Plate XVII · nine sheets of film
Layers of the Body
One standing figure, printed in the way the old school atlases did it: each system of the body on its own sheet of clear film, laid one over another on a white card.
Scroll, and the sheets turn one at a time, front to back. At every layer, the things that commonly go wrong there are marked on the film in red grease pencil.
The figure faces you, so its right side is on your left. A general picture, not medical advice.
Scroll to turn the first sheet
1 Sheet one
Skin
The outside of you is an organ: waterproof, self-repairing, and full of sensors. It keeps water in and germs out, makes vitamin D in sunlight, and cools you by sweating.
It is also thinner than it feels. Most of it is between one and two millimetres thick.
1 A block cut from the forearm
Two layers, glued to a third
The epidermis on top is a sheet of cells with no blood supply. New cells are made at its base and pushed upward for about a month, dying on the way, so the surface you touch is dead and is always being shed.
The dermis under it is the living leather: collagen, blood vessels, nerve endings, hair roots, sweat and oil glands. Under that is fat, the next sheet.
what goes wrong here
Sunburn
Ultraviolet light breaks the DNA of living cells in the epidermis. The redness is not heat: it is extra blood sent to the damage, and it peaks half a day to a day after the sun.
Cells too damaged to repair destroy themselves, and a few days later the dead layer peels. Damage that is repaired wrongly is how skin cancers start, which is why burns add up over a lifetime.
what goes wrong here
A burn, by how deep
Superficial burns stop in the epidermis: red, sore, healed in about a week. Partial-thickness burns reach the dermis and blister.
Full-thickness burns go through both layers. They can hurt less in the middle, because the nerve endings are gone, and with the hair roots and glands destroyed there is nothing left to regrow skin from. They need a graft.
what goes wrong here
Acne
Every hair grows from a pit with an oil gland beside it. In the teenage years hormones turn the oil up. If dead cells plug the pit, oil backs up behind the plug, bacteria that live there multiply, and the wall of the pit becomes inflamed: a spot.
It clusters on the face, chest and back because that is where the oil glands are largest and most crowded.
what goes wrong here
A cut that needs stitches
A graze takes off epidermis, and the base regrows it without a mark. A cut that goes right through the dermis is different: the skin is under tension, so the edges pull apart, and you may see yellow fat at the bottom.
Stitches hold the edges together so the gap to be bridged is small. The body fills it with scar, which is collagen laid down in a hurry: strong enough, but never quite as strong as the skin it replaced.
2 Sheet two
Fat, and the veins in it
Under the skin is a quilted layer of fat held in small pockets by fibres. It is padding, insulation and the body's main store of energy, and it is also what fixes the skin to what lies beneath.
Its thickness varies more than any other layer: almost nothing on the shin, the eyelid and the back of the hand; centimetres on the belly and thigh.
The veins you can see through your skin run here. The blue is a trick of light on skin. The blood in them is dark red.
- Cephalic vein
- Basilic vein
- Vein of the elbow crease, where blood is drawn
- Great saphenous vein, the longest in the body
what goes wrong here
A bruise
A knock that does not break the skin can still burst small vessels under it. Blood leaks into the fat and has nowhere to go.
The colours are that blood being taken apart. Fresh and red, then dark blue-purple as it loses oxygen; then, as the red pigment is broken down, green and finally yellow, before it is carried away. About two weeks.
3 Sheet three
Surface muscle
The meat of you. Each muscle is a bundle of fibres that can do exactly one thing: get shorter. The lines on the film are the direction each one pulls.
Red is muscle. Cream is tendon, the tough cord or sheet that ties a muscle to bone and does not stretch.
- Deltoid
- Pectoralis major
- Biceps
- Rectus abdominis
- External oblique
- Sartorius, the longest muscle
- Quadriceps
- Tibialis anterior
3 The upper arm
Muscles can only pull
So they work in opposing pairs. The biceps on the front bends the elbow. To straighten it again, the biceps has to let go while the triceps on the back pulls the other way.
