The speculum, otherwise known as the "gag", is used for opening the horse's mouth.  Without this key piece of apparatus, comprehensive examination and treatment would be extremely difficult.  There are a variety of speculums - my personal preference is the Houseman's gag.  The ratchet system allows for easy opening and closing as well as ensuring the least amount of discomfort for the horse.  It is easy to disassemble and allows for interchanging of parrot plates (for parrot-mouthed horses) and gum plates.