slippery fluid sealed inside the joint capsule. It lubricates and also feeds the cartilage, which has no vessels of its own.",{"label":621,"big":622,"value":623},"Capsule","seal","The tough bag enclosing the whole joint and keeping the fluid where it belongs.",{"label":625,"big":626,"value":627},"Disc","spacer","Between vertebrae: a tough ring with a soft, water-rich middle, acting as both cushion and pivot.",{"id":629,"type":62,"variant":630,"title":631,"markdown":632},"u-misconception-tendon-ligament","misconception","\"Tendon and ligament are two words for the same thing\"","They are not, and the difference is easy to keep straight once you notice it.\n\n- A **tendon** connects a **muscle** to a bone. Its job is to *transmit a pull*. When a muscle contracts, the tendon is the cord that passes that pull along to the skeleton.\n- A **ligament** connects a **bone** to a bone. Its job is to *limit movement*. It does not pull anything; it stops the joint going too far.\n\nFeel the difference on yourself. The thick cord behind your ankle is a tendon — flex your foot and you can feel it tighten as the calf muscle pulls. The straps inside your knee are ligaments; they never tighten because you decided to tighten them, only because the joint reached its limit.\n\nBoth are made mostly of collagen, which is why they look similar. What differs is what sits on each end.",{"id":634,"type":53,"title":635,"eyebrow":636,"navLabel":637},"u-ch-muscle-types","Three kinds of muscle, and who is in charge of each","Chapter 07","7 Three muscles",{"id":639,"type":162,"caption":640,"columns":641,"rows":646},"u-table-three-muscles","The three muscle tissues compared in detail",[642,643,644,645],"Feature","Skeletal","Smooth","Cardiac",[647,651,655,660,665,670],[568,648,649,650],"Attached to bones; also tongue, face, diaphragm","Walls of gut, blood vessels, airways, bladder","The heart wall only",[652,653,654,654],"Control","**Voluntary** — you decide","**Involuntary** — automatic",[656,657,658,659],"Appearance","Striped (striated); long parallel fibres","Not striped; spindle-shaped cells","Striped, but branched and joined end to end",[661,662,663,664],"Speed","Fast and strong","Slow and sustained","Steady and rhythmic",[666,667,668,669],"Tiring","Tires quickly under load","Can hold a squeeze for a long time","Effectively never tires",[671,672,673,674],"Example job","Lifting a bag; smiling; speaking","Pushing food along the gut; narrowing a vessel","Squeezing blood out of the heart",{"id":676,"type":46,"markdown":677},"u-voluntary","\"Voluntary\" and \"involuntary\" are about **who gives the order**, not about how important the muscle is.\n\nYou can decide to clench your fist, and skeletal muscle obeys. You cannot decide to push food along your intestine, and it happens anyway. This division is a very good arrangement: imagine having to remember to keep your heart beating during a maths test.\n\nThe **diaphragm** is the interesting exception. It is skeletal muscle, it runs automatically while you sleep, and yet you can take over and breathe deliberately whenever you choose — hold your breath, blow out a candle, sing a long note. It sits on the boundary between the two worlds, which is why it is worth knowing about.\n\nSmooth muscle deserves one more note: it is why your gut keeps working and why your blood vessels can widen and narrow. When you blush, smooth muscle in tiny vessels in your face has relaxed and let more blood through. Nobody chose that either.",{"id":679,"type":89,"component":90,"componentVersion":5,"config":680,"objective":729,"textAlternative":730,"help":731},"u-lab-sort-muscle",{"prompt":681,"bins":682,"items":692,"seconds":144},"Sort each action into the kind of muscle doing the work — and notice which ones you never chose.",[683,686,689],{"id":684,"label":685},"skeletal","Skeletal (voluntary)",{"id":687,"label":688},"smooth","Smooth (involuntary)",{"id":690,"label":691},"cardiac","Cardiac (involuntary)",[693,697,701,705,709,713,717,721,725],{"id":694,"label":695,"bin":684,"why":696},"write","Writing