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The uterus (from Latin "uterus", plural ''uteri'') or womb is a major female hormone-responsive reproductive sex organ of most mammals, including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, (uterine tubes) depending on the species. It is within the uterus that the fetus develops during gestation, usually developing completely in placental mammals such as humans and partially in marsupials such as kangaroos and opossums. Two uteri usually form initially in a female and usually male fetus, and in placental mammals they may partially or completely fuse into a single uterus depending on the species. In many species with two uteri, only one is functional. Humans and other higher primates such as chimpanzees, usually have a single completely fused uterus, although in some individuals the uteri may not have completely fused. Horses, on the other hand, have bipartite uteri. In English, the term ''uterus'' is used consistently within the medical and related professions, while the Germanic-derived term ''womb'' is more common in everyday usage. Most animals that lay eggs, such as birds and reptiles, including most ovoviviparous species, have an oviduct instead of a uterus. Note however, that recent research into the biology of the viviparous (not merely ovoviviparous) skink ''Trachylepis ivensi'' has revealed development of a very close analogue to eutherian mammalian placental development. In monotremes, mammals which lay eggs, namely the platypus and the echidnas, either the term ''uterus'' or ''oviduct'' is used to describe the same organ, but the egg does not develop a placenta within the mother and thus does not receive further nourishment after formation and fertilization. Marsupials have two uteri, each of which connect to a lateral vagina and which both use a third, middle "vagina" which functions as the birth canal. Marsupial embryos form a ''choriovitelline'' "placenta" (which can be thought of as something between a monotreme egg and a "true" placenta), in which the egg's yolk sac supplies a large part of the embryo's nutrition but also attaches to the uterine wall and takes nutrients from the mother's bloodstream. == Structure == The uterus is located inside the pelvis immediately dorsal (and usually somewhat rostral) to the urinary bladder and ventral to the rectum. The human uterus is pear-shaped and about 3 in. (7.6 cm) long, 4.5 cm broad (side to side) and 3.0 cm thick (anteroposterior).〔Manual of Obstetrics. (3rd ed.). Elsevier 2011. pp. 1–16. ISBN 9788131225561.〕 A nonpregnant adult uterus weighs about 60 grams. The uterus can be divided anatomically into four segments: The fundus, corpus, cervix and the internal os. === Regions === From outside to inside, the path to the uterus is as follows: * Cervix uteri – "neck of uterus" * * External orifice of the uterus * * Canal of the cervix * * Internal orifice of the uterus * corpus uteri – "Body of uterus" * * Cavity of the body of the uterus * * Fundus (uterus) 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「uterus」の詳細全文を読む スポンサード リンク
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