Back to the whole execution thing (now that I've had a chance to catch up on this), I'm surprised that people are calling the guillotine humane. I'm pretty sure a cleanly severed head stays conscious for long enough to be horrifying.
I think the only way to approach execution humanely is to first have the subject completely knocked out, and then ensure as instant destruction of the brain as possible.
There have been some claims circulating but it's frequently dubious sourcing. For example, there's a story about this French lady who murdered a journalist and the executioner held her head up and slapped it and witnesses claimed her eyes looked at him and her face had an indignant expression. But,
1: The source may just be someone writing about it around that time,
2: People regularly invent memories about things that everyone else said happened (such as X or Y television personality saying "that ought to hold the little shits" after believing the cameras were turned off),
3: It's just the kind of story you would expect from Revolution-era France,
4: There was a lot of curiosity about whether the head would remain conscious, meaning everyone would be excited to read stories about it - meaning there was a very clear profit motive for anyone fabricating such a story.
We also have contradicting accounts of various stories such as executioners asking the victim to blink after beheading, where one account says he did for 30 seconds and another says he never did. Doctors claiming the eyes moved in response to calling the victim's name up to an hour after death - which is just stupid and impossible from the standpoint of what we know today, but may have seemed possible to a liar back then. We have a lot of inconsistency and a lot of outright falsehood and it's tough to sort the wheat from the chaff.
Being cut off from the blood supply, and thus oxygen, knocks a brain out very quickly. Consider that when you hold your breath, your blood still holds oxygen, and is still circulating it. When you're strangled, the blood flow is not cut off perfectly and the brain still holds some blood. In decapitation the blood flow stops completely and instantly and the brain's blood also begins to drain out. It's also reasonable to assume that the shock and trauma could contribute to a blackout. Fear itself may also contribute.
Apparently there should be a split second where the victim can feel the blade cutting, and possibly up to a couple seconds after that. But it's reasonable to assume that there's a problem with motor control and perhaps some muscles spasm automatically, and is blinking on command really the most important thing to the victim at that moment? So it's hard to tell what's going through a person's head in the seconds after beheading (an axe! har har).
However, apparently it was the concern that the victim feels the pain of the death that made it unacceptable as a form of execution. I suspect someone could invent a quicker machine, but that's probably a tough pitch to the shareholders at GM.
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I think the reason behind the humane execution thing is similar to why we don't televise it, and why the perp's victims don't get to perform the execution or choose the method. It's supposed to be equivalent to destruction of a mad dog, a sad event but one that must be done for everyone's safety. By stripping it as much as possible of anger, by making it as painless as possible, it makes the executioner (and the people in charge telling him to do it) feel that the execution is not an act of murder.
Which is kinda weird.