What structure serves as the ring of muscles at the neck of the uterus?

The structure that serves as the ring of muscles at the neck of the uterus is the cervix. The cervix is located at the lower end of the uterus and acts as a barrier, controlling the passage between the uterus and the vagina. Its muscular composition allows it to open and close, facilitating menstrual flow, supporting the developing fetus during pregnancy, and playing a vital role during childbirth by dilating to allow for passage through the birth canal.

In contrast, the ovary is involved in the production of eggs and hormones but does not have a muscular structure. The uterus is the organ that houses the developing fetus but does not serve as a muscular ring; it is more of a muscular chamber. The fallopian tubes are responsible for transporting eggs from the ovaries to the uterus but do not have a muscular ring function. Thus, the cervix uniquely fulfills the specific role described in the question.