Attack on Titan / Shingeki No Kyojin

[John Cribati]

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Or rather, where their spinal cord and brain would be. At least the anime keeps making a big deal about how it's one meter by ten centimeters.

That was actually my first guess; That's approximately where the brain-stem would be on an actual human. Since it controls almost all of the body's involuntary processes, I just assumed it included the titans' regenerative properties.