Appearance
5.3 — The Spine
The spine has to do four things that pull against each other. It carries the entire weight of the head, arms and trunk down to the pelvis. It protects the spinal cord, which cannot be repaired if damaged. It permits bending in every direction and rotation. And it absorbs the shock of every step you take.
A rigid column would protect the cord perfectly and let you do nothing. A flexible one would let you move and would leave the cord unprotected. The spine is 33 short segments, and the compromise between them is why it works and why it fails.
The regions
Cervical (C1–C7) — the neck. Smallest bodies, greatest mobility. Every mammal has seven cervical vertebrae, from a mouse to a giraffe. The giraffe's are simply enormous; there are still seven of them. The rule is broken by only a handful of species, and it is a good illustration of how strongly a developmental pattern can be conserved.
Thoracic (T1–T12) — the chest. Each articulates with a pair of ribs, which restricts movement considerably.
Lumbar (L1–L5) — the lower back. Largest bodies, because they carry the most weight.
Sacrum (5 fused) — a solid triangular block wedged between the hip bones, transmitting the entire body weight to the pelvis.
Coccyx (3–5 fused) — the vestigial tail (Chapter 3.1). It anchors pelvic floor muscles and takes weight when you sit leaning back.
The counting is 7, 12, 5, 5, 4.
The curves, and why they alternate
A newborn's spine is a single C-shaped curve, concave forward — the shape it had in the womb. The adult curves develop in response to activity.
The cervical curve appears at about three months, when a baby starts lifting its head. The lumbar curve appears at around 12 months, when the child starts walking. Both are secondary curves, acquired; the thoracic and sacral curves are primary, present from the start.
The alternating curves act as a spring. A straight column transmits shock directly; a curved one flexes slightly and absorbs it. The mechanical result is that a curved column can resist several times the compressive load a straight one of the same material could, and it is why you can jump without your skull being hammered.
Three abnormal curvatures:
Kyphosis — excessive backward curve of the thoracic spine, the rounded upper back. Common in the elderly from vertebral compression fractures (Chapter 5.9), and the reason people lose height with age.
Lordosis — excessive forward curve of the lumbar spine.
Scoliosis — a sideways curve with rotation. Affects around 2 to 3 percent of adolescents, more often girls, and most cases have no identifiable cause. Screening looks for asymmetry when bending forward, because rotation makes the ribs on one side more prominent — the deformity is easier to see bent over than standing. Mild curves are observed, moderate curves braced, and severe curves operated on, the threshold being roughly 45 to 50 degrees.
A typical vertebra
Each vertebra has the same parts, sized differently by region.
The body — the thick cylindrical block at the front. This carries the load, and it grows larger going down the column.
The vertebral arch — the ring of bone behind the body. Together with the body it encloses the vertebral foramen, and the stack of foramina forms the vertebral canal containing the spinal cord.
Seven processes project from the arch: one spinous process pointing backward, which is what you feel down your back; two transverse processes pointing sideways; and four articular processes forming joints with the vertebrae above and below.
Those joints — the facet joints — are what limit movement, and they are angled differently in each region. In the neck they are near-horizontal, permitting rotation. In the thorax they are set to allow rotation but limit flexion. In the lumbar spine they face nearly front-to-back, which permits flexion and extension and almost completely blocks rotation.
That last fact is the mechanical reason for a very large number of back injuries. The lumbar spine is built to bend forward and back, not to twist. Lifting a heavy object while twisting loads the disc in exactly the direction the facet joints cannot protect, and it is the classic mechanism of disc herniation.
The two odd vertebrae
C1, the atlas, has no body at all — it is a ring. It supports the skull and its joint with the occipital bone allows the nodding "yes" movement.
C2, the axis, has a peg — the dens — projecting upward through the atlas's ring, and the atlas rotates around it. This is the "no" movement, and it accounts for about half of all neck rotation.
The dens is held against the atlas by a strong transverse ligament, and the spinal cord runs immediately behind it. If that ligament ruptures or the dens fractures, the peg can be driven backward into the cord at the level where the brainstem controls breathing. This is the mechanism of death in judicial hanging with a long drop, and in some high-speed injuries. It is why anyone with a suspected neck injury is immobilised until the neck is cleared, and why the assessment protocol for that is one of the most standardised in emergency medicine.
Rheumatoid arthritis can erode the same ligament, which is why patients with long-standing rheumatoid disease need their neck assessed before any procedure requiring the head to be manipulated for airway management. Chapter 21.5.
The intervertebral disc
Between every pair of vertebral bodies from C2 down to the sacrum sits a disc. There are 23 of them, and together they make up about a quarter of the spine's total height.
Two parts:
The nucleus pulposus at the centre — a gel about 80 percent water, and the remnant of the notochord (Chapter 4.4). It is incompressible, like any fluid.
