The cervix is the central part of the female reproductive system. It is the part of the uterus that opens into the vagina. It secretes a mucus called cervical mucus that protects the uterus from infection and plays a role in reproduction. Cervical mucus is thinner at the time of ovulation to facilitate the passage of sperm and promote fertilization; during the rest of the cycle, the cervical mucus is thicker and blocks the passage of spermatozoa. The cervix is composed of 2 parts: the ectocervix, which is the lower part near the vagina, and the endocervix, which is the upper part near the uterus. At the border of the endocervix and the ectocervix is the junction zone. This is where most cancers start. The cervix is completely covered by a mucous membrane, which consists of a surface tissue called epithelium and a connective tissue deep inside. The border between the two tissues is called the basement membrane.