Which structure acts as a barrier between the uterus and the vagina in horses?

The cervix serves as a crucial barrier between the uterus and the vagina in horses. It functions primarily to regulate the passage of substances between these two structures and plays a vital role in maintaining pregnancy. During certain phases of the reproductive cycle, the cervix will either open to allow sperm to enter the uterus or close to protect the uterus from bacterial contamination and external influences. This selective permeability is essential for both reproductive success and overall health.

In contrast, the uterus itself envelops the developing fetus but is not a barrier but rather the organ containing the embryo during pregnancy. The oviducts, also known as fallopian tubes, are the pathways through which ova travel from the ovaries to the uterus, thus they do not function as a barrier. The horns refer to the two extensions of the uterus but similarly do not act as a barrier between the vagina and uterus. Therefore, the cervix is the correct structure that fulfills this protective role.