your name","Skeletal muscles in the hand and forearm, under your direct control.",{"id":698,"label":699,"bin":684,"why":700},"chew","Chewing a chapati","Jaw muscles are skeletal and voluntary — you can stop chewing whenever you like.",{"id":702,"label":703,"bin":687,"why":704},"gut-push","Food being pushed along the intestine","Smooth muscle squeezes in waves along the gut wall, and never asks your permission.",{"id":706,"label":707,"bin":687,"why":708},"blush","Blushing","Smooth muscle in small blood vessels relaxes, letting more blood into the skin of the face.",{"id":710,"label":711,"bin":687,"why":712},"pupil","The pupil of your eye shrinking in bright light","Smooth muscle in the iris, working automatically. You cannot decide to change your pupil size.",{"id":714,"label":715,"bin":687,"why":716},"airway","An airway in the lung narrowing","Smooth muscle rings around the airways tighten and relax automatically.",{"id":718,"label":719,"bin":690,"why":720},"heartbeat","A single heartbeat","Cardiac muscle, found nowhere else in the body, beating on its own rhythm.",{"id":722,"label":723,"bin":690,"why":724},"heart-fast","Your heart speeding up when you run","Still cardiac muscle. The rate changes automatically; you never issue the order.",{"id":726,"label":727,"bin":684,"why":728},"breathe-hold","Holding your breath on purpose","The diaphragm is skeletal muscle — which is exactly why you can take deliberate control of it.","Sort nine everyday actions by muscle type, and see how much of your body runs without you.","Three bins — skeletal, smooth and cardiac — and nine action cards. Drag each card to the muscle doing the work.\n\nThe lesson lands when you finish and look at the board. Only three of the nine actions were ones you chose. Blushing, shrinking your pupil, pushing food along, narrowing an airway and every heartbeat happened without a decision from you. Smooth and cardiac muscle run the quiet background of a body.\n\nThe card that catches most people is **holding your breath**. Breathing feels automatic, so people file it under smooth muscle — but the diaphragm is skeletal muscle, which is precisely why you can take charge of it when you want to.",{"simplerExplanation":732,"hints":733},"Three bins. For each action ask: did I decide to do this? If yes, it is skeletal. If not, is it the heart (cardiac) or somewhere else (smooth)?",[734,735],"Anything in the heart wall is cardiac, and nothing else is.","If it happens inside a tube — gut, vessel, airway, bladder — think smooth muscle.",{"id":737,"type":53,"title":738,"eyebrow":739,"navLabel":740},"u-ch-pairs","How a movement is actually produced","Chapter 08","8 Muscles in pairs",{"id":742,"type":46,"markdown":743},"u-pairs-intro","Say you want to lift a glass of water to your mouth. What actually happens?\n\nA muscle contracts by getting **shorter and fatter**. It has two ends: the **origin**, on the bone that stays still, and the **insertion**, on the bone that moves. Between them the muscle crosses at least one joint. When the muscle shortens, the two ends are pulled towards each other, and the bone that is free to move — the insertion end — moves.\n\nLifting the glass: the **biceps** has its origin up on the shoulder blade and its insertion on the radius, one of the forearm bones, just past the elbow. It crosses the elbow. When it contracts, the forearm swings up towards the shoulder, and the glass comes with it.",{"id":745,"type":46,"markdown":746},"u-antagonists","Now put it down again. The biceps cannot push; it can only stop pulling. So a second muscle does the opposite job.\n\nThe **triceps** runs down the back of the upper arm and inserts on the ulna just behind the elbow, on the other side of the joint. When it contracts, it pulls the forearm back down and straightens the elbow.\n\nTwo muscles pulling opposite ways across the same joint are called an **antagonistic pair**. At any moment one is the **agonist** — the one doing the work — and the other is the **antagonist**, relaxing in a controlled way so the movement is smooth instead of a jerk.