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buttocks
The buttocks (singular: ''buttock'') are two rounded portions of the anatomy, located on the posterior of the pelvic region of apes (including humans), and many other bipeds or quadrupeds, and comprise a layer of fat superimposed on the gluteus maximus and gluteus medius muscles. Physiologically, the buttocks enable weight to be taken off the feet while sitting. In many cultures, they play a role in sexual attraction. Many cultures have also used them as a target for corporal punishment. There are several connotations of buttocks in art, fashion, culture and humor, and the English language is replete with many popular synonyms. In humans the buttocks are located between the lower back and the perineum.
==Anatomy==
The buttocks are formed by the masses of the gluteal muscles or "glutes" (the gluteus maximus muscle and the gluteus medius muscle) superimposed by a layer of fat. The superior aspect of the buttock ends at the iliac crest, and the lower aspect is outlined by the horizontal gluteal crease. The gluteus maximus has two insertion points: superior portion of the linea aspera of the femur, and the superior portion of the iliotibial tractus. The masses of the gluteus maximus muscle are separated by an intermediate intergluteal cleft or "crack" in which the anus is situated.
The buttocks allow primates to sit upright without needing to rest their weight on their feet as four-legged animals do. Females of certain species of baboon have red buttocks that blush to attract males. In the case of humans, females tend to have proportionally wider and thicker buttocks due to higher subcutaneous fat and proportionally wider hips. In humans they also have a role in propelling the body in a forward motion and aiding bowel movement.〔Foundations of Osteopathic Medicine, Page 586, Anthony G. Chila - 2010〕〔Recent Advances in Pediatrics, 2013 Suraj Gupte, p 141〕
Some baboons and all gibbons, though otherwise fur-covered, have characteristic naked callosities on their buttocks. While human children generally have smooth buttocks, mature males and females have varying degrees of hair growth, as on other parts of their body. Females may have hair growth in the gluteal cleft (including around the anus), sometimes extending laterally onto the lower aspect of the cheeks. Males may have hair growth over some or all of the buttocks.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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