One of the major reasons AI (artificial insemination) is more difficult in sheep, than it is in other livestock, is the anatomical structure of the ewe's cervix.  The cervix is 4-7 cm long and connects the uterus to the anterior vagina.  The interior wall of the cervix has a number of ridges and crypts which helps to keep infection out.  In the ewe, however, these folds are more pronounced, irregular in shape, and fit so closely together that an inseminating pipette is virtually impenetrable.