The Cervix

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The cervix of the uterus is the tapered inferior region of the uterus. Its name, cervix, comes from the Latin word meaning "neck" due to its role as the narrow connection between the larger body of the uterus above the vagina below. The cervix plays vital roles in the control of movement into and out of the uterus, protection of the fetus during pregnancy, and the delivery of the fetus during childbirth.

The cervix makes up the lower one-third of the uterus and its tissues are continuous with the tissues that make up the rest of the uterus. Its outer covering is made of a simple squamous epithelium known as the perimetrium. Inside the perimetrium is the myometrium, a thick region of smooth muscle tissue that gives the cervix its ability to expand and contract. Compared to the rest of the uterus that contracts forcefully during childbirth, the myometrium of the cervix is thinner and used only as a sphincter to regulate the opening of the uterus. Lining the inside of the cervix is a thin layer of endometrium containing the epithelial cells that constantly produce cervical mucus.

The cervix can be broken down into several anatomically distinct regions.

• The cervical canal is the hollow orifice through the cervix that connects the uterine cavity to the hollow lumen of the vagina.
• Connecting the cervical canal to the lumen of the vagina is the external os, a small circular opening surrounded by the external tissue of the cervix. The tissue of the cervix surrounding the external os is rounded and convex, causing the external os to protrude slightly into the vagina.
• Connecting the cervical canal to the uterine cavity is the internal os, a small circular opening where the cervical canal narrows before opening into the uterus.

The cervix of the uterus acts as the gatekeeper of the uterus by controlling when substances can pass into and out of the uterus. To assist in this role, the epithelial lining of the cervix produces thick cervical mucus that fills the cervical canal and forms a mucus plug blocking the flow of material between the uterus and the vagina. Around the time of ovulation, the consistency of the cervical mucus becomes much thinner, allowing the passage of sperm into the uterus for fertilization. During pregnancy the cervix and its mucus plug protect the developing fetus by sealing the uterus from possible contamination by external pathogens.

During menstruation, the smooth muscle tissue in the myometrium of the cervix dilates to allow the passage of menstrual flow and may cause sensations of pain and discomfort known as menstrual cramps.

The process of childbirth requires the cervix to dilate to around ten centimeters in diameter in order to accommodate the head of the fetus as it passes into the birth canal. To achieve this feat, the cervix begins to dilate several days prior to the formal start of labor.