Cervical specula consists of tubes or bi-valve instruments, mounted on shanks, or handles, of sufficient length to admit of introduction through or by means of a vaginal speculum. They are little employed except as a shield to prevent instrumental injury to the cervix, or misapplication of caustics intended for points within the fundus. Vaginal specula are used to spread, retract or dilate the vagina, either for purposes of examination or for treatment. It is also used for anal surgery, although several other forms of anal specula exist. One common form, the sigmoidoscope, resembles a tube that has a removable bullet shaped insert. When the speculum is inserted into the rectum, the insert dilates the rectum to the diameter of the tube. The insert is then removed, leaving the tube to allow examination of the rectum.