A TV program did an experiment a few years ago. They determined that the Aztecs and Maya most likely an incision below the sternum, reached under the breast plate and pulled out the heart. This would be much easier than trying to cut through the ribs with an obsidian knife. However, their goal was to extract the still beating heart to offer to the gods. The body was of no importance and was thrown down the stairs of the temple.
If the goal is to kill the victim to offer their life to the gods or Satan or whatever, and a metal knife is being used, then a stab into the chest would be effective; although, if the blade is long enough, stabbing under the sternum into the heart is the best method.
Some of these pictures show throat slitting. This is the traditional method of animal sacrifice in many cultures, including ancient Judaism. The blood is collected in a bowl and poured over the altar. It's the technique I chose for my depiction of the sacrifice of Jephthah's daughter.
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