The annulus fibrosus around it — 15 to 25 concentric rings of tough fibrocartilage, with the fibres in each ring running at about 60 degrees to the vertical and in the opposite direction to the neighbouring ring. This crossed-fibre arrangement resists twisting, and it is the same principle as the crossed collagen in an osteon (Chapter 5.1) and the plies in a tyre.
How it works: the disc is a hydraulic cushion. Compress it and the gel centre pushes outward in all directions, and the fibrous rings contain that pressure and convert it into tension. Load is therefore spread evenly across the whole vertebral surface rather than concentrated where the force was applied.
Discs have almost no blood supply. They are nourished by fluid moving in and out as the load changes — squeezed out during the day and drawn back in overnight when you lie down. This is why you are about 1 to 2 centimetres taller in the morning than in the evening, and why astronauts gain several centimetres in weightlessness.
It is also why discs heal so badly and why they degenerate with age. From around 30 onward the nucleus loses water content, the disc thins, and it can no longer distribute load properly.
Disc herniation
Pressure inside a lumbar disc depends heavily on posture, and the measured figures make the practical advice obvious. Taking standing upright as the baseline, lying down is roughly a quarter of it, sitting upright is around 40 percent higher, and sitting leaning forward while holding a weight is several times the standing value.
Herniation happens when the annulus tears and the nucleus pushes out. It almost always goes backward and slightly to one side, because the annulus is thinnest there and because a ligament reinforces the direct midline.
And immediately behind that point is a nerve root.
The symptoms are therefore not primarily back pain. They are pain, numbness, tingling and weakness travelling down the leg in the exact distribution of the compressed nerve root — sciatica. The level can be identified from the examination alone: an L5 root problem causes weakness lifting the foot and numbness on the top of the foot and the big toe; an S1 problem causes weakness pushing the foot down, numbness on the outer border and sole, and a lost ankle reflex.
Around 90 percent of disc herniations improve without surgery within 6 to 12 weeks, as the extruded material is gradually resorbed. Treatment is analgesia and staying active — bed rest makes outcomes worse, not better, which reversed decades of advice.
One presentation is a genuine emergency, and it is worth memorising. A large central herniation can compress the whole bundle of nerve roots below the cord's end. Cauda equina syndrome presents with:
- Loss of bladder control — usually retention first, then overflow incontinence.
- Numbness in the saddle area — the inner thighs, buttocks and genitals.
- Bilateral leg weakness or sciatica.
- Loss of bowel control or anal tone.
This needs surgical decompression within hours, because delay causes permanent incontinence and sexual dysfunction. Any back pain with new bladder symptoms or saddle numbness is an emergency department problem tonight, not a physiotherapy appointment next week. Every doctor is taught to ask about these specifically, and it is the single most important thing to know about back pain.
Where the spinal cord ends
The spinal cord does not run the full length of the spine. It ends at about the level of L1 or L2 in an adult.
The reason is developmental. In the fetus the cord fills the whole canal, but the vertebral column then grows faster, so the cord's lower end is left behind relative to the bones. The nerve roots that must still reach their exit points below therefore run downward inside the canal as a loose bundle — the cauda equina, horse's tail, which is what it looks like.
This anatomy is why a lumbar puncture is safe. A needle inserted between L3 and L4, or L4 and L5, enters a space containing cerebrospinal fluid and floating nerve roots that drift out of the way, with no cord to injure. The landmark used is the line joining the tops of the hip bones, which crosses at about L4.
And it is why spinal anaesthesia is given at the same level, and why an epidural in labour is placed in the space just outside the dural sac.
Spinal cord injury
The consequences are determined almost entirely by the level.
Above C3 — the phrenic nerve supply to the diaphragm is lost (Chapter 4.4) and the person cannot breathe unaided. Ventilation is permanent.
C4 to C5 — tetraplegia with diaphragm preserved. Breathing is possible; arms and legs are not.
C6 to C8 — some arm and hand function preserved, and the exact level makes an enormous practical difference: preserved wrist extension allows a passive grip, which is the difference between feeding yourself and not.
Thoracic — paraplegia, with trunk stability improving as the level descends.
Lumbar — varying leg weakness, bladder and bowel involvement.
Two things are true of all levels and are often overlooked. Bladder, bowel and sexual function are affected at every level, and patients consistently rate these as more important to quality of life than walking. And autonomic dysreflexia — a dangerous surge in blood pressure triggered by something as minor as a blocked catheter, in injuries above T6 — is a life-threatening emergency that is frequently missed because it looks like nothing much.
Complete versus incomplete matters enormously for prognosis. Any preserved sensation or movement below the level, including sensation around the anus, indicates an incomplete injury with meaningful recovery potential.
What the next page fixes
The spine carries the trunk, and attached to it is a cage that protects the heart and lungs while expanding and contracting twenty thousand times a day, plus a shoulder girdle that is barely attached to the skeleton at all. Chapter 5.4 covers the ribs, the sternum, the clavicle and the scapula, and why the arm's freedom of movement is bought at the price of the most frequently dislocated joint in the body.