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The cervix or cervix uteri (''(ラテン語:neck of the uterus)'') is the lower part of the uterus in the human female reproductive system. In a non-pregnant woman, the cervix is usually between 2 and 3 cm long and roughly cylindrical in shape. The narrow, central cervical canal runs along its entire length, connecting the uterine cavity and the lumen of the vagina. The opening into the uterus is called the internal os and the opening into the vagina is called the external os. The lower part of the cervix, known as the vaginal portion of the cervix (or ectocervix), bulges into the top of the vagina. The cervix has been documented anatomically since at least the time of Hippocrates, over 2,000 years ago. The cervical canal is a passage through which sperm must travel to fertilize an egg cell after sexual intercourse. Several methods of contraception, including cervical caps and cervical diaphragms aim to block or prevent the passage of sperm through the cervical canal. Cervical mucus is used in several methods of fertility awareness, such as the Creighton model and Billings method, due to its changes in consistency throughout the menstrual period. During vaginal childbirth, the cervix must flatten and dilate to allow the fetus to progress along the birth canal. Midwives and doctors use the extent of the dilation of the cervix to assist decision-making during childbirth. The endocervical canal is lined with a single layer of column-shaped cells, while the ectocervix is covered with multiple layers of cells topped with flat cells. The two types of epithelia meet the squamocolumnar junction. Infection with the human papillomavirus (HPV) can cause changes in the epithelium, which can lead to cancer of the cervix. Cervical cytology tests can often detect cervical cancer and its precursors, and enable early successful treatment. Ways to avoid HPV include avoiding sex, using condoms, and HPV vaccination. HPV vaccines, developed in the early 21st century, reduce the risk of cervical cancer by preventing infections from the main cancer-causing strains of HPV. == Structure == The cervix is part of the female reproductive system. Around in length,〔 it is the lower narrower part of the uterus continuous above with the broader upper part—or body—of the uterus.〔 The lower end of the cervix bulges through the anterior wall of the vagina, and is referred to as the vaginal portion of cervix (or ectocervix) while the rest of the cervix above the vagina is called the supravaginal portion of cervix.〔 A central canal, known as the cervical canal, runs along its length and connects the cavity of the body of the uterus with the lumen of the vagina.〔 The openings are known as the internal os and external orifice of the uterus (or external os) respectively.〔 The mucosa lining the cervical canal is known as the endocervix and the mucosa covering the ectocervix is known as the exocervix. The cervix has an inner mucosal layer, a thick layer of smooth muscle, and posteriorly the supravaginal portion has a serosal covering consisting of connective tissue and overlying peritoneum.〔 In front of the upper part of the cervix lies the bladder, separated from it by cellular connective tissue known as parametrium, which also extends over the sides of the cervix. To the rear, the supravaginal cervix is covered by peritoneum, which runs onto the back of the vaginal wall and then turns upwards and onto the rectum forming the recto-uterine pouch.〔 The cervix is more tightly connected to surrounding structures than the rest of the uterus. The cervical canal varies greatly in length and width between women and over the course of a woman's life,〔 and can measure 8 mm (0.3 in) at its widest diameter in premenopausal adults.〔 It is wider in the middle and narrower at each end. The anterior and posterior walls of the canal each have a vertical fold, from which ridges run diagonally upwards and laterally. These are known as palmate folds due to their resemblance to a palm leaf. The anterior and posterior ridges are arranged in such a way that they interlock with each other and close the canal. They are often effaced after pregnancy.〔 The ectocervix has a convex, elliptical surface and is divided into anterior and posterior lips. The size and shape of the external opening and the ectocervix can vary according to age, hormonal state, and whether natural or normal childbirth has taken place. In women who have not had a vaginal delivery, the external os is a small circular opening, and in women who have had a vaginal delivery, the external os is slit-like. On average, the ectocervix is long and wide.〔 Blood is supplied to the cervix by the descending branch of the uterine artery and drains into the uterine vein.〔 The pelvic splanchnic nerves, emerging as S2–S3, transmit the sensation of pain from the cervix to the brain.〔 These nerves travel along the uterosacral ligaments, which pass from the uterus to the anterior sacrum.〔 Three channels facilitate lymphatic drainage from the cervix. The anterior and lateral cervix drains to nodes along the uterine arteries, travelling along the cardinal ligaments at the base of the broad ligament to the external iliac lymph nodes and ultimately the paraaortic lymph nodes. The posterior and lateral cervix drains along the uterine arteries to the internal iliac lymph nodes and ultimately the paraaortic lymph nodes, and the posterior section of the cervix drains to the obturator and presacral lymph nodes.〔 However, there are variations as lymphatic drainage from the cervix travels to different sets of pelvic nodes in some people. This has implications in scanning nodes for involvement in cervical cancer.〔 After menstruation and directly under the influence of estrogen, the cervix undergoes a series of changes in position and texture. During most of the menstrual cycle, the cervix remains firm, and is positioned low and closed. However, as ovulation approaches, the cervix becomes softer and rises to open in response to the higher levels of estrogen present. These changes are also accompanied by changes in cervical mucus,〔 described below. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Cervix」の詳細全文を読む スポンサード リンク
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