Originally answered by K9dogluvr.....
"Internal decapitation is the breaking of the spinal cord without the loss of one's head, breaking the essential nerves controlling the body. It is commonly seen in car accidents in which there was no headrest or one that was not properly adjusted."
While the spinal cord is often completely severed due to this injury, that is actually a secondary condition resulting from the internal decapitation, not the definition of the term. Internal decapitation with severence or other immediate spinal cord trauma is actually the intent of hanging a person. If hanging is used to exceute a person, the body's weight actually causes this particular injury, often along with separating the spinal cord from the brain stem in the process, which is what causes the desired instant death.
The spinal cord is attached to the brain by way of the brain stem, and usually sits within the spinal column in a bath of cerebrospinal fluid. The spinal cord actually does nothing to hold your head to your body, since any pulling on or compression of the cord itself can cause devestating problems. What actually holds your head to your neck is a combination of tendons, ligaments, muscles and other soft tissues that connect the base of the skull to the two top verterbrae. This two bones are shaped differently than the rest of the verterbrae and provides you with the ability to turn your head without paralyzing yourself.
The true clinical definition of internal decapitation actually involves the tendons and ligaments that hold the skull to the top of the spinal column. Essentially what happens is that the shearing forces caused by a trauma to the upper neck cause a type of dislocation of the neck at the worst possble place. One way to think about it is that following this injury the only thing that may be holding a person's head on is skin and the spinal cord, along with some muscles and other soft tissues. Any movement at all could cause tearing, compression, or other trauma to the spinal cord and or brain stem, leading to devestating neurological consequences or death.
Believe it or not there have been several people who have survived an internal decapitation. If the condition is quickly suspected or diagnosed, surgical intervention can reattach the skull to the neck, and provided the spinal cord remained intact meaningful recovery is possible. I believe there have been 6 known cases of recovery from internal decapitation, and more than one of those cases resulted in a full recovery with no long term neurological damage.
Overall, internal decapitation is an exceedingly rare injury that requres the "perfect storm" of circumstances to occur. Just another example of how amazingly resilient the human body can be. :)
Sorry to be long winded, but I am fascinated by medical science.