Follow the biceps down: the red stops well short of the elbow. The last part is tendon, crossing the joint to fix onto the forearm. A muscle moves the bone beyond the joint it crosses.
what goes wrong here
A pulled muscle
A strain is torn muscle: a few fibres, or many. It happens when a muscle is made to lengthen while it is trying to shorten, which is why it strikes in a sprint or a kick, usually where the red meets the cream.
Muscle has a rich blood supply, so it heals well: a couple of weeks for a mild tear, a few months for a bad one. The commonest of all, the hamstring, is on the back of the thigh, behind this sheet.
what goes wrong here
Tennis elbow
The muscles that lift the wrist and fingers all hang from one small knob of bone on the outside of the elbow. Hours of gripping and twisting load that one attachment again and again, and the tendon frays faster than it is rebuilt.
Tendon is pale because it has little blood in it. That is why it is strong, and also why it mends slowly: months, not weeks.
4 Sheet four
Deep muscle
Under the muscles that move you are the ones that hold you together. This sheet is mostly clear: where there is no deep muscle, you look straight through to the bones.
Between each pair of ribs are thin muscles that swing the ribs up and out when you breathe in. Along the spine and across the hip runs the muscle that lifts the thigh. A sheet of muscle wraps the belly like a belt.
- Muscles between the ribs
- Pectoralis minor
- Brachialis
- Transversus, the belt
- Psoas and iliacus
- Vastus intermedius
what goes wrong here
The rotator cuff
The shoulder is the most mobile joint in the body because its socket is nearly flat: a ball resting on a saucer. What holds the ball on is four small deep muscles whose tendons wrap round it like a cuff.
One of those tendons runs through a narrow gap under the tip of the shoulder blade. Years of rubbing there fray it. A torn cuff is the usual reason an older shoulder hurts at night and will not lift the arm sideways.
5 Sheet five
Nerves
The wiring. The brain and the spinal cord are the centre. From the cord, nerves leave in pairs between the bones of the spine, one pair for each strip of the body.
Where the nerves for a limb leave, they braid together, then separate again into the named nerves of the arm or leg. Each carries signals both ways: orders out to muscle, sensation back in.
- Brain
- Spinal cord
- Braid of nerves for the arm
- Median nerve
- Ulnar nerve
- Nerves under each rib
- Femoral nerve
- Sciatic nerve
5 Toe to brain
Why a stubbed toe hurts twice
Nerve fibres are not all the same. The thick ones are wrapped in a fatty insulation: they carry touch at about 50 metres a second and sharp pain at about 15. Thin bare ones carry dull, aching pain at about one.
All three set off together from your toe. You know you have hit something almost at once, the sharp pain lands a tenth of a second later, and the ache arrives after more than a second.
what goes wrong here
Carpal tunnel
At the wrist, the small bones form a trench and a tough strap roofs it over. Through that tunnel go nine tendons to the fingers, and one nerve: the median.
The tunnel cannot stretch. If anything in it swells, the nerve is what gets squeezed. The result is numbness and pins and needles in exactly the fingers that nerve serves: the thumb, index, middle and half of the ring finger. Never the little finger.
what goes wrong here
The funny bone
It is not a bone. On the inner side of the elbow the ulnar nerve runs in a groove with only skin over it. Knock it against the bone and you have struck a nerve directly: a jolt down the forearm into the little finger and half the ring finger, the part of the hand that nerve serves.
A “dead arm” is the same idea higher up: a blow or a long squeeze on a nerve stops it carrying signals for a while.
what goes wrong here
Sciatica
The sciatic nerve is the thickest in the body, about as wide as your thumb. It is made of roots that leave the bottom of the spine, and it runs down the back of the thigh to the foot.
If one of those roots is pinched where it leaves the spine, usually by a bulging disc, the pain is felt wherever that root's fibres go: the buttock, the back of the thigh, the calf, the foot. The leg hurts. The problem is in the back.