\n\n- Bending the elbow: biceps is the agonist, triceps the antagonist.\n- Straightening the elbow: triceps is the agonist, biceps the antagonist.\n\nEvery joint you can move in two directions has at least one such pair. Front of the thigh (quadriceps, straightens the knee) against back of the thigh (hamstrings, bends it). Front of the shin against the calf. Chest against upper back.",{"id":748,"type":68,"title":749,"items":750},"u-steps-lift","Lifting a glass, step by step",[751,755,759,762,766,769],{"title":752,"tag":753,"text":754},"Decision","brain","Your brain sends a signal down a nerve to the biceps.",{"title":756,"tag":757,"text":758},"Contraction","biceps shortens","Sliding filaments inside the muscle cells pull past each other, so the whole muscle gets shorter and thicker.",{"title":760,"tag":341,"text":761},"Transmission","The pull passes through the biceps tendon into the radius, just beyond the elbow joint.",{"title":763,"tag":764,"text":765},"Movement","the elbow bends","The forearm swings up around the elbow hinge. The upper arm, anchored by the shoulder, stays put.",{"title":652,"tag":767,"text":768},"triceps relaxes","The triceps pays out gradually rather than going slack all at once, so the glass rises smoothly and does not fly upwards.",{"title":770,"tag":771,"text":772},"Reversal","swap the roles","To lower the glass, the triceps takes over and the biceps pays out under control. The same pair, the jobs exchanged.",{"id":774,"type":62,"variant":775,"title":776,"markdown":777},"u-observation-feel-pair","observation","Feel the pair swap over","Sit at a table with your forearm resting on it, palm up.\n\n1. Grip the front of your upper arm with your other hand. Now curl your forearm up. You will feel the biceps bunch and harden.\n2. Keep your hand there and slowly lower the forearm back down. The biceps stays firm the whole way — it is paying out, not switching off. That controlled lengthening is why you can lower things gently instead of dropping them.\n3. Now move your grip to the **back** of the upper arm. Press the back of your hand up against the underside of the table and push. The triceps hardens under your fingers, and the biceps goes soft.\n\nYou have just felt an antagonistic pair swap roles. Nothing pushed; both muscles only ever pulled.",{"id":779,"type":405,"prompt":780,"options":781,"explanation":790},"u-predict-tendon-position","The muscles that move your fingers are mostly not in your hand at all — they sit in your forearm and reach the fingers through long tendons. Why would the body put the engine so far from the part it moves?",[782,784,786,788],{"id":409,"label":783},"Because there is no room for muscle in the hand",{"id":412,"label":785},"To keep the hand slim and light, so it can fit into narrow places and move quickly",{"id":415,"label":787},"Because tendons are stronger than muscles",{"id":418,"label":789},"To keep the fingers warm","**To keep the hand slim and light.** If the muscles needed to grip hard were packed into the hand itself, it would be a fat, clumsy lump. Instead the bulky muscles live in the forearm and reach the fingers through thin tendons — exactly the trick used in a bicycle brake, where the lever is at the handlebar, the pad is at the wheel, and a thin cable connects the two.\n\nThere **is** some muscle in the hand — the small ones that spread the fingers and work the thumb — but the powerful gripping muscles are all upstream.",{"id":792,"type":793,"conceptId":794,"relation":795,"explanation":796},"u-conn-body-systems","connection","body-systems","helps_understand","Knowing where each organ sits and which tissue it is made from is the groundwork. Body systems then follows the work as it passes from organ to organ.",{"id":798,"type":53,"title":799,"eyebrow":800,"navLabel":801},"u-ch-cavities","The cavities, and every organ in its place","Chapter 09","9 Cavities & organs",{"id":803,"type":46,"markdown":804},"u-cavities","Organs are not loose inside you. They sit in **cavities**: spaces with defined walls, lined by smooth slippery membranes so that neighbouring organs can slide against each other without sticking.