The Aztec and Inca caught on pretty quick that the new comers didn't fight by their rules-getting prisoners for sacrifices. And the surrounding peoples, such as the Tlaxcala nearly defeated the Spanish on first contact. It was against the Tlaxcala that the report of the severed horse head is written.
The sad truth is, obsidian simply shatters when slammed into steel and if that weren't enough, the favored weapon of the Mezo world, the "machuatl", was made of wood. It was designed like a saw, so, a power man could chop and pull with would actually 'saw' the head off a horse.
I believe the Aztec, Maya, and Inca were performing quite elaborate surgeries with obsidian tools before the conquest.
Also**The Nazca culture apparently practiced many different types of surgery. They seemed to have been very successful at brain surgery and used gold and silver implements to drill, cut and suture brain tissue. From the examination and autopsies of mummies it appears that the Nazca culture had a success rate of brain surgery similar to what we have now. The Nazca culture also practiced C-sections, set bones and performed internal medicine and surgery.**(quote)
