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breast
The breast is one of two prominences found on the upper ventral region of the torso of male and female primates. In females, it serves as the mammary gland, which produces and secretes milk and feeds infants.〔(【引用サイトリンク】title=Breast – Definition of breast by Merriam-Webster )〕 Both males and females develop breasts from the same embryological tissues. At puberty, estrogens, in conjunction with growth hormone, cause breast development. Males do not develop pronounced or well-matured breasts because their bodies produce lower levels of estrogens and higher levels of androgens, namely testosterone, which suppress the effects of estrogens in developing breast tissue. The breasts of females are typically far more prominent than those of males.
Subcutaneous fat covers and envelops a network of ducts that converge to the nipple, and these tissues give the breast its size and shape. At the ends of the ducts are lobules, or clusters of alveoli, where milk is produced and stored in response to hormonal signals.〔(【引用サイトリンク】url=http://training.seer.cancer.gov/breast/anatomy/ )〕 During pregnancy, the breast responds to a complex interaction of hormones, including estrogens, progesterone, and prolactin, that mediate the completion of its development, namely lobuloalveolar maturation, in preparation of lactation and breastfeeding. Upon childbirth, the alveoli are stimulated to produce and secrete milk for infants.
Along with their function in feeding infants, female breasts have social and sexual characteristics. Breasts have been featured in notable ancient and modern sculpture, art, and photography. Female breasts can figure prominently in a woman's perception of her body image and sexual attractiveness. A number of Western cultures associate breasts with sexuality and tend to regard bare breasts as immodest or indecent. Breasts and especially the nipples are an erogenous zone on women. Given the emphasis of some cultures on breast size and attractiveness, some women seek breast augmentation or other kinds of surgery to enlarge or reduce their breast size or to reverse sagging breasts.
==Etymology==
The English word ''breast'' derives from the Old English word ''brēost'' (breast, bosom) from Proto-Germanic ''breustam'' (breast), from the Proto-Indo-European base bhreus– (to swell, to sprout).〔(Indo-European Lexicon: Pokorny Master PIE Etyma )〕 The ''breast'' spelling conforms to the Scottish and North English dialectal pronunciations.〔(Online Etymology Dictionary )〕
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