\n\n- The **cranial cavity** is inside the skull and holds the brain, floating in a thin layer of fluid.\n- The **spinal canal** runs down through the vertebrae and holds the spinal cord.\n- The **thoracic cavity** is inside the ribcage: the two lungs in their own sealed sacs, and the heart in a space between them.\n- The **abdominal cavity** lies below the diaphragm: liver, stomach, spleen, pancreas, intestines, and the kidneys pressed against its back wall.\n- The **pelvic cavity** is the bowl below that, holding the bladder and the lower intestine.\n\nThe **diaphragm** is the wall between the last two groups. Above it: heart and lungs. Below it: everything to do with food and waste. That single sheet of muscle is the most useful landmark in the whole trunk.",{"id":806,"type":162,"caption":807,"columns":808,"rows":812},"u-table-organs-detail","Each major organ: its cavity, its exact position and its size",[80,809,810,811],"Cavity and position","Size or mass","One structural detail",[813,818,823,828,833,838,843,848,853,858,863,868,873],[814,815,816,817],"Brain","Cranial; inside the skull","≈ 1.4 kg","Floats in cushioning fluid, wrapped in three membranes.",[819,820,821,822],"Heart","Thoracic; centre of the chest, apex pointing down and to the **left**","≈ 300 g; fist-sized","Cardiac muscle chambers inside a tough sac of its own.",[824,825,826,827],"Lungs","Thoracic; one each side of the heart","Right 3 lobes, left 2","Spongy and light. Left has a notch for the heart.",[829,830,831,832],"Diaphragm","The floor of the thoracic cavity","A sheet, not a lump","Skeletal muscle: automatic and controllable.",[834,835,836,837],"Liver","Abdominal; upper **right**, under the ribs, crossing a little to the left","≈ 1.5 kg","Largest internal organ; dual blood supply.",[839,840,841,842],"Stomach","Abdominal; upper **left**, under the ribs","50 mL empty, up to ≈ 1500 mL full","Stretches about 30× as folded ridges flatten.",[844,845,846,847],"Small intestine","Abdominal; coiled in the centre","≈ 6.0 m long, narrow","Folded, fuzzy lining multiplies the area.",[849,850,851,852],"Large intestine","Abdominal; framing the small intestine — up the right, across, down the left","≈ 1.5 m, wider","Shorter but wider, with pouches.",[854,855,856,857],"Kidneys","Abdominal; **posterior**, against the back wall, near the lowest ribs","Each ≈ 11 cm, ≈ 150 g","Right sits lower; the liver is above it.",[859,860,861,862],"Bladder","Pelvic; low at the front, behind the pubic bone","Comfortable at ≈ 400 mL","Smooth muscle bag; wall thins as it fills.",[864,865,866,867],"Pancreas","Abdominal; lying across the back, behind and below the stomach","≈ 15 cm long","Soft and pale, tucked into the gut’s curve.",[869,870,871,872],"Spleen","Abdominal; far **left**, behind the stomach, under the ribs","≈ 150 g","Dark red and soft, against the diaphragm.",[874,875,876,877],"Skin","Not in a cavity — it is the boundary","1.5–2.0 m², ≈ 4.0 kg","Three layers, each with a different job.",{"id":879,"type":62,"variant":469,"title":880,"markdown":881},"u-nuance-six-metres","Is the small intestine really 6.0 m?","You will see figures from about 3 m to about 7 m for the small intestine, and both can be right, because they are measuring different things.\n\nIn a **living** body the intestine is held in a state of gentle muscular tone, and endoscopic measurements often give around 3 to 5 m. After death that tone is lost and the intestine relaxes and stretches, which is why measurements taken in anatomy labs — the ones most textbooks were originally based on — come out at 6 to 7 m.\n\nWe use **about 6.0 m** here because that is the figure school syllabuses use, and because the important point survives either way: the tube is several times longer than the body that contains it. But notice the lesson. When a measurement disagrees between sources, the first question is not \"who is wrong?