6 Sheet six
Arteries and veins
Two sets of pipes. Arteries carry blood out from the heart under pressure, so their walls are thick and they lie deep and protected. Veins bring it back at almost no pressure, helped by one-way valves and the squeeze of muscles around them.
Red for arteries and blue for veins is the atlas's code for direction, not for the colour of what is inside.
- Aorta
- Carotid artery
- Jugular vein
- Brachial artery
- Radial artery
- Vena cava
- Femoral artery
- Great saphenous vein
6 Where an artery comes up for air
Your pulse
Each heartbeat sends a wave of pressure down every artery. You can feel it wherever an artery runs just under the skin with bone behind it to press against.
The usual place is the wrist, on the thumb side, where the radial artery crosses the end of the radius. The side of the neck is the other. Count the beats for a minute: at rest most adults are between 60 and 100.
7 Sheet seven
The skeleton
Bone is not dry and it is not dead. It is a living tissue with its own blood supply, and it is taken apart and rebuilt all the time, a little faster wherever it is loaded.
Pale blue on the film is cartilage: the springy ends of the ribs, and the discs between the bones of the spine. Behind the ribs you can already see what the cage is for.
- Skull
- Collar bone
- Breastbone
- Humerus
- Radius
- Ulna
- Hip bone
- Femur
- Kneecap
- Tibia
- Fibula
what goes wrong here
A broken wrist
Put a hand out to break a fall and the end of the radius takes it. It is among the commonest breaks there is.
Bone mends with bone, not scar. Blood clots in the gap; within weeks a soft collar of cartilage bridges it; that is turned into rough bone, which is why a healing break has a lump; then, over a year or more, the lump is carved back to the old shape.
what goes wrong here
A sprained ankle
A ligament is a short strap from bone to bone that stops a joint going where it should not. A sprain is a ligament stretched or torn. (A strain, with a t, is muscle or tendon.)
Roll your foot inward off a kerb and the straps on the outside of the ankle take the whole load. Like tendon, ligament has a poor blood supply, so a bad sprain can take longer to come right than a clean break.
what goes wrong here
A torn knee ligament
The knee is two rounded knuckles standing on a nearly flat table. Nothing about the bones holds it together. Four ligaments do: one down each side, and two crossed in the middle.
The front one of the crossed pair, the ACL, stops the shin sliding forward. A sudden stop and twist can snap it. Bathed in joint fluid, the two ends do not knit, so it is usually rebuilt with a strip of the person's own tendon.
what goes wrong here
Worn joints
Where two bones meet, each is capped with a few millimetres of smooth cartilage, slipperier than ice on ice. It has no blood vessels and no nerves, and it is fed only by the fluid squeezed through it as the joint moves.
So when it wears thin it cannot repair itself and gives no warning. The pain of osteoarthritis begins once the bone beneath is taking the load. Knees, hips and the joints of the hand go first.
what goes wrong here
Thinning bone
Inside the ends of a bone is not solid but a lattice of fine struts, lined up along the loads the bone carries. One set of cells is always removing bone and another replacing it.
Up to about thirty, more is added than taken. Later, and fastest in women after the menopause, the balance tips the other way. Struts thin, then disconnect. The bone looks the same from outside until a fall that should not have broken it does: the hip, the wrist, the spine.
8 Sheet eight
The organs
Two rooms, one above the other, with a dome of muscle between them. Above the dome, inside the ribs: the lungs, and the heart between them, tipped to the left. Below it: the liver under the right ribs, the stomach under the left, and about five metres of gut folded into the space that is left.
- Brain
- Windpipe
- Right lung
- Heart
- Diaphragm
- Liver
- Gall bladder
- Stomach
- Spleen
- Large bowel
- Small bowel
- Bladder
what goes wrong here
Asthma
The windpipe divides again and again, about twenty-three times, into tubes too small to see. Each of the smaller tubes is wrapped in a thin band of muscle.
