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There are 20 feet of blood vessels in each square inch of human skin.
It is believed that the Incas used anesthesia. Evidence supports the theory that shamans chewed cocoa leaves and drilled holes into the heads of patients (letting evil spirits escape), spitting into the wounds they made. The mixture of cocaine, saliva, and resin numbed the site enough to allow hours of drilling.
Intradermal injections are somewhat difficult to correctly administer because the skin layers are so thin that it is easy to accidentally punch through to the deeper subcutaneous layer.
The human body produces and destroys 15 million blood cells every second.
After 5 years of being diagnosed with rheumatoid arthritis, one every three patients will no longer be able to work.