The cervix (pl.: cervices) or uterine cervix (Latin: cervix uteri) is a dynamic fibromuscular sexual organ of the female reproductive system that connects the vagina with the uterine cavity. The human female cervix has been documented anatomically since at least the time of Hippocrates, over 2,000 years ago. The cervix is approximately 4 cm (1.6 in) long with a diameter of approximately 3 cm (1.2 in) and tends to be described as a cylindrical shape, although the front and back walls of the cervix are contiguous. The size of the cervix changes throughout a female's life cycle. For example, females in the fertile years of their reproductive cycle tend to have larger cervixes than postmenopausal females; likewise, females who have produced offspring have a larger cervix than those who have not.
In relation to the vagina, the part of the cervix that opens into the uterus is called the internal os while the opening of the cervix into the vagina is called the external os. Between those extremes is the conduit commonly called the cervical canal. The lower part of the cervix, known as the vaginal portion of the cervix (or ectocervix), bulges into the top of the vagina. The endocervix borders the uterus. The cervical conduit has at least two types of epithelium (lining): the endocervical lining is glandular epithelia that lines the endocervix with a single layer of column-shaped cells; while the ectocervical part of the conduit contains squamous epithelium. Squamous epithelia line the conduit with multiple layers of cells topped with flat cells. These two linings converge at the squamocolumnar junction (SCJ). This junction changes location dynamically throughout a female's life. The cervix is the organ that allows epithelia to flow from a female's uterus and out through her vagina at menstruation. Menstruation releases epithelia from a female’s uterus with every period of her fertile years, unless pregnancy occurs.