The circulatory system delivers oxygen and other necessary particles via blood to the brain so that it can carry out its necessary functions. Deprived of oxygen or blood, the brain's function deteriorates rapidly. Circulation takes place in a closed system based on a pressurized environment; blood is pumped in and out of the heart and past the lungs, where it is refreshed once more. Decapitation opens this closed system irrevocably, causing a full and massive drop in blood pressure, leaving the brain starved of both blood and oxygen.
Depending upon how the head is removed from the body, this loss of blood and ultimately consciousness can take longer in some modes of decapitation than in others. Several blows to the back of the neck with a sword or axe can lead to blood loss before the head is fully severed from the body. But the guillotine's design in particular makes severing the head cleaner and quicker. The blade and mouton (weight) assembly of the guillotine weighed more than 175 pounds (80 kilograms) and was dropped from a height of 14 feet (4.3 meters) from ground level onto the back of the victim's neck [sources: Guillotine.info, Davies].
Moreover, the guillotine's blade was set within a track leading in a direct line down to the back of the victim's neck, improving the chances that a head will drop rather than be sent flying toward the crowd. A wooden screen called a shield further prevented any potential trajectory for a flying head. Instead, the victim's head generally went into the basket situated handily beneath the victim's head.
This made for quick and easy retrieval of the head by the executioner -- who merely pulled a lever -- after it was cut off. Picking up the head to show to the crowd was customary, and occasionally the executioner showed disrespect to the head as well. This was the case with Charlotte Corday, a woman executed by guillotine in France in 1793 after she assassinated the revolutionary leader Jean-Paul Marat.
After her head was severed, the executioner smacked its cheeks while he held it aloft. To the astonishment of the crowd, Corday's cheeks flushed and her facial expression changed into the "unequivocal marks of indignation" [source: Ernle, et al].
Corday was the first, though not the last, severed head reported to show the signs of consciousness following decapitation.