That is called the cervix.
internal OS
Although, there is also the cervical cavity, but since you were asking for the "main" cavity, I would say it's the uterine cavity.
The uterine cavity is the inner space of the uterus where fertilized eggs implant and develop into fetuses during pregnancy. It is lined with a specialized tissue called endometrium, which thickens and sheds during each menstrual cycle if pregnancy does not occur. The uterine cavity plays a crucial role in supporting the growth and development of a fetus.
Sperm must penetrate through the cervical mucus, the cervix, the uterine cavity, and the fallopian tubes to reach the egg for fertilization.
uterine agenesis
The cervical spine, or the neck.
A gastrovascular cavity is a digestive sac with only a single opening. A complete digestive tract (otherwise known as an Alimentary canal) has two openings (a mouth and anus) between its digestive tubes unlike the gastrovascular cavity in which food is ingested and eliminated through the same opening (example would be hydra)
Nourished by glycogen rich uterine secretions. -Pearson Text Book
A tiny ball of 16 cells found freely floating in the uterine cavity is called a blastocyst. It is an early stage of development in the embryo.
Intrauterine part is the Intestitial part: passes through the uterine muscle to the uterine cavity Extrauterine part consists of some segments : Fimbrial, Infundibular, Ampullary and the Ishtmic. Ishtmic portion is the meet point between intra, and extra. Isthmus means neck
D & C (Dilatation and Curettage.)
eustachian tube it starts from the middle ear cavity and terminates on nasopharynx.