\" but \"what exactly did each of them measure?\"",{"id":883,"type":269,"title":884,"problem":885,"steps":886,"help":893},"u-we-organ-shares","What fraction of a body is the parts we have named?","Take a 60 kg adult. The skeleton is about 15% of body mass and skeletal muscle about 40%. Add the brain (1.4 kg), the liver (1.5 kg), the heart (300 g) and the skin (4.0 kg). What share of the whole body have we accounted for?",[887,888,889,890,891,892],"Skeleton: 60 × 0.15 = **9.0 kg**.","Skeletal muscle: 60 × 0.4 = **24.0 kg**.","Heart: 300 g = 0.3 kg.","Add everything: 9.0 + 24.0 + 1.4 + 1.5 + 0.3 + 4.0 = **40.2 kg**.","As a share of 60 kg: 40.2 ÷ 60 × 100 = **67.0%**.","So six named things are about two-thirds of a person. The remaining 33.0% is everything else: blood, lungs, gut, kidneys, fat stores, the brain's fluid, and the water inside every cell.",{"simplerExplanation":894,"anotherExample":895},"Muscle and bone are much heavier than the organs people think about. Muscle alone (24.0 kg) outweighs the brain, liver, heart and skin put together.","For a 40 kg child the same percentages give a skeleton of about 6.0 kg and muscle of about 16.0 kg.",{"id":897,"type":53,"title":898,"eyebrow":899,"navLabel":900},"u-ch-skin","Skin: the organ that is also the boundary","Chapter 10","10 Skin and senses",{"id":902,"type":46,"markdown":903},"u-skin-layers","Skin is the largest organ, at 1.5 to 2.0 square metres in an adult, and it is built in three layers, each with a clear job.\n\nThe **epidermis** is the thin outer layer, made of epithelial tissue in flat sheets. New cells are made at its base and pushed steadily upwards, filling with a tough protein and flattening as they go, until the outermost ones are no longer alive at all — a dry, sealed shield of flat cells that rub off constantly and are replaced from below. This layer also holds the cells that make the pigment that gives skin its colour and helps protect it from the Sun's ultraviolet light.\n\nThe **dermis** below is the thick living layer, made of connective tissue and packed with equipment: touch and pressure and temperature sensors, tiny blood vessels, sweat glands, hair roots, and a mesh of collagen and stretchy fibres that lets skin be pulled and spring back.\n\nBeneath that is the **subcutaneous layer**, mostly fat. It cushions knocks, insulates against cold and stores energy.",{"id":905,"type":62,"variant":536,"title":906,"markdown":907},"u-example-skin-thickness","The same organ, five different thicknesses","Skin is not one thickness everywhere. It is tuned, place by place, to the job it has to do there.\n\n- **Eyelids**: about half a millimetre, thinner than a sheet of card. It has to fold completely, several times a minute, for your whole life.\n- **Back of the hand**: thin and loose, so your knuckles can bend fully.\n- **Palm of the hand**: thick, hairless, ridged and firmly anchored, so it grips and does not slide.\n- **Back**: thick and tough, protecting a large area that is hard to see and easy to knock.\n- **Soles of the feet**: the thickest of all, several millimetres, because you stand on them.\n\nEvery one of those is the same organ with the same three layers. What changes is how thick each layer is made. Structure fits function again.",{"id":909,"type":46,"markdown":910},"u-sense-organs","The sense organs are worth one careful paragraph each, because each is a piece of physics built out of tissue.\n\nThe **eye** is an optical instrument. Light passes through a clear window at the front, then through a hole (the pupil) whose size is adjusted automatically by smooth muscle, then through a lens that can change shape to focus, and lands on a light-sensitive layer at the back where it becomes a nerve signal. Lens, aperture, screen: a camera uses the same three parts in the same order.