Plate from 18th century encyclopedia showing illustrations of various surgical instruments for lithotomy and trepanning. Lithotomy is a surgical method for removal of calculi, stones formed inside certain hollow organs, such as the kidneys (kidney stones), bladder (bladder stones), and gallbladder (gallstones), that cannot exit naturally through the urinary system or biliary tract. Trepanning, also known as trephination, trephining or making a burr hole, is a surgical intervention in which a hole is drilled or scraped into the human skull, exposing the dura mater to treat health problems related to intracranial diseases. It may also refer to any "burr" hole created through other body surfaces, including nail beds. It is often used to relieve pressure beneath a surface. A trephine is an instrument used for cutting out a round piece of skull bone. Also shown is a tooth drawing instrument.