The term breast can refer to the upper ventral region of the human torso. Alternatively the term is used for each of two parts of that, especially for women: the breasts are parts of the female human body that contain the organs that secrete milk used to feed babies. Maless also have breasts and are born with the main milk ducts intact, but while the gland that produces milk is present in the male, it remains undeveloped unless stimulated by the female hormone, estrogen. Milk production can also occur in both men and women as a rare side-effect of some medicinal drugs (such as some antipsychotic medication). Both sexes have a large concentration of blood vessels and nerves in their nipples.
A woman's breasts sit over the pectoralis major muscle and usually extend from the level of the 2nd rib to the level of the 6th rib anteriorly. The superior lateral quadrant of the breast extends diagonally upwards in an 'axillary tail'. A thin layer of mammary tissue extends from the clavicle above to the seventh or eight ribs below and from the midline to the edge of the latissimus dorsi posteriorly.
Others believe that the human breast evolved in order to prevent infants from suffocating while feeding. Since human infants do not have a protruding jaw like our ancestors and the other primates, the infant's nose might be blocked by a flat female chest while feeding. According to this theory, as the human jaw became recessed, so the breasts became larger to compensate.
A common misconception is that women have breasts so that they can feed babies by producing milk. The mammary glands that secrete the milk from the breasts make up a relatively small fraction of the overall breast tissue. Most of the human female breast is actually adipose tissue (fat) and connective tissue. Breast size does not make any difference to a woman's ability to nurse a baby.
Because some cultures place a high value on symmetry of the female human form, and because women often identify their femininity and sense of self with their breasts, many women in developed countries undergo breast reconstruction after mastectomy for breast cancer.
- http://www.puberty101.com/p_stages_breasts.shtml -- Stages of breast development, from Puberty101