In asthma the lining of those tubes is touchy. Set off by cold air, pollen, smoke or exercise, the muscle tightens, the lining swells and makes mucus, and the tube narrows. Air goes in more easily than it comes out. The wheeze is air squeezing through.
what goes wrong here
Pneumonia
The tubes end in air sacs, hundreds of millions of them, with walls so thin that oxygen passes straight through into the blood. Spread flat, they would cover the floor of a small flat: about 70 square metres.
Pneumonia is an infection down in those sacs. They fill with fluid and pus, and that part of the lung stops being a sponge and becomes solid. Each lung is divided into lobes, and the infection often fills one of them, which shows as a white patch on a chest X-ray.
what goes wrong here
A heart attack
The heart pumps blood to everything, but it cannot feed on the blood inside it. It has its own arteries, running over its surface. You can see two of them on the film.
If a fatty patch in the wall of one of them splits, a clot forms on it and can block the artery. The muscle beyond the block is starved of oxygen, and after about twenty minutes it starts to die. That is a heart attack, and it is why speed matters more than anything.
The pain is often not where the heart is. It can spread to the left arm, the neck or the jaw.
what goes wrong here
Heartburn
Nothing to do with the heart. The stomach makes hydrochloric acid, strong enough to kill most of what you swallow, and protects itself with a lining of mucus. The gullet above it has no such lining.
A ring of muscle where the two meet is supposed to stay shut except when you swallow. If it lets acid back up, it burns the gullet, and the gullet runs up the chest directly behind the heart. Hence the name, and hence the worry.
what goes wrong here
Appendicitis
Low on the right, where the small bowel joins the large, a blind tube the size of a finger hangs off the bowel. If its opening blocks, it swells and becomes infected.
The pain moves, and the way it moves is the clue. It begins as a vague ache round the navel, because the gut's own pain nerves are bad at saying where. Hours later the swollen appendix touches the lining of the belly wall, which has precise nerves, and the pain becomes sharp and settles low on the right.
what goes wrong here
Gallstones
The liver makes bile, a green detergent that breaks fat into droplets. Between meals it is stored and concentrated in a small bag tucked under the liver.
Concentrated too far, it can crystallise into stones. Most sit there for life unnoticed. But after a fatty meal the bag squeezes, and if a stone jams in the outlet the pain comes under the right ribs, and often seems to be in the right shoulder blade as well.
9 The card itself
The back wall
With the last film turned, what is left is printed on the card: the spine, the back of the ribs, the shoulder blades, and the things that lie flat against them, behind everything else.
The body's two largest vessels run down the front of the spine. The kidneys sit either side, high up, half under the lowest ribs. The pancreas lies across, behind the stomach.
- Gullet
- Aorta
- Vena cava
- Kidney
- Pancreas
- First loop of the small bowel
- Ureter
- Spine
what goes wrong here
A kidney stone
The kidneys filter the blood, about 180 litres of it a day, and take back all but a litre or two. What is left is urine, carrying dissolved waste. Too concentrated, and some of that waste crystallises.
A stone that stays in the kidney may never be felt. One that starts down the ureter, a tube only a few millimetres wide, is another matter: the tube clamps on it in waves. The pain starts in the back under the ribs and travels round and down to the groin as the stone does.
what goes wrong here
Type 2 diabetes
Scattered through the pancreas are islands of cells that make insulin, the signal that tells muscle, fat and liver to take sugar out of the blood after a meal.
In type 2 diabetes the body's cells have stopped answering that signal properly. The pancreas makes more insulin to compensate, and in time cannot keep up. Sugar stays high in the blood, and over years that damages the smallest vessels and nerves: in the eyes, the kidneys and the feet.
× Without turning a sheet
How each layer is seen
A doctor cannot peel you. Each way of looking inside picks out some layers and not others.
X-rays are stopped by the calcium in bone and pass through most of the rest: the picture is the skeleton sheet, with the lungs as dark air. Ultrasound echoes off the boundaries between soft organs: gallstones, a baby. MRI listens to the water in soft tissue, so it shows what X-rays cannot: brain, discs, ligaments, muscle.
Below: the numbers, a map of where pain misleads, and the sources.