These tubes are, believe it or not, mentioned in Othello. Shakespeare, however, was, as we all well know, a very dirty old man. See porn in Hamlet if you don't believe me. There is also an oblique reference to these in 'The Flea' (by John Dunne, a comparatively famous metaphysical poet, and a contemporary of Shakespeare's).
However, their true purpose was (I am assured by my English teacher, who, being a filthy minded Freudian, is certain to know) not for bowel enemas and colonnic irrigation, as Webster suggests, but for cleansing of the female genitalia. From inside. These devices were shaped, well, rather as you would expect them to be. And water was pumped up the inside. With warm water, this vaginal douche would be similar to being on the recieving end of a man's orgasm. For this reason, the device was often used to 'teach' virgins.
I swear I am not making this up.