How dangerous is a blow to the head?
As Michael Schumacher remains in a critical condition after a ski accident, experts warn that even a seemingly minor head injury can have major consequences.
Any knock to the head can be dangerous, although thankfully most are not.
The brain is a delicate, blancmange-like organ that is vulnerable.
Although it is protected by the skull, rapid forward, back or twisting movements of the head can send the brain crashing against this bony box.
Even mild injuries of this sort can produce diffuse injury.
How the brain is susceptible to damage after impact
At first there may be no noticeable damage - at least on the outside.
Indeed, people who have been in serious collisions may appear lucid and unscathed on first inspection.
But it can take up to 48 hours for symptoms to appear.
Anyone who has had a blow to the head should seek immediate medical attention regardless of how mild it might seem.
The force of the blow can cause the brain to twist, damaging nerves and tissue.
The skull's sharp interior ridges can tear arteries and veins running through the brain, allowing blood to leak.
As with any injured body part, there will be swelling and bruising too.
With nowhere to escape, the pressure can build and squeeze the brain.
This is a medical emergency.
Luke Griggs of the charity Headway says: "There is no hard and fast rule with head injuries.
"A seemingly innocuous injury can be fatal, as we have seen before with the sad situation with Natasha Richardson."
Liam Neeson's wife died from head injuries sustained in a skiing accident. She initially showed no sign of injury but about an hour later was taken to hospital after feeling unwell.
Mr Griggs says: "With severe head injuries one of the difficulties we have got is that the symptoms can be delayed.
"Bruising and swelling does not happen immediately and it may take time for the damage to occur.
"That's why it is important to seek medical help, even if there are no symptoms."
Doctors say people who have had a head injury should be monitored for at least two days for any signs that their condition might be worsening.
Any drowsiness or loss of consciousness, balance problems, vomiting or severe headache is a warning that should not be ignored.
Dr Mike Langran, a GP and expert in ski injury research, said most ski resorts had medics on hand to deal promptly with these sorts of injuries.
"Accidents of this nature are, thankfully, rare events amongst skiers and snowboarders although of course they usually receive substantial media attention. As with any recreational activity however, it is impossible to completely remove all elements of risk when participating in snow sports."
He said wearing a helmet could help skiers to avoid or lessen such injuries.
Michael Schumacher remains in a critical condition following his skiing accident on Sunday.