\n\nThe **ear** is a mechanical instrument in three sections. The visible outer part collects sound and funnels it down a tube to a stretched membrane, the eardrum. Behind that, three tiny bones — including the **stapes**, at about 3.0 mm the smallest bone you own — pass the vibration inwards and concentrate it. Deep inside, a fluid-filled spiral turns different frequencies into signals on different nerve fibres. A separate set of three fluid-filled loops in the same bony region handles balance.\n\nThe **nose** and **tongue** are both chemical detectors: patches of specialised cells that respond to particular molecules, in air for smell and dissolved in saliva for taste. Most of what you call the \"taste\" of food is actually smell, which is why food is dull when your nose is blocked.\n\nThe **skin**, as we have just seen, carries the touch senses in its dermis.",{"id":912,"type":793,"conceptId":913,"relation":914,"explanation":915},"u-conn-light","light","applied_in","The eye focuses, controls an aperture and forms an image on a screen. The Light topic explains refraction, lenses and images — the physics the eye is doing.",{"id":917,"type":793,"conceptId":918,"relation":914,"explanation":919},"u-conn-sound","sound","The ear catches vibrating air with a stretched drum and passes it on through three of the smallest bones in the body. The Sound topic explains what those vibrations are.",{"id":921,"type":53,"title":922,"eyebrow":923,"navLabel":924},"u-ch-close","Check yourself","Chapter 11","11 Check yourself",{"id":926,"type":927,"title":928,"questions":929},"u-quiz","quiz","Twelve questions on how the body is put together",[930,943,956,969,982,995,1007,1020,1033,1046],{"itemId":931,"prompt":932,"options":933,"correct":412,"why":942},"human-body-anatomy.understand-q-tissues","How many main tissue types is the whole body built from?",[934,936,938,940],{"id":409,"label":935},"Two",{"id":412,"label":937},"Four",{"id":415,"label":939},"Twelve",{"id":418,"label":941},"Over two hundred","Four: epithelial (covers and lines), connective (supports and transports), muscle (shortens) and nervous (signals). There are over two hundred *cell* types, which is a different count.",{"itemId":944,"prompt":945,"options":946,"correct":412,"why":955},"human-body-anatomy.understand-q-blood-tissue","Blood is classed as which type of tissue?",[947,949,951,953],{"id":409,"label":948},"Epithelial",{"id":412,"label":950},"Connective",{"id":415,"label":952},"Muscle",{"id":418,"label":954},"It is not a tissue at all","Connective tissue, because it has scattered cells in a large amount of non-living material between them. In blood that material happens to be a liquid rather than a solid.",{"itemId":957,"prompt":958,"options":959,"correct":415,"why":968},"human-body-anatomy.understand-q-anatomical-position","In the anatomical position, which way do the palms face?",[960,962,964,966],{"id":409,"label":961},"Backwards",{"id":412,"label":963},"Towards the thighs",{"id":415,"label":965},"Forwards",{"id":418,"label":967},"Upwards, with arms raised","Palms forward. It matters because turning the palm crosses the two forearm bones over each other, which would change what \"the bone on the thumb side\" means.",{"itemId":970,"prompt":971,"options":972,"correct":412,"why":981},"human-body-anatomy.understand-q-bone-composite","A chicken bone soaked in vinegar becomes bendy. What has the vinegar removed?",[973,975,977,979],{"id":409,"label":974},"The collagen",{"id":412,"label":976},"The mineral",{"id":415,"label":978},"The marrow",{"id":418,"label":980},"The water","The acid dissolves the calcium mineral (about 65% of dry bone), leaving the flexible collagen behind. Baking a bone does the opposite: it destroys the collagen and leaves something hard but crumbly.",{"itemId":983,"prompt":984,"options":985,"correct":412,"why":994},"human-body-anatomy.understand-q-marrow","What does red bone marrow do?",[986,988,990,992],{"id":409,"label":987},"Stores calcium",{"id":412,"label":989},"Makes blood cells",{"id":415,"label":991},"Lubricates joints",{"id":418,"label":993},"Connects muscle to bone","Red marrow is a blood-cell factory, making several million cells every second. In an adult it is mostly in