In other ways the uterus is a sandwich, a muscle hero. The cervix and fundus are both composed of three tissue types. The meat in the middle is the thick myometrium, built of three interwrapping sheets of muscle. On the outside of the myometrium is a slick covering, the serous membrane, which is similar in texture and function to the sacs surrounding the heart and lungs. Like those sacs, the uterine serous membrane keeps the organ wet and cushioned.
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On the other side of the myometrium is the uterine lining, the endometrium. The body likes to work in threes, and so the endometrium is made of three layers of mucous membrane. Unlike serous membrane, mucous membrane breathes and snorts and secretes. It absorbs water, salts, and other compounds. It releases mucus, a mixture of white blood cells, water, the sticky protein known as mucin, and cast-off tissue cells. Menstruation is in part a mucus discharge. During menstruation, two of the mucous sheaths are shed, thence to be reconstructed when the cycle begins anew. Like one who has reached enlightenment, the third, deepest endometrial layer escapes the wheel of death and rebirth, and it is to this stable foundation that a placenta moors itself if a fetus should be favored with a home.
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Hippocrates thought that the womb wandered, and he meant wandered , took a transcorporeal journey up to the breastbone, even to the throat, becoming particularly frantic when it wasn't fed on a regular basis with semen. (By Hippocrates' estimation, the uterus of a whore would be far calmer than that of a virgin.) He was wrong, of course, but that does not mean the uterus is an immobile stone. In fact, it is springy and fungible. It is held loosely in its pelvic girdle by six ligaments, flexible bands of fibrous tissue that offer support for the organ and also enclose the blood vessels that nourish it. The position of the uterus shifts in the pelvis depending on whether you're prone or upright, your bladder is full or empty, and other such unremarkable circumstances. If you're sitting down now, not particularly in need of a toilet break, and not pregnant, your uterus is probably tipped slightly forward, its fundus leaning toward a spot an inch or two above your pubis, that hard bone in your crotch. If you were to stand up, again with an empty bladder, and push your shoulders back with military crispness, your uterus would assume a nearly horizontal position, like a pear that's fallen over.
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The uterus is at its most physiologically flamboyant when preg-
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