Great site about obstructed defecation. Includes flash movies that show different prolapses and bowl movements really clearly:

http://www.obstructed-defaecation.com/

NHS information about pelvic organ prolapse:

http://www.nhs.uk/Conditions/Prolapse-of-the-uterus/Pages/Introduction.aspx

Voices for Pelvic Floor Dysfunction – community site about PFD:

http://www.voicesforpfd.org/

Really good pelvic floor physio website:

www.pelvicphysiotherapy.com