What this organ is, where it runs, what it is made of inside, and which paths the signal takes. Without this the rest of the map does not read.
This is educational context. It is not a diagnosis, not a prognosis, not a treatment plan and not advice about any individual person. For anything urgent, seek professional help rather than an answer from a website.
The brain in profile with its folds, the cerebellum behind it and the brainstem narrowing into the spinal cord, which runs down inside the vertebral canal through thirty-three bones and ends in a cone above the small of the back. Two arrows along it: commands travelling down, sensation travelling up. Beside it, a magnified window on the last of those bones, where the cone ends and the loose bundle of roots continues below it.
A long column of nervous tissue running from the base of the brain down into the lower back. Not a nerve and not bone: it is part of the central nervous system, and nearly all traffic between brain and body goes through it.
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Where it runs
Inside the vertebral canal, a tunnel formed by the vertebrae. Between the bone and the cord itself are the meninges and the fluid it floats in.
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The link between brain and body
Commands for movement travel down; sensation travels up. Both streams run in the same column, which is why one injury can change movement and feeling at the same time.
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Where it begins and ends
It begins at the base of the skull and, in an adult, ends at about the first or second lumbar vertebra. Below that the canal holds not cord but a bundle of roots.
What may change after an injuryThe level in the bone and the level in the cord are different numbers. Below the end of the cord it is roots that are injured, and that is a different picture from an injury to the cord itself.
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The brain
The organ inside the skull where a command to move begins and where a signal from the body becomes a sensation. The spinal cord continues it downwards; it is not a separate organ.
The brainstem
The part between the brain and the spinal cord. Every long pathway passes through it, and it is where most of them cross to the opposite side of the body.
Where this goes in the corpus
Nothing in this corpus is filed under the axes of this plate yet.
This plate explains the axes the corpus is indexed by. None of them holds a record here today, which is a fact about what has been read so far and not about the science.
The whole vertebral column in profile along the curves it actually has, every bone drawn with its body, its arch around the canal and its spinous process, labelled C1 to the coccyx, with thirty-one pairs of nerve roots leaving it, the cord ending in a cone and the cauda equina below. Beside it, a ring of the three coverings — dura, arachnoid and pia — with the fluid space between them.
Eight segments, C1–C8. The nerves to the diaphragm, the shoulders and the arms leave from here.
What may change after an injuryAn injury at this level can affect the arms and breathing, not only what is below. The higher the level, the more of the body lies below it.
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Thoracic region
Twelve segments, T1–T12. The nerves to the chest, the abdomen and the intercostal muscles leave from here.
What may change after an injuryThe arms are usually spared; the level decides how much trunk control and seated balance remain.
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Lumbar region
Five segments, L1–L5. The nerves to the hips and legs leave from here.
What may change after an injuryMostly the legs. Bladder and bowel function can change too, because their control comes from segments lying alongside and below.
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Sacral region
Five segments, S1–S5. The nerves to the feet, the bladder, the bowel and sexual function leave from here.
What may change after an injuryThe lowest sacral segments decide the classification: feeling or a contraction preserved exactly there is what a clinician calls sacral sparing.
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Cauda equina
The bundle of nerve roots running in the canal below the end of the cord. These are peripheral nerves, not the cord itself.
What may change after an injuryAn injury here is described as a root injury. That is a different picture from an injury to the cord, and the two are worth keeping apart.
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The meninges
Three layers around the cord — dura, arachnoid and pia mater. Between them is fluid that works as a cushion.
The coccygeal region
The lowest part of the spine — three to five fused vertebrae, the tailbone. There is no spinal cord here at all: it ends much higher up, and only nerve roots continue downwards.
Conus medullaris
The cone-shaped end of the cord at about L1–L2. The sacral segments are packed into that small volume.
The vertebral canal
The tunnel formed by the openings of the vertebrae stacked one on another. The spinal cord lies inside it, and the bone is what protects it.
Dura mater
The outermost of the three coverings: dense and tough. It separates the cord from the bone of the canal and holds the shape of a sac around it.