the breastbone, ribs, pelvis, skull and the ends of the long bones.",{"itemId":996,"prompt":997,"options":998,"correct":415,"why":1006},"human-body-anatomy.understand-q-axial","Which of these belongs to the axial skeleton rather than the appendicular?",[999,1001,1003,1005],{"id":409,"label":1000},"The collarbone",{"id":412,"label":1002},"The shoulder blade",{"id":415,"label":1004},"The sternum",{"id":418,"label":126},"The sternum is part of the central column: skull, spine, ribs and breastbone, 80 bones in all. Collarbone, shoulder blade and femur all belong to the 126 appendicular bones of the limbs and girdles.",{"itemId":1008,"prompt":1009,"options":1010,"correct":412,"why":1019},"human-body-anatomy.understand-q-ligament","What does a ligament join?",[1011,1013,1015,1017],{"id":409,"label":1012},"Muscle to bone",{"id":412,"label":1014},"Bone to bone",{"id":415,"label":1016},"Muscle to muscle",{"id":418,"label":1018},"Skin to muscle","Bone to bone, across a joint, limiting how far it can travel. A tendon is the one that joins muscle to bone and transmits a pull.",{"itemId":1021,"prompt":1022,"options":1023,"correct":412,"why":1032},"human-body-anatomy.understand-q-thumb","Which joint type is at the base of your thumb, and why does it matter?",[1024,1026,1028,1030],{"id":409,"label":1025},"A hinge, which lets the thumb bend",{"id":412,"label":1027},"A saddle joint, which lets the thumb face the fingers",{"id":415,"label":1029},"A pivot, which lets the thumb rotate",{"id":418,"label":1031},"A fixed joint, which keeps the thumb stable","A saddle joint. It lets the thumb swing across the palm and oppose each fingertip — the movement behind gripping, writing and tool use.",{"itemId":1034,"prompt":1035,"options":1036,"correct":412,"why":1045},"human-body-anatomy.understand-q-antagonist","When you straighten your elbow, which muscle is the agonist (the one doing the work)?",[1037,1039,1041,1043],{"id":409,"label":1038},"The biceps",{"id":412,"label":1040},"The triceps",{"id":415,"label":1042},"Both equally",{"id":418,"label":1044},"Neither; gravity does it","The triceps pulls the forearm straight. The biceps is the antagonist, paying out under control so the movement is smooth. Bending the elbow swaps the two roles.",{"itemId":1047,"prompt":1048,"options":1049,"correct":415,"why":1058},"human-body-anatomy.understand-q-diaphragm","Which statement about the diaphragm is correct?",[1050,1052,1054,1056],{"id":409,"label":1051},"It is smooth muscle, so it is entirely involuntary",{"id":412,"label":1053},"It is cardiac muscle, like the heart",{"id":415,"label":1055},"It is skeletal muscle that runs automatically but can also be controlled",{"id":418,"label":1057},"It is not a muscle at all, but a sheet of cartilage","Skeletal muscle, which is exactly why you can hold your breath or blow out a candle — and yet it keeps working while you sleep. It forms the floor of the chest cavity and the ceiling of the abdominal one.",{"id":1060,"type":1061,"prompt":1062},"u-reflect","reflection","Pick any organ and write down which of the four tissue types it must contain, and why each one is needed there. For example, a bladder needs smooth muscle to squeeze, epithelial tissue to line it and keep urine in, connective tissue to hold its shape, and nervous tissue to report when it is full.\n\nThen try the harder version: is there any organ in the body that could manage with only *one* tissue type? Argue your answer.",{"id":1064,"type":1065,"title":1066,"points":1067},"u-summary","summary","Cheat sheet",[1068,1069,1070,1071,1072,1073,1074,1075,1076,1077,1078,1079,1080],"**Levels:** cell → tissue → organ → organ system → organism. Over 200 cell types, but only **4 tissue types**.","**Tissues:** epithelial (covers and lines), connective (bone, cartilage, tendon, fat, **blood**), muscle (shortens), nervous (signals). Connective tissue is defined by the material *between* its cells.","**Collagen** is the body's rope: the main protein of tendon, ligament, cartilage, skin and the flexible half of bone.","**Anatomical position:** standing, arms down, **palms forward**. Anterior\u002Fposterior, superior\u002Finferior, medial\u002Flateral, proximal\u002Fdistal, superficial\u002Fdeep. **Left and right belong to the body.