Iowa State University Veterinarian Records, RS 22/6/0/9, University Archives, Special Collections Department, Iowa State University, http://www.add.lib.iastate.edu/spcl/arch/rgrp/22-6-0-9.html
A standardbred Palamino yearling filly was brought to the Stange Memorial Clinic on January 3, 1945. Five to six weeks before admission the owner noticed that during urination a structure similar in color and consistency to the vulvar mucous membrane was protruded between the lips of the vulva. Following urination this structure would disappear. At about three weeks of age he noticed that the prolapsed tissue required more time after urination to disappear. About ten days ago he noticed the filly straining following urination and it now required two to three hours for the proIapsed tissue to disappear from sight. At no time did he attempt manual replacement of the prolapsed structure. The filly's straining increased and the prolapsed organ showed more redness so he decided to seek professional assistance.
Riley, G. E.
"Persistent Hymen in a Mare,"
Iowa State University Veterinarian: Vol. 7
, Article 8.
Available at: https://lib.dr.iastate.edu/iowastate_veterinarian/vol7/iss3/8