Arachnoid mater
The middle covering, thin and translucent. Beneath it is the space filled with fluid, in which the cord effectively floats.
Pia mater
The thinnest, innermost covering. It lies directly on the tissue of the cord and follows every groove in it.
Cerebrospinal fluid
The clear fluid in the space beneath the arachnoid. It bathes the brain and the cord and cushions them against a jolt.
Thirty-one pairs of spinal nerves
Thirty-one pairs of nerves leave the cord, one pair per segment, left and right. Each pair serves its own strip of skin and its own muscles.
The cord in cross-section at reading size: butterfly-shaped grey matter with its dorsal, lateral and ventral horns; white matter around it; the groove behind and the fissure in front; the sensory rootlets entering behind through their ganglion and the motor rootlets leaving in front, joining as one spinal nerve that then divides. Two arrows: sensation in, movement out.
The butterfly-shaped centre. Nerve cell bodies sit here: the signal arrives, is processed, and a command leaves.
What may change after an injuryCells lost here do not come back on their own. That is what one direction of research aims at — rebuilding tissue, not rebuilding a wire.
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White matter
The ring around the grey matter. Long fibres in a myelin sheath — the wires the signal travels up and down.
What may change after an injuryA fibre cut here does not regrow on its own: the scar and the chemistry around it prevent it. Regrowing the fibre and routing around the gap are two different directions.
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Dorsal horn
The back part of the butterfly. Sensory information from the body arrives here.
What may change after an injuryChanges here are about feeling and about pain, including pain that appears later.
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Ventral horn
The front part of the butterfly. The motor neurons that drive muscles directly sit here.
What may change after an injuryDamage here affects the muscles of that very level, not only everything below it.
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Spinal nerve
The dorsal and ventral roots join and leave the spine as one nerve carrying both sensory and motor fibres. There are 31 pairs.
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The cross-section
Cut across, the cord shows butterfly-shaped grey matter in the middle and white matter around it. That arrangement is why one injury at one level affects different functions differently.
Lateral horn
The side projection present in the thoracic and upper lumbar cord. Autonomic control leaves from here: blood pressure, temperature, internal organs.
Dorsal root
The bundle of fibres entering the cord from behind, carrying sensation from the body inward.
Dorsal root ganglion
The swelling on the dorsal root where the cell bodies of the sensory neurons sit.
Ventral root
The bundle of fibres leaving the cord in front, carrying motor commands out to the muscles.
The same cross-section with three territories filled in — the dorsal columns behind, the anterolateral tract in front and to the side, the corticospinal tract behind it — and beside it a schematic of brain, brainstem and cord in which each of the three is followed as a line to the height at which it crosses to the other side.
An ascending path in the back of the white matter. It carries fine touch, vibration and the sense of where a limb is; it crosses to the other side in the brainstem, not in the cord.
What may change after an injuryLosing the sense of position makes movement possible but not usable — those are different things.
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Spinothalamic tract
An ascending path carrying pain and temperature. It crosses to the other side almost at once, within a segment or two of entering.
What may change after an injuryBecause it crosses early, damage to one half of the cord takes pain away on the opposite side of the body — the pattern a named syndrome describes.
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Corticospinal tract
A descending path from the motor cortex to the motor neurons. It carries voluntary movement and crosses to the other side in the brainstem.
What may change after an injuryThis is the path people mean when they say the command does not arrive. Routing around a break in it is a research direction of its own.
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Where a pathway crosses
Every long pathway crosses to the opposite side somewhere along its course. That is why damage on one side can change sensation or movement on the other.
This layer names the axes the corpus is indexed by. Where an axis holds no records, the panel says so and says what would change it — the gap is part of the answer.
The same corpus, entered a different way: by question, by place, by connection, and by what changed recently. Every one of these reads the records this step has been pointing at.
124 medical statements · 6 authorities · 124 stand on an address that answered · 0 have been read against the source by a person
The last of those numbers is nought, and it is printed rather than left to be assumed. These explanations are ordinary anatomy and ordinary clinical vocabulary, written for this product, and each one names the authority it stands on; nobody has yet sat down and compared them, sentence by sentence, with the text at those addresses.