**","**Bone is alive** and is a composite: about 35% collagen (tough, bendy) and 65% mineral (hard, stiff). Compact shell, spongy lattice, **marrow** inside making blood cells, periosteum outside, growth plates near the ends.","**Axial skeleton, 80 bones:** skull 22 + ossicles 6 + hyoid 1 + spine 26 + ribs 24 + sternum 1. **Appendicular, 126:** girdles 4 + 2, limbs 60 + 60. Total **206**.","**Spine:** 7 + 12 + 5 = 24 movable, plus sacrum and coccyx. 4 curves, formed as a baby learns to lift its head and to stand. Vertebrae get bigger as the load increases.","**Hands and feet hold 106 of your 206 bones** (51.5%), because many bones mean many joints mean fine control.","**Joints:** hinge, ball-and-socket, pivot, gliding, saddle, fixed. Synovial joints have cartilage caps, a capsule, synovial fluid, ligaments outside and tendons crossing. **Ligament = bone to bone; tendon = muscle to bone.**","**Three muscles:** skeletal (striped, voluntary, tires), smooth (unstriped, involuntary, tireless, in gut and vessel walls), cardiac (striped, involuntary, heart only, never rests). The **diaphragm** is skeletal — automatic *and* controllable.","**Movement:** a muscle shortens and pulls its insertion towards its origin, crossing a joint on the way. It can never push, so joints have **antagonistic pairs** — biceps and triceps, quadriceps and hamstrings.","**Cavities:** cranial (brain), spinal (cord), thoracic (heart, lungs), abdominal (liver, stomach, spleen, pancreas, gut, kidneys at the back), pelvic (bladder). The **diaphragm** divides thoracic from abdominal.","**Skin:** epidermis (thin, tough, outermost cells dead), dermis (living, with sensors, vessels, glands and collagen), subcutaneous fat (cushion, insulation, store). 1.5–2.0 m², thickest on the soles, thinnest on the eyelids.",{"id":1082,"type":1083,"sourceIds":1084},"u-sources","sources",[1085,1086,1087,1088,1089,1090,1091,1092],"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-ncert-textbooks",[1085,1086,1087,1088,1089,1090,1092],"needs_review",{"generatedBy":1096,"notes":1097},"claude-code","Draft generated locally; pending owner review. Every figure is computed and asserted in scratchpad\u002Fhuman-body-anatomy\u002Ffacts.py.","c514db840b5936c3d16308e450e16601b1eb6bd1751eccb7f1bc8882fa10cf29",{"component:sort-game@1":1100,"component:match-pairs@1":1101,"component:body-explorer@1":1102,"logic:practice":1103,"source:human-body-anatomy-bianconi-cell-count":1104,"source:human-body-anatomy-britannica-human-body":1105,"source:human-body-anatomy-britannica-muscle":1106,"source:human-body-anatomy-britannica-skeleton":1107,"source:human-body-anatomy-britannica-skin":1108,"source:human-body-anatomy-ncert-textbooks":1109,"source:human-body-anatomy-wikipedia-bone-list":1110,"source:human-body-anatomy-wikipedia-organ-list":1111},"b164f45a2c8ca08f26c450768ff0231e113e9fe45381eddb34dc6d0548596c38","2a8ee4ac87460b4e1175a4bb13c96b03d577db06dde95670eb7fcfe4ad787899","b44b6ef91c082fccbb2437932390f32c3cdec3d3b00c1261c62e6f7a421e644b","3d6b0fe1b15255975a32b0fcd94e8019bc959ad45cbf12e136e86149549c6878","86a46797301346e8f555b991be692fef026b03adafcce61373363e59a5835b1d","d8eb8accac4fca64e6711169db72028b260fb8b81c937c86516d9009896fa34c","b2397d7be01ef8cf5211da5eca1573ad62d46085f36d4db2ed08f21c53056557","193bc0eb450e6145811c970ee092582c5438e4c1a9b77f43068da210057b43d4","2e0b8f545b04515ce65fc2b664b763d136911c4156af3a9d3b82fe3c668f5a95","efd8ff1189b2fa80028f5c70765a1028db96825546b54365d07ce2b79ce43928","12c5d62ebb8a2544cf16cbcbe8105c8a9cf1eccbc9b2131099e56d49e47bc134","6ae52bf16d9cf93ff90c426eeb0f7cb7f36e504d5e893cbcecd97a44a5fb6c9e",{"state":1113,"reviewer":1114,"selfReview":1115,"reviewedAt":1116,"method":1117},"approved","The library owner",true,"2026-09-20T10:18:37.581Z","owner_bulk","preview-7e1cbbcc4f",1789899597571]