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The hymen is a membrane that surrounds or partially covers the external vaginal opening. It forms part of the vulva, or external genitalia, and is similar in structure to the vagina.〔Emans, S. Jean. "Physical Examination of the Child and Adolescent" (2000) in ''Evaluation of the Sexually Abused Child: A Medical Textbook and Photographic Atlas'', Second edition, Oxford University Press. 61-65〕 In children, although a common appearance of the hymen is crescent-shaped, many shapes are possible.〔 The effects of sexual intercourse and childbirth on the hymen are variable. If the hymen is sufficiently elastic, it may return to nearly its original condition. In other cases, there may be remnants (carunculae myrtiformes), or it may appear completely absent after repeated penetration. Additionally, the hymen may be lacerated by disease, injury, medical examination, masturbation or physical exercise. For these reasons, the state of the hymen is not a conclusive indicator of virginity.〔〔 ==Development and histology== The genital tract develops during embryogenesis, from the third week of gestation to the second trimester, and the hymen is formed following the vagina. At week seven, the urorectal septum forms and separates the rectum from the urogenital sinus. At week nine, the Müllerian ducts move downwards to reach the urogenital sinus, forming the uterovaginal canal and inserting into the urogenital sinus. At week twelve, the Müllerian ducts fuse to create a primitive uterovaginal canal called unaleria. At month five, the vaginal canalization is complete and the fetal hymen is formed from the proliferation of the sinovaginal bulbs (where Müllerian ducts meet the urogenital sinus), and becomes perforate before or shortly after birth. The hymen has no nerve innervation. In newborn babies, still under the influence of the mother's hormones, the hymen is thick, pale pink, and redundant (folds in on itself and may protrude). For the first two to four years of life, the infant produces hormones that continue this effect.〔McCann, J; Rosas, A. and Boos, S. (2003) "Child and adolescent sexual assaults (childhood sexual abuse)" in Payne-James, Jason; Busuttil, Anthony and Smock, William (eds). Forensic Medicine: Clinical and Pathological Aspects, Greenwich Medical Media: London, a)p.453, b)p.455 c)p.460.〕 Their hymenal opening tends to be annular (circumferential). Past neonatal stage, the diameter of the hymenal opening (measured within the hymenal ring) widens by approximately 1 mm for each year of age.〔 During puberty, the hymenal opening can also be enlarged by tampon or menstrual cup use, pelvic examinations with a speculum, regular physical activity or sexual intercourse.〔 Once a girl reaches puberty, the hymen tends to become very elastic. In one survey, only 43% of women reported bleeding the first time they had intercourse, indicating that the hymens of a majority of women are sufficiently open to prevent tearing.〔〔 Prepubescent girls' hymenal openings come in many shapes, depending on hormonal and activity level, the most common being crescentic (posterior rim): no tissue at the 12 o'clock position; crescent-shaped band of tissue from 1–2 to 10–11 o'clock, at its widest around 6 o'clock. From puberty onwards, depending on estrogen and activity levels, the hymenal tissue may be thicker, and the opening is often fimbriated or erratically shaped.〔 In younger children, a torn hymen will typically heal very quickly. In adolescents, the hymenal opening can naturally extend and variation in shape and appearance increases.〔 A glass or plastic rod of 6 mm diameter having a globe on one end with varying diameter from 10 to 25 mm, called Glaister Keen rod, is used for close examination of the hymen or the degree of its rupture. In forensic medicine, it is recommended by health authorities that a physician who must swab near this area of a prepubescent girl avoid the hymen and swab the outer vulval vestibule instead.〔 In cases of suspected rape or child sexual abuse, a detailed examination of the hymen may be performed, but the condition of the hymen alone is often inconclusive.〔 After giving birth, the vaginal opening usually has nothing left but hymenal tags (carunculae myrtiformes) and is called ''parous introitus.'' 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「Hymen」の詳細全文を読む